UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Occurrence and utilization of fog Hayes, Derek 1970

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Notice for Google Chrome users:
If you are having trouble viewing or searching the PDF with Google Chrome, please download it here instead.

Item Metadata

Download

Media
831-UBC_1970_A8 H38.pdf [ 10.56MB ]
Metadata
JSON: 831-1.0103983.json
JSON-LD: 831-1.0103983-ld.json
RDF/XML (Pretty): 831-1.0103983-rdf.xml
RDF/JSON: 831-1.0103983-rdf.json
Turtle: 831-1.0103983-turtle.txt
N-Triples: 831-1.0103983-rdf-ntriples.txt
Original Record: 831-1.0103983-source.json
Full Text
831-1.0103983-fulltext.txt
Citation
831-1.0103983.ris

Full Text

THE OCCURRENCE AND UTILIZATION OF FOG. by DEREK WILLIAM HAYES B.Sc, U n i v e r s i t y of H u l l , 19-69. A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OE ARTS i n the Department of Geography. We accept this ! thesis as; conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA May, 1970 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced degree at the U n i v e r s i t y o f B r i t i s h Co lumb i a , I a g r e e tha t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s tudy . I f u r t h e r ag ree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y purposes may be g r a n t e d by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . It i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f GEOGRAPHY  The U n i v e r s i t y o f B r i t i s h Co lumbia Vancouver 8, Canada 16th June, 1970. i i ABSTRACT. The t h e s i s i s p r o p o s e d t h a t , u n d e r c e r t a i n f a v o u r a b l e c o n d i t i o n s , w a t e r may be d e p o s i t e d f r o m f o g s on» t o p l a n t s , and u t i l i z e d b y t h e m . Where o t h e r s o u r c e s o f w a t e r a r e s c a r c e , t h i s ; a d d i t i o n may be c r i t i c a l f o r t h e i r s u r v i v a l . The p h y s i c a l c h a r a c t e r i s t i c s , and t h e s p a t i a l and t e m p o r a l o c c u r r e n c e o f f o g s a r e d i s c u s s e d . A n a t t e m p t i s made t o d e f i n e some o f t h e a s s o c i a t e d m e t e o r o l o g i c a l c o n d i t i o n s , and some o f t h e c a u s a l m e c h a n i s m s , o f c e r t a i n common f o g t y p e s , b y a d e t a i l e d a n a l y s i s o f t h e t e m p o r a l c o i n c i d e n c e o f f o g and c e r t a i n m e t e o r o l o g i c a l p a r a m e t e r s a t V a n c o u v e r I n t e r n a t i o n a l A i r p o r t . T h i s ; i n f o r m a t i o n is? t h e n u s e d t o c o n s t r u c t a c o n d i t i o n a l p r o b a b i l i t y mode l f o r ' t h e p r e d i c t i o n o f f o g o c c u r r e n c e . The- t e c h n i q u e s ; o f m e c h a n i c a l l y m e a s u r i n g i n t e r c e p t i o n o f w a t e r f r o m f o g s a r e n e x t c o n s i d e r e d . E x p e r i m e n t s ; w i t h s c r e e n e d r a i n g a u g e s , i n E n g l a n d , and a gauze - c y l i n d e r t y p e r e c o r d i n g f o g g a u g e , a t V a n c o u v e r I n t e r n a t i o n a l A i r p o r t , a r e d e s c r i b e d , and t h e s i g n i f i c a n c e o f t h e r e s u l t s a s s e s s e d . I t i s h y p o t h e s i z e d t h a t t h e r e a r e two m a i n p a t h w a y s o f f o g w a t e r u t i l i z a t i o n b y v e g e t a t i o n ; t h e e v i d e n c e f o r e a c h i s a s s e s s e d . T h e s e a r e d i r e c t a b s o r p t i o n o f d e p o s i t e d w a t e r , w h i c h u s u a l l y o c c u r s u n d e r m o i s t u r e s t r e s s e d c o n d i t i o n s , and d r i p t o t h e g r o u n d , w i t h r e p l e n i s h m e n t o f s o i l w a t e r , and s u b s e q u e n t n o r m a l r o o t a b s o r p t i o n . I t i s c o n s i d e r e d t h a t c e r t a i n p l a n t m o r p h o l o g i e s , a t v a r i o u s s c a l e s , i n f l u e n c e t h e amount o f w a t e r t h a t may be i n t e r c e p t e d f r o m f o g . T h e s e a r e d i s c u s s e d , and a t t e m p t s a r e made a t e x p e r i m e n t a t i o n , u s i n g l a b o r a t o r y f o g i i i s i m u l a t i o n . The d i f f i c u l t i e s o f a c c u r a t e l y m e a s u r i n g f o g d r i p amounts f r o m c o m p l e x v e g e t a t i o n t y p e s a r e e v a l u a t e d , and some s i m p l e m o d e l s o f f o g d r i p a s s e s s e d . i v CONTENTS. Page CHAPTER 1. INTRODUCTION 1 CHAPTER 2. PHYSICAL ASPECTS OF FOG . . . 3 T y p e s o f f o g 3 ( i ) R a d i a t i o n f o g 4 ( i i ) A d v e c t i o n f o g 6 ( i i i ) H i l l f o g 6 C o n d e n s a t i o n and n u c l e i 7 H y g r o s c o p i c n u c l e i 8 Non - h y g r o s c o p i c 10 S p a t i a l d i s t r i b u t i o n o f c o n d e n s a t i o n n u c l e i 11 F o g d r o p l e t s i z e s 13 L i q u i d w a t e r c o n t e n t 14 CHAPTER 3. FOG OCCURRENCE AND FREQUENCY 16 F o g o c c u r r e n c e . 16 ( i ) The d a t a 16 ( i i ) S p a t i a l and a l t i t u d i n a l v a r i a t i o n o f f o g i n B r i t i s h C o l u m b i a 17 The m e t e o r o l o g i c a l f a c t o r s a s s o c i a t e d w i t h f o g . . 22 A n a l y s i s o f f o g a t V a n c o u v e r I n t e r n a t i o n a l A i r p o r t 22 Time o f y e a r 23 F o g and t e m p e r a t u r e 24 F o g and r e l a t i v e h u m i d i t y 31 H o u r s o f s u n s h i n e , 34 F o g and w i n d 37 W i n d d i r e c t i o n 44 F o g p r e d i c t i o n : a m o d e l 47 CHAPTER 4. FOG INTERCEPTION INSTRUMENTATION 64 V F o g i n t e r c e p t i o n 1 b y s c r e e n s 6k B a l a n c e i n s t r u m e n t s 69 F o g i n t e r c e p t i o n by c y l i n d r i c a l - mesh m o d i f i e d r a i n g a u g e s •. 70 C o n s t r u c t i o n and e x p o s u r e o f a f o g gauge a t V a n c o u v e r I n t e r n a t i o n a l A i r p o r t • • • • 75 Some r e s u l t s f r o m a f o g gauge i n s t a l l e d a t V a n c o u v e r I n t e r n a t i o n a l A i r p o r t 81 CHAPTER 5. DIRECT ABSORPTION OF FOG WATER BY THE A E R I A L PARTS OF PLANTS ®7 T h e o r e t i c a l c o n s i d e r a t i o n s 87 P h y s i o l o g i c a l c o n s i d e r a t i o n s : p a t h w a y s o f w a t e r e n t r y i n t o a e r i a l p l a n t o r g a n s 91 1. The s t o m a t a 91 2. The c u t i c l e and t h e e p i d e r m i s 96 3. S p e c i a l i z e d c e l l s f o r w a t e r i n t a k e 98 P h y s i c a l c o n s i d e r a t i o n s 102 1. The e f f e c t o f t e m p e r a t u r e 102 2. The w e t t a b i l i t y o f t h e c u t i c l e 103 A b s o r p t i o n and t r a n s l o c a t i o n : e x p e r i m e n t a l e v i d e n c e 105 A e r i a l a b s o r p t i o n i n D o u g l a s F i r 110 I n t r a - p l a n t w a t e r f l o w I l 4 C h a p t e r c o n c l u s i o n 115 CHAPTER 6 . FOG DRIP AND FORESTS 118 T e c h n i q u e s and r e s u l t s 119 The e f f e c t o f e l e v a t i o n above s e a l e v e l 122 E f f e c t o f e l e v a t i o n o f v e g e t a t i o n 125 D e p t h o f p e n e t r a t i o n o f f o g i n t o a f o r e s t 126 CHAPTER 7. FORM, WATER CATCH, AND FOG DRIP 129 v i S c a l e s o f c o n s i d e r a t i o n 1 2 9 The f o g s i m u l a t o r 1 3 1 E x p e r i m e n t s 1 3 ^ I n t e r c e p t i o n f r o m n a t u r a l f o g s . 1 ^ 3 P r e l i m i n a r y m o d e l s o f f o g d r i p 1 ^ 7 CHAPTER 8 . CONCLUSION 1 5 2 APPENDIX I . S i g n i f i c a n c e t e s t 1 5 5 APPENDIX I I . Mon - p a r a m e t r i c t e s t f o r t r e n d 1 5 6 BIBLIOGRAPHY 1 5 8 v i i T A B L E S . Page 1. P e r c e n t a g e f r e q u e n c y o f f o g d r o p l e t n u c l e i 11 2. S p a t i a l d i s t r i b u t i o n o f c o n d e n s a t i o n n u c l e i 12 3. C o r r e l a t i o n o f f o g day f r e q u e n c y , d i s t a n c e f r o m s e a , and a l t i t u d e i n B r i t i s h C o l u m b i a 18 4. Min imum t e m p e r a t u r e s , days w i t h m o r n i n g f o g and d a y s w i t h o u t f o g , V a n c o u v e r I n t l . A . , 1965 - 1969 • 28 5. T e m p e r a t u r e c h a r a c t e r i s t i c s on f o g da3 rs and n o n -f o g d a y s i n t h e montane f o r e s t of. E c u a d o r 30 6. R e l a t i v e h u m i d i t y ( p e r c e n t ) w i t h and w i t h o u t f o g a l l d a y , V a n c o u v e r I n t l . A . , 1965 - 19^9 31 7. D u r a t i o n s o f h i g h r e l a t i v e h u m i d i t i e s , w i t h and w i t h o u t f o g , i n montane f o r e s t i n E c u a d o r 32 8 . R e l a t i v e h u m i d i t y ( p e r c e n t ) b e f o r e , d u r i n g , and a f t e r t h e o c c u r r e n c e o f A . M . f o g , w i t h c o m p a r i s o n . o f n o n - f o g d a y s . V a n c o u v e r I n t l . A . , 1965 - 19^9 33 9. Mean number o f h o u r s o f b r i g h t s u n s h i n e w i t h and w i t h o u t f o g , V a n c o u v e r I n t l . A . , 1965 - 1969 34 10. Hours ; o f s u n s h i n e a s s o c i a t e d w i t h m o r n i n g f o g o c c u r r e n c e , V a n c o u v e r I n t l . A . , 1965 - 1969 36 11. A v e r a g e h o u r l y w i n d r u n a t V a n c o u v e r I n t l . A . , 1965 - 1969 40 12. Maximum w i n d s p e e d s a t V a n c o u v e r I n t e r n a t i o n a l A i r p o r t , 1965 - 1969 • •• 40 13. F r e q u e n c i e s o f w i n d speeds p r e c e d i n g f o r m a t i o n o f n i g h t fogs ; a t Kew, E n g l a n d , 1900 - 1905 44 l i . : ; l . ? e T O ? n c i e s , fey mbnth's7 ; , b' ' f ; ; days when t h e d i f f e r e n c e b e t w e e n t h e a c t u a l t e m p e r a t u r e and t h e d e w p o i n t was o f t h e m a g n i t u d e i n d i c a t e d , a t 1600 PST 52 15. F r e q u e n c i e s , b y m o n t h s , o f d a y s when t h e d i f f e r e n c e b e t w e e n t h e a c t u a l t e m p e r a t u r e and t h e d e w p o i n t was o f t h e m a g n i t u d e i n d i c a t e d , a t 2200 PST 53 v i i i 16. F r e q u e n c i e s , b y m o n t h s , o f d a y s when t h e w i n d s p e e d was o f t h e m a g n i t u d e i n d i c a t e d , a t 2200 PST . . . . . . . 54 17. F o g p r o b a b i l i t i e s i n s p e c i f i e d p a r a m e t e r ranges ; , V a n c o u v e r I n t l . A . , I965 - 1969 56 18. A v e r a g e p r o b a b i l i t y o f m o r n i n g f o g : 3 v a r i a b l e s , V a n c o u v e r I n t l . A . , 1958 - 1969 6 l 19. A v e r a g e p r o b a b i l i t y o f m o r n i n g f o g : 2 v a r i a b l e s , V a n c o u v e r I n t l . A . , 1958 - 1969 63 20. A b s t r a c t o f r e s u l t s o f s c r e e n e d gauge e x p o s u r e s on B e a c h y H e a d , E n g l a n d , D e c . 19^7 - J a n . I968 67 21. F o g d r i p measurements on t h e San F r a n c i s c o P e n i n s u l a 120 22. A n n u a l r a i n f a l l and n e g a t i v e i n t e r c e p t i o n a t s e l e c t e d l o c a t i o n s i n t h e Cascade Head E x p e r i m e n t a l F o r e s t , O r e g o n 123 23. Mean amounts o f w a t e r (grams) e x t r a c t e d b y c i r c l e s . 139 24. Mean amounts o f w a t e r (grams) e x t r a c t e d b y s h a p e s . . l 4 l 25. R e l a t i o n s h i p s o f f o g w a t e r c a t c h i n an hour, t o w e i g h t i n Thu. ja p l i c a t a Dorm 144 26. Amounts o f w a t e r c a u g h t f r o m a n a t u r a l f o g , U . B . C . , J a n u a r y , 1970 . ... l 4 5 i x PLATES. 1. Gauge f i t t e d with, mesh m o d i f i e r , Beachy Head, England, J u l y , 1969 72 2. V e r t i c a l w i r e gauge m o d i f i e r as o r i g i n a l l y c o n s t r u c t e d lh 3 . Fog gauge, Vancouver I n t e r n a t i o n a l A i r p o r t , W i nter 1969 - 1970 76 k. D e t a i l o f f o g c a t c h i n g d e v i c e , f o g gauge, Vancouver I n t e r n a t i o n a l A i r p o r t , Winter 1969 - 1970 77 5. Douglas F i r s e e d l i n g s . (A and B) used f o r a e r i a l a b s o r p t i o n experiments^ 113 6. Douglas F i r s e e d l i n g s (C) used f o r a e r i a l a b s o r p t i o n experiments 113 7. The f o g s i m u l a t o r 132 8 . Two a r t i f i c i a l l e a v e s ready to be exposed 135 9. Technique o f a r t i f i c i a l l e a f exposure 135 X FIGURES. 1. A g e n e t i c f o g c l a s s i f i c a t i o n s y s t e m 5 2. H o u r s o f f o g w i t h e l e v a t i o n , s o u t h e r n B . C 20 3u Mean a n n u a l f r e q u e n c y o f A . M . f o g , V a n c o u v e r I n t e r n a t i o n a l A i r p o r t 25 4 . T o t a l d a y s \ t f i th a l l day f o g , by m o n t h s , V a n c o u v e r I n t e r n a t i o n a l A i r p o r t 26 5 . Mean d a i l y t e m p e r a t u r e r a n g e s , V a n c o u v e r I n t e r n a t -- i o n a l A i r p o r t 29 6. F r e q u e n c y o f f o g a t d i f f e r e n t w i n d s p e e d s , o v e r s e a o f f N e w f o u n d l a n d 39 7. -Mean w i n d speeds; p r e c e d i n g and n o t p r e c e d i n g m o r n i n g f o g , V a n c o u v e r I n t l . A . , 19&5 - 19^9 42 8 . Mean w i n d speeds p r e c e d i n g and n o t p r e c e d i n g m o r n i n g f o g , by months-, V a n c o u v e r I n t l . A . , 1965 -1969 43 9. Mean w i n d speeds p r e c e d i n g and n o t p r e c e d i n g m o r n i n g f o g , V i c t o r i a I n t l . A . , 19^5 -I969 ^5 10. E m p i r i c a l p r o b a b i l i t y o f f o g w i t h v i s i b i l i t y l e s s t h a n 0 . 5 m i l e s and c e i l i n g l o w e r t h a n 200 f e e t , b y h o u r s , V a n c o u v e r I n t l . A . , 1950 - 19^9 48 11. S e t m o d e l o f f o g and n o n - f o g i n t e r s e c t i o n s w i t h t h e p a r a m e t e r s v , y , and z . 58 12. A n g l e o f f a l l o f f o g and r a i n d r o p s i n v a r y i n g w i n d s p e e d s 80 13. C l i m a t i c p a r a m e t e r s and f o g d r i p ( i ) 85 1.4. C l i m a t i c p a r a m e t e r s and f o g d r i p { i l ) 86 1 5 . L o n g i t u d i n a l l e a f s e c t i o n o f a s p e c i a l i z e d e p i d e r m a l c e l l o f a l e a f o f Chaetacme a r i s t a t a . . . . 99 16. U p p e r l e a f s u r f a c e s t r u c t u r e o f M c i n t o s h a p p l e . . . . 101 17. Shapes o f a l u m i n u m l e a v e s u s e d i n e x p e r i m e n t s 137 18. A r e a and d r i p f r o m a l u m i n u m e l i d e s 138 19. A r e a and d r i p f r o m a l u m i n u m shapes 140 x i Pathways! o f f o g w a t e r use by p l a n t s 151 x i i ACKNOWLEDGEMENTS. I s h o u l d e s p e c i a l l y l i k e t o t h a n k D r . J . E . H a y , my a d v i s o r , f o r h i s e n c o u r a g e m e n t , h e l p , and a d v i c e a t a l l s t a g e s . T h a n k s a r e a l s o due t o o t h e r members o f t h e D e p a r t m e n t o f G e o g r a p h y , U . B . C . , p a r t i c u l a r l y D r . G . R . G a t e s , f o r e n c o u r a g e m e n t , and f i n a n c i a l h e l p w i t h i n s t r u m e n t a t i o n ; P r o f . K . G . D e n i k e , f o r c o n s i d e r a b l e h e l p w i t h s t a t i s t i c a l p r o b l e m s ; D r . H .0. S l a y m a k e r , f o r r e a d i n g t h e f i n a l d r a f t and g i v i n g a d v i c e , and P r o f . M . N o r t h , f o r r e a d i n g and h e l p f u l c r i t i c i s m ! . D r . A . B l a c k , o f t h e D e p a r t m e n t o f S o i l S c i e n c e , U . B . C , r e a d and o f f e r e d a d v i c e on c h a p t e r f i v e . D r . J . D . Chapman and t h e U . B . C . G e o g r a p h y D e p a r t m e n t a r e t h a n k e d f o r f i n a n c i a l a s s i s t a n c e w i t h i n s t r u m e n t a t i o n , and t h e U . B . C . G e o l o g y D e p a r t m e n t i s t h a n k e d f o r t h e l o n g l o a n o f a m i c r o s c o p e . « The D e p a r t m e n t o f T r a n s p o r t , p a r t i c u l a r l y M r . W . M a c k i e , M r . N . P e n n y , and M r . H i . C h r i s t i a n s e n , i s t h a n k e d f o r a l l o w i n g me t o s i t e my f o g gauge a t V a n c o u v e r I n t e r n a t i o n a l A i r p o r t . F i n a l l y , I s h o u l d l i k e t o t h a n k D r . R . W a r d , o f t h e D e p a r t m e n t o f G e o g r a p h y , U n i v e r s i t y o f H u l l , E n g l a n d , who i n i t i a l l y f o s t e r e d my i n t e r e s t i n t h e p o s s i b i l i t i e s o f f o g p r e c i p i t a t i o n . -1 CHAPTER ONE. INTRODUCTION. I n t e r n a t i o n a l l y d e f i n e d , f o g i s a d i s p e r s e s y s t e m o f w a t e r d r o p l e t s s u s p e n d e d i n t h e a i r s u c h t h a t t h e v i s i b i l i t y i s r e d u c e d t o 1 0 0 0 m e t r e s o r l e s s . I t i s u s u a l l y t h o u g h t o f , b e c a u s e o f t h i s - r e d u c t i o n i n v i s i b i l i t y , t o be d e t r i m e n t a l , and t o be d i s p e r s e d i f p o s s i b l e . The m a i n o b j e c t o f t h i s t h e s i s i s t o show t h a t f o g c a n a l s o be b e n e f i c i a l ; w h i l s t h i n d e r i n g e c o n o m i c a c t i v i t y i n one a r e a , i t may make i t p o s s i b l e i n a n o t h e r . I t i s c o n s i d e r e d t h a t f o g i s u n d e r e s t i m a t e d as a s o u r c e o f w a t e r a v a i l a b l e f o r t h e g r o w t h o f p l a n t s i n many a r e a s where f o g s a r e p r e v a l e n t . T h i s i s p a r t i c u l a r l y t h e ca se where t h e n o r m a l p r e c i p i t a t i o n i s l o w . R e s e a r c h i n t o t h i s l a r g e l y u n t a p p e d w a t e r r e s o u r c e i s a t t h e p r e s e n t t i m e m i n i m a l , and more i s n e e d e d . P l a n t g r o w t h i n many s e m i - a r i d c o a s t a l a r e a s i s t h o u g h t t o be p o s s i b l e o n l y due t o t h e o c c u r r e n c e o f f o g . W a t e r s u p p l i e s may be augmented and u t i l i z e d f o r i r r i g a t i o n . W i r e n e t s s t r u n g a c r o s s t h e mouths o f some C h i l e a n 1 c o a s t a l d e s e r t v a l l e y s b e a r w i t n e s s t o t h i s u t i l i z a t i o n . F o r e s t f i r e s may be m i n i m i z e d i n o t h e r a r e a s . C h a p t e r s two and t h r e e c o n s i d e r how and w h e r e , and w i t h what f r e q u e n c y , f o g i s f o r m e d , f o r l i t t l e i s known "about t h e s p a t i a l and t e m p o r a l d i s t r i b u t i o n s o f f o g . The s p a t i a l and t e m p o r a l d i s c o n t i n u i t i e s i n f o g o c c u r r e n c e mean t h a t i t i s v e r y d i f f i c u l t t o a c c u r a t e l y d e f i n e , and i t s d i s t r i b u t i o n d i f f i c u l t t o d e p i c t . C h a p t e r f o u r c o n s i d e r s t h e p o s s i b l e t e c h n i q u e s 2 f o r m e a s u r i n g w a t e r c o n t r i b u t e d f r o m f o g s , as an e s s e n t i a l p r e l i m i n a r y t o any q u a n t i t a t i v e a s s e s s m e n t . The s u b s e q u e n t c h a p t e r s a r e c o n c e r n e d xtfith t h e a l t e r n a t i v e ways i n w h i c h v e g e t a t i o n may u t i l i z e w a t e r o b t a i n e d f r o m f o g . I t i s h y p o t h e s i z e d t h a t t h e r e a r e two p a t h w a y s o f w a t e r u s e . The f i r s t i s a c o m p a r a t i v e l y m i n o r one i n t e r m s o f t h e a b s o l u t e q u a n t i t i e s o f w a t e r i n v o l v e d , and r e q u i r e s w a t e r s t r e s s e d c o n d i t i o n s ; . T h i s i s t h e d i r e c t a b s o r p t i o n o f w a t e r i n t o t h e p l a n t t h r o u g h t h e a e r i a l o r g a n s , and i s d i s c u s s e d i n c h a p t e r f i v e . T h i s may, h o w e v e r , be o f g r e a t s i g n i f i c a n c e where o t h e r s u p p l i e s o f w a t e r a r e l i m i t e d . The s e c o n d pa thway i s when v e g e t a t i o n i n t e r c e p t s f o g w a t e r d r o p l e t s w h i c h s u b s e q u e n t l y d r i p t o t h e g r o u n d . T h i s augments t h e s o i l m o i s t u r e , and may t h e n be a b s o r b e d n o r m a l l y t h r o u g h t h e p l a n t r o o t s . I n c h a p t e r s i x , t h i s p r o c e s s i s c o n s i d e r e d , e s p e c i a l l y i n r e l a t i o n t o f o r e s t s , s i n c e h e r e t h e v e g e t a l s u r f a c e e x p o s e d t o t h e f o g , and p o t e n t i a l l y a b l e t o i n t e r c e p t w a t e r f r o m i t , i s a t a maximum'. I n c h a p t e r s e v e n i t i s h y p o t h e s i z e d t h a t t h e r e a r e c e r t a i n p l a n t and t r e e m o r p h o l o g i e s w h i c h a r e most e f f i c i e n t i n i n t e r c e p t i n g w a t e r f r o m f o g . An a t t e m p t i s made t o d e m o n s t r a t e t h e e f f e c t o f l e a f a r e a and shape b y l a b o r a t o r y e x p e r i m e n t a t i o n . I t i s c o n c l u d e d ( C h . 8) t h a t , u n d e r c e r t a i n c o n d i t i o n s and i n c e r t a i n l o c a t i o n s , f o g i s o f g r e a t p o t e n t i a l b e n e f i c i a l u s e , and i t i s a t p r e s e n t u n d e r - u t i l i z e d as a w a t e r r e s o u r c e . 3 CHAPTER TWO PHYSICAL ASPECTS OF FOG. F o g i s t h e r e s u l t o f t h e c o n d e n s a t i o n o f w a t e r v a p o u r i n t h e a i r , f o r m i n g m i n u t e w a t e r d r o p l e t s . The r a t e o f v a p o u r c o n d e n s a t i o n , and t h u s t h e r a t e o f f o g d r o p l e t g r o w t h , a r e a f u n c t i o n o f many i n t e r r e l a t e d f a c t o r s , some o f w h i c h a r e s t i l l b e i n g i d e n t i f i e d b y c l o u d p h y s i c i s t s . I t may be s t a t e d : dq "dt" = f ( T, p, w, .... ) + f ( n, d, .... ) where d q / d t i s t h e r a t e o f d r o p l e t g r o w t h , and t h e f i r s t e x p r e s s i o n i n v o l v e s m e t e o r o l o g i c v a r i a b l e s : T = t e m p e r a t u r e p =' v a p o u r p r e s s u r e w = a measure o f t u r b u l e n c e ; and t h e s e c o n d e x p r e s s i o n i s t o i n c l u d e e n v i r o n m e n t a l v a r i a b l e s , s u c h as n = c o n c e n t r a t i o n o f s u i t a b l e c o n d e n s a t i o n n u c l e i d = d i a m e t e r s o f d r o p l e t s a l r e a d y f o r m e d . I t w i l l be c o n v e n i e n t t o d i s c u s s t h e e f f e c t o f t h e s e two s e t s o f v a r i a b l e s s e p a r a t e l y , a l t h o u g h i t w i l l be r e a d i l y a p p r e c i a t e d t h a t i n r e a l i t y t h e two c a n n o t be s e p a r a t e d , and t h e f o r m a t i o n o f f o g w i l l r e l y l a r g e l y on t h e i r s p a t i a l and t e m p o r a l c o i n c i d e n c e . TYPES OF FOG. Fogs may be c l a s s i f i e d on t h e b a s i s o f t h e i r method o f f o r m a t i o n . T h e r e a r e v e r y many ways i n w h i c h f o g may be f o r m e d , b u t t h e r e a r e two m a j o r o n e s ; b y 4 c o o l i n g o f t h e a i r s u c h t h a t t h e d e w p o i n t i s r e a c h e d , and b y t h e e v a p o r a t i o n o f w a t e r i n t o t h e a i r w i t h no n e c e s s a r y change i n t h e a m b i e n t a i r t e m p e r a t u r e . A l t h o u g h most f o g c l a s s i f i c a t i o n s t o d a t e have b e e n p r e s e n t e d on a g e n e t i c b a s i s ( B y e r s , 1965b; W i l l e t t , 1 9 4 4 ) , t h i s d i s t i n c t i o n ha s n o t b e e n e m p h a s i z e d . A f o g c l a s s i f i c a t i o n has b e e n d e r i v e d ( F i g . 1, o v e r l e a f ) w h i c h , w i t h i n t h i s b a s i c s u b d i v i s i o n , t a k e s a c c o u n t o f t h e method o f c o o l i n g o f t h e a i r , o r o f e v a p o r a t i o n i n t o i t . A l l known n a t u r a l l y o c c u r r i n g f o g t y p e s a r e c o v e r e d i n t h i s c l a s s i f i c a t i o n . H o w e v e r , t h r e e t y p e s o f f o g a r e most f r e q u e n t l y e n c o u n t e r e d , and t h e s e w i l l be c o n s i d e r e d f u r t h e r . ( i ) R a d i a t i o n F o g . R a d i a t i o n f o g s a r e c a u s e d by c o o l i n g o f t h e a i r i n c o n t a c t w i t h a c o m p a r a t i v e l y c o l d g r o u n d s u r f a c e b y r a d i a t i v e f l u x d i v e r g e n c e . The a i r i s t h u s c o o l e d , and i f t h e t e m p e r a t u r e a p p r o a c h e s t h e d e w p o i n t t e m p e r a t u r e , r a d i a t i o n f o g may f o r m . The c o o l i n g o f t h e l a y e r o f a i r i n c o n t a c t w i t h t h e g r o u n d p r o d u c e s an i n v e r s i o n o f t e m p e r a t u r e w h i c h t e n d s t o s t a b i l i z e any t h i n l a y e r o f f o g as i t f o r m s . D e e p e n i n g w i l l o n l y o c c u r i f t h e r e i s - some w i n d , t o c r e a t e enough t u r b u l e n c e t o t h i c k e n t h e f o g l a y e r . R a d i a t i o n f o g i s n o r m a l l y " y o u n g " ( i t ha s n o t e x i s t e d f o r more t h a n a few h o u r s ) ; c o n s e q u e n t l y t h e moda l d r o p l e t s i z e i s n o r m a l l y s m a l l , i n t h e r a n g e 2 - 2 0 yu . T h i s means t h a t i t ha s a t e n d e n c y n o t t o i m p a c t on t o v e g e t a t i o n , b u t t o f l o w a r o u n d i t ( see C h . 7 ) . A l s o , w i n d s p e e d s a r e u s u a l l y l i g h t , and t h e r e i s n o t t h e n e c e s s a r y movement t o a l l o w " c o m b i n g " o f t h e 5 A . CAUSED BY COOLING OF A I R AND CONSEQUENT SATURATION. 1. A d v e c t i v e c o o l i n g . i i . L a n d & s e a b r e e z e f o g . w a r m a i r b l o w i n g t o c o l d e r l a n d o r s e a s u r f a c e . i i i . T r o p i c a l a i r f o g . . . . . . l a r g e s c a l e t r o p i c a l a i r movement t o h i g h e r l a t i t u d e s , t h u s c o o l i n g a i r i n summer o v e r c o l d w a t e r 2!. A d i a b a t i c c o o l i n g . . u p s l o p e m o t i o n c a u s i n g c o o l i n g ; i n c l u d e s c l o u d on g r o u n d , . f l u x o f a i r mass a c r o s s i s o b a r s t o l o w e r p r e s s u r e a r e a i . c o o l i n g due t o f a l l o f p r e s s u r e 3. R a d i a t i v e c o o l i n g . i . G r o u n d r a d i a t i o n f o g . . r a d i a t i o n f l u x d i v e r g e n c e i n f a i r l y s t i l l a i r i i . H i g h i n v e r s i o n f o g . . . . c o n t i n e n t a l s c a l e r a d i a t i o n o f h e a t , c o o l i n g o f a i r , c r e a t i o n o f h i g h l e v e l i n v e r s i o n . (100 - 600m.) i i i . R a d i a t i o n - a d v e c t i o n f o g n i g h t r a d i a t i o n a l c o o l i n g o f a i r a d v e c t e d i n l a n d i n day 4. M i x a t i v e c o o l i n g . i . H o r i z o n t a l m i x i n g f o g i i . F r o n t pa s s age f o g . . . . . h o r i z . m i x i n g o f a i r , c o o l i n g . c o o l i n g a t a f r o n t due t o m i x i n g o f a i r masses i B . CAUSED BY SATURATION DUE TO EVAPORATION. (No n e c e s s a r y t e m p e r a t u r e change) 1. Downward v a p o u r f l u x ( f r o n t a l ) i . P r e warm; f r o n t f o g . . . i i . P o s t warm f r o n t f o g . . e v a p o r a t i o n f r o m warmer r a i n f a l l i n g t h r o u g h c o l d e r a i r b e l o w 2. Upward v a p o u r f l u x ( s team) . warm w a t e r e v a p o r a t i o n i n t o v e r y c o l d a i r . l o c a l s a t u r a t i o n b y a n i m a l h e r d s & s e t t l e m e n t i n v e r y c o l d a i r F i g . 1. A g e n e t i c f o g c l a s s i f i c a t i o n s y s t e m . 6 water from them by vegetation. ( i i ) Advection Fog. Advection fogs are formed by the cooling of the a i r to i t s dewpoint by transportation over a colder surface. Advection fog i s t y p i c a l l y formed by warm a i r moving offshore from a warm land mass, to be cooled by a colder current. The cause of fog along the coast of western North America, South America, and South West A f r i c a , where there are "foggy deserts" (see Ch. 5«) i s advectiorj of a i r from warm water to cold water i n an onshore d i r e c t i o n . The cold water along the coasts of the west coast deserts i s the r e s u l t of the c h a r a c t e r i s t i c cold currents of those regions. The theory of Sverdrup (l<?42) , as to the cause of the fog on the B r i t i s h Columbia coast i s that during part of the year north-westerly winds* i n ad d i t i o n to the G o r i o l i s force cause a south-westerly movement of water away from the coast. This i s replaced by cold water upwelling to the surface, and provides the necessary cooling medium f o r onshore winds. (Pincock and Turner, 1956) | ( i i i ) H i l l Fog. H i l l fog, also termed mountain or upslope fog, i s formed when a i r i s forced to r i s e , and as a consequence i s cooled a d i a b a t i c a l l y to i t s dewpoint. True h i l l fog i s probably quite rare, (George, 1 9 5 l ) , but: i n the present study any cloud type' touching the 1 surface i n an upland region w i l l be considered as h i l l fog. In the mountain water catchment areas 7 considered l a t e r (Ch. 6) t h i s i s probably the main Tog type. CONDENSATION AND NUCLEI. Whilst the preceding fog c l a s s i f i c a t i o n ( F i g . 1, p1'. 5) i n d i c a t e s that meteorologic fac t o r s play a dominant r o l e in' determining fog formation, p h y s i c a l environmental f a c t o r s , such as the a v a i l a b i l i t y of n u c l e i f o r the condensation process, also play an important, though l e s s well-defined, r o l e . Some of the non - meteorologic f a c t o r s w i l l now- be considered. In a i r a r t i f i c i a l l y freed from aerosols and free ions, condensation, cannot occur u n t i l about an e i g h t f o l d supersaturation has been reached. (Relative humidity = 800 per cent.) Condensation at t h i s supersaturation has. been termed spontaneous condensation by Landsberg ( 1 9 3 8 ) . In a i r pure except f o r free ions, condensation may occur when a f o u r f o l d supersaturation has been reached. (Relative humidity = 400 per cent.) However, the air. i s i n r e a l i t y never t o t a l l y free from aerosols, and i t i s generally considered that the formation of fog and cloud droplets occurs around the m i l l i o n s of hygroscopic n u c l e i normally found i n the a i r . Condensation occurs because of the zone of lower saturation vapour pressure found around the hygroscopic nucleus. The discovery of t h i s f a c t was due to the cla s s i c , experiments of Aitken (1888 - 1892) and Wilson ( 1 8 9 7 ) . Growth of droplets-occurs because of the e f f e c t of surface tension. Small droplets i n cloud and fog have s l i g h t l y higher vapour pressures: than the la r g e r droplets. The smaller droplets thus evaporate to give vapour which condenses again on the l a r g e r ones. 8 (Langmuir, 1 9 4 8 ) . This pressure diff e r e n c e i s very-small, but i s very important i n the growth of fog^and cloud drbplets up to 20 - 3 0 / * diameter. Whilst other processes have to be considered i n droplet growth above t h i s diameter, (Mason, 1957) , t h i s process probably accounts! f o r the growth of most types of fog, where the modal droplet diameter is- 20 - 30 /A- . Okita (1962) concluded from a study i n Hokkaido, Japan, that ordinary fog droplets grow mainly by condensation. There are many types of condensation n u c l e i . Atmospheric condensation n u c l e i are believed to o r i g i n a t e i n three main ways: by condensation and sublimation of vapours during smoke formation, and i n gaseous; reactions;; by the mechanical d i s r u p t i o n and d i s p e r s a l of matter, such as the formation of dust and spray; and by the coagulation of n u c l e i , which may y i e l d mixed p a r t i c l e s as well as simply l a r g e r ones. (Mason, 1 9 5 7 ) . Since condensation n u c l e i play an important r o l e i n the condensation process, i t w i l l be of value to b r i e f l y discuss the d i f f e r e n t types of n u c l e i , and - consider where they might occur. Hygroscopic n u c l e i . Several i n v e s t i g a t i o n s have shown that sea s a l t s , i n c l u d i n g NaCl, MgCl 2 , MgSO^ , and CaSO^ , are the most frequent types of condensation. (Simpson, 194l) Results of chemical composition analyses c a r r i e d out by Gindel ( 1 9 6 6 ) , at Rehovot, I s r a e l , suggested that at l e a s t i n a semi - a r i d environment, chlorides and sulphates formed the most abxindant n u c l e i types. S o i l p a r t i c l e s and cosmic dust have also been found as hygroscopic n u c l e i i n fogs. (Myers,1968)i; Simpson (l9 4 l ) c a l c u l a t e d that i f sea s a l t s 9 were e n t i r e l y responsible f o r condensation, n u c l e i would have to be produced from the ocean surface at the 2 —1 u n l i k e l y rate of 50,000 cm sec . Mason (1957) pointed out that, although Simpson probably overestimated, i t ; is-; l i k e l y that other sources of hygroscopic condensation n u c l e i are important. Ogiwara and Okita (1952) concluded, from a study at Sendai, Japan, that combustion products were the main source of hygroscopic condensation n u c l e i i n t h e i r samples. They considered that sea s a l t s played only a minor r o l e , despite the fa c t that Sendai, where the samples were taken, i s only 12 km. from the sea. Combustion products, from factories;, automobiles, and the l i k e , produce b i l l i o n s of hygroscopic, n u c l e i , i p a r t i c u l a r l y sulphur t r i o x i d e , that r a p i d l y disperse and become a s i g n i f i c a n t fog - aiding f a c t o r i n many areas. L o c a l l y , man's a c t i v i t i e s strongly augment the number of a v a i l a b l e condensation n u c l e i . (Byers, 1965a) The famous study of Hrudieka (193&) i s of relevance here. Hrudieka analysed h i s t o r i c a l records, and produced f i g u r e s f o r 20 year i n t e r v a l s of the average number of foggy days i n Prague1, Czechoslovakia: Date 1801-1820 1821-1840 1841-1860 1861-1880 1881-1900 1901-1920 Av.No. foggy days/ year. 83 80 87 79 158 217 The incre a s i n g number of foggy days was a t t r i b u t e d by Hrudieka to the e f f e c t of increas i n g i n d u s t r i a l i z a t i o n a f t e r about 1880, and the consequent p r o v i s i o n of condensation n u c l e i r e s u l t i n g from the burning of c o a l . Beyond 1920, there was apparently no further increase, due 10 to, according to Geiger (1965), "improvements! i n the construction of f i r e p l a c e s , " and "replacement of steam locomotives by e l e c t r i c ones." (Geiger, 1965, p. ^92.) In many i n d u s t r i a l i z e d parts of the world, fog frequency has now decreased with the a v a i l a b i l i t y of more sophisticated technology to remove po l l u t a n t s , increased p u b l i c awareness, and l e g i s l a t i o n , such as the? Glean A i r Act (1956) passed f o r London, England, as a r e s u l t of public outcry against the 1952 smog. Household chimneys i n that c i t y now/ produce only 13' per cent of the smoke that they produced i n 1952. (Province, Vancouver, B.C. newspaper, 1. 12. 69., p. 2.) The absorptive properties of hygroscopic n u c l e i allow condensation to begin at comparatively low r e l a t i v e humidities, down to about 65 per cent, although normally condensation w i l l not occur u n t i l the r e l a t i v e humidity i s over 90 per cent. Non - hygroscopic n u c l e i . One author, Kuroiwa (1953) found that condensation could occur on non - hygroscopic n u c l e i . These were derived from combustion products such as carbon - black and t a r s , but the physics of t h i s condensation process i s > l u n c e r t a i n . Kuroiwa also found fog droplets apparently without a nucleus at a l l . These he ( explained by hypothesizing that l a r g e r drops accumulating around non - hygroscopic n u c l e i may s p l i t i n t o two pieces, making one a no - nucleus; droplet. I t i s possible, Kuroiwa states, that«when a cloud i s formed i n v i o l e n t a i r streams, such as that i n cap clouds over the summit; of a mountain, or i n the clouds produced i n steeply r i s i n g a i r currents, l a r g e r water droplets may o c c a s i o n a l l y be broken up by purely mechanical 1 1 causes. Table 1 , below, from Kuroiwa ( 1 9 5 3 ) and Nakaya ( 1 9 5 7 ) , gives an i n d i c a t i o n of the r e l a t i v e frequencies of d i f f e r e n t nucleus types. Type of Combustion S o i l Sea No Sample Fog. ~" product. material s a l t nucleus t o t a l . A.Sea fog 4 9 . 1 2 1 4 . 0 3 28 . 0 7 8 . 7 7 57 B.Mt. fog "51.28 28.20 12.82 7 . 6 9 3 9 C. Both 5 0 . 0 0 1 9 . 7 9 2 1 . 8 7 8 . 3 3 9 6 D.Sea fog 5 0 . 0 0 1 0 . 0 0 40 . 0 0 not given 2 5 5 Table. 1 . Percentage frequency of fog droplet n u c l e i . 1 Includes hygroscopic and non - hygroscopic n u c l e i . A,B, and C are cal c u l a t e d from figures given by Kuroiwa, ( 1 9 5 3 ) , p. 3 7 2 ; D i s from Nakaya ( 1 9 5 7 ) . SPATIAL DISTRIBUTION OF CONDENSATION NUCLEI. L i t t l e work has been c a r r i e d out on analysing the s p a t i a l d i s t r i b u t i o n of condensation n u c l e i ; from what has been said previously, this? iss of s i g n i f i c a n c e to the present study. One study that has been made is; that of Landsberg ( 1 9 3 8 ) . He analysed -the nucleus) contents of the atmosphere i n d i f f e r e n t l o c a l i t i e s , and found that the average concentration i s smallest over the oceans and i n the upper a i r , and i s greater i n a i r subject to i n d u s t r i a l p o l l u t i o n . Landsberg's results; are shown i n Table 2 , overleaf. I n t e r p r e t a t i o n of these nucleus; concentration f i g u r e s i s very d i f f i c u l t , as there.are so many va r i a b l e s 12 L o c a l i t y No. of places No. of observations Average nucleus concentration, (cm^ ) C i t y 28 2,500 147,000 Town 15 4,700 34,300 Country, i n l a n d 25 3,500 9,500 Country, shore 21 2,700 9,500 Mt., 500-1000m. 13 870 6,000 Mt.,1000-2000m. 16 1,000 2,130 Mt., 2000m. + 25 190 950 Islands 7 480 9,200 Ocean 21 6oo 940 Table. 2 . S p a t i a l d i s t r i b u t i o n of condensation n u c l e i . (Adapted-, from Landsberg, 1938) that need.to be considered. Landsberg attempted some correlations; with c l i m a t i c parameters, i n c l u d i n g fog frequency, without success, and he concluded that the nucleus count at a given s t a t i o n i.s; c o n t r o l l e d too much by l o c a l influences to permit general inferences. In West Pakistan', and also i n Mexico, measurements of condensation nucleus concentrations by Fournier D'Albe (1957) showed that i n these locations the main n u c l e i source was the sea and coastal zones. The main agent; of nucleus removal from the a i r was found to be p r e c i p i t a t i o n , but Fournier D<r'Albe found that i f the n u c l e i escaped p r e c i p i t a t i o n near the coasts they could e a s i l y penetrate f a r inland. I t i s c l e a r that i t ±s-. not at present possible to draw any fir m conclusions as to the r e l a t i o n s h i p of fog to condensation nucleus d i s t r i b u t i o n s ; n u c l e i are 13 normally u n l i k e l y to be the l i m i t i n g f a c t o r on fog or cloud formation, however. (Mason, 1957) FOG DROPLET SIZES. Before considering fog d i s t r i b u t i o n , and the meteorologic, f a c t o r s associated with t h i s d i s t r i b u t i o n , (Ch. 3 ) , i t w i l l be of value here to consider b r i e f l y the droplet size c h a r a c t e r i s t i c s : and l i q u i d water contents of fogs, since t h i s i s of obvious s i g n i f i c a n c e i n any consideration of fog water use by plants. The l a r g e r the water droplet, the greater the momentum i t may possess to impact on to vegetal surfaces. Nucleus sizes do not a f f e c t the size d i s t r i b u t i o n of the fog droplets themselves. (Kuroiwa, 1953) Woodcock (1950)» however, found that large n u c l e i (diameter 7 1 /A, ) induced more rapid growth of a fog droplet than smaller n u c l e i . Natural fogs are r a r e l y monodisperse, that i s , with droplets of equal s i z e s , but have a range of diameters varying from 2 - 3 5 / 4 . up to 60 - 8 0 . Natural fogs may thus c o r r e c t l y be termed polydisperse. The normal droplet s i z e d i s t r i b u t i o n of fog can be approximated by a normal curve, with a modal frequency around 20 - 30 y U . The range of droplet sizes i s due to the f a c t that the factors causing droplet growth are almost never exactly s p a t i a l l y equal; s l i g h t v a r i a t i o n s i n vapour pressure y i e l d a polydisperse fog. (Coalescence of fog droplets i s not normally by c o l l i s i o n ; see p. 8 ) Modal droplet size appears to increase with time, that i s , with the "age", or persistence of the fog. Malrous ( 1 9 5 4 ) , i n a study of fogs o f f the E n g l i s h east coast, found that the only f a c t o r increasing the mean 14 droplet, diameter i n sea fog was the length of persistence. Thus i t may be concluded that where fog p e r s i s t s , the p r o b a b i l i t y of the growth of l a r g e r droplets i s increased; t h i s i n turn w i l l increase the chance pf impaction on to vegetation, due to increase of momentum with mass. LIQUID WATER CONTENT. The problems; associated with the measurement, of fog l i q u i d water content are considerable, (Mason, 1957), mainly r e l a t i n g to sampling procedures. From the evidence that e x i s t s , l i q u i d water content of fog appears to increase with temperature, since the a i r i s capable of holding more water at higher temperatures. (Zaitsev, 1950.) This assumption i s important since i t means that i n hot a r i d or semi-arid areas, fogs w i l l contain more water f o r the same volume of a i r at a given r e l a t i v e humidity. Possible d r i p of fog water from leaves of plants, or absorption i n t o the leaves (see CH. 5 and 6) w i l l also increase due to the decrease i n water v i s c o s i t y with temperature increase. The r e l a t i o n s h i p of l i q u i d water content to droplet size d i s t r i b u t i o n i n the fog i s unclear. Increase of momentum with mass; (see above) i s important f o r vegetal impaction. Grunow (1959), s u b s t i t u t i n g a wire mesh f o r vegetation, studied the amounts of water c o l l e c t e d i n r e l a t i o n to the fog droplet spectrum. (Since Grunow measured t h i s on Mt. Hohenpeissenberg, i n Bavaria, he was dealing with cloud touching the surface, which is- included here: under the term 'fd>g' ; see p. 6 ) He found that the e f f i c i e n c y of cold polar a i r f o r depositing water was very poor. This was characterized 15 by small diameter droplets, ranging from 2 to 15 / t • Increasing water catch was found to r e s u l t from maritime a i r masses, both temperate and subtropical i n o r i g i n , when the cloud droplet spectrum was characterized by a wider range of droplet diameters, from k. to 25 /« , with a modal frequency of 8 - 14 /? . Deposits were found to be heaviest from a i r masses that had degenerated due to continental influence. In t h i s case, the droplet spectrum was found to range from 5 to 60/* , with a modal frequency of 12 to 1 8 ^ diameter. Thus i t seems that the e f f e c t i v e l i q u i d water content, from the point of 'view of impaction on to vegetation, increases with age, increase of temperature, and i s also a function of a i r mass o r i g i n . In the next chapter, the s p a t i a l occurrence of fog, and the meteorological factors associated with i t s occurrence, w i l l be considered. 16 CHAPTER THREE POG OCCURRENCE AND FREQUENCY. No s i g n i f i c a n t amounts of water are l i k e l y to be gained from fogs i f the phenomenon occurs only r a r e l y . I t i s therefore of value to consider where, and with what frequency, fog may form. FOG OCCURRENCE. i . The Data. For the present purpose, that of considering the contribution to the water economy that may be gained from fogs, most of the e x i s t i n g maps of fog frequency are inadequate, i n that they do not give fog duration. Fog duration represents the time during which fog i s a v a i l a b l e f o r possible u t i l i z a t i o n by the vegetation. There have been comparatively few maps of fog frequency, but those that do e x i s t record "fog - days" as the unit of frequency, whereas t h i s study requires the use of a much smaller time i n t e r v a l . Hourly data i s probably much more j u s t i f i a b l e here. In North America, the f i r s t map of "fog - day^> frequency appears to have been that of Stone ( 1936) . Five years l a t e r , i n 19^1, a map of the "Average Annual Number of Days with Dense Fog" i n the United States was published (U.S.D.A., 1 9 4 l ) . Both of these maps, however, f a i l e d to take account of the changing d e f i n i t i o n s of the terms " fog " and " dense fog " that had occurred i n the 17 p e r i o d f r o m w h i c h t h e y d e r i v e d t h e i r mean v a l u e s . I n 1966, i n a n a t t e m p t t o overcome d i f f i c u l t i e s o f d e f i n i t i o n s , a map was p r o d u c e d b y C o u r t a n d Gears t o n (1966) o f f o g f r e q u e n c y i n t h e U n i t e d S t a t e s , w h i c h r i g o r o u s l y d e f i n e d t h e t e r m s u s e d i n v i s i b i l i t y d e f i n i t i o n s t o make up a c o m p o s i t e map f o r 60 y e a r means o f " f o g - d a y s . " However, f r o m t h e p r e s e n t p o i n t o f view/, t h i s map was s > t i l l o f l i m i t e d v a l i d i t y b e c a u s e i t a g a i n f a i l e d t o d e l i m i t a ny t e m p o r a l m i n i m a t o t h e o c c u r r e n c e o f f o g w i t h i n a p a r t i c u l a r d a y . A map o f f o g f r e q u e n c y b y h o u r s i s r e q u i r e d t o g i v e a more r e a l i s t i c p i c t u r e o f f o g d u r a t i o n . A c o m p a r a t i v e l y s m a l l number o f m e t e o r o l o g i c a l s t a t i o n s r e c o r d h o u r l y w e a t h e r , when compared t o t h e t o t a l number r e c o r d i n g w e a t h e r d a i l y . T h i s a p p l i e s t o N o r t h A m e r i c a a n d e l s e w h e r e . I n Ca n a d a , h o u r l y d a t a a r e a v a i l a b l e ( H o u r l y D a t a Summaries, f o r i n d i v i d u a l s t a t i o n s ) , b u t t h e h o u r s o f " f o g " r e c o r d e d i n c l u d e a l l v i s i b i l i t i e s , i c e f o g i n c l u d e d , o f l e s s t h a n s i x m i l e s . T h i s d e f i n i t i o n d o e s n o t a g r e e w i t h t h e i n t e r n a t i o n a l d e f i n i t i o n o f v i s i b i l i t y i n f o g . ( s e e p. l ) F o r t h e p u r p o s e h e r e , t h e a s s e s s m e n t o f p o t e n t i a l a v a i l a b i l i t i e s o f f o g f o r p l a n t u s e , t h i s d a t a s o u r c e i s n o t v e r y h e l p f u l w i t h o u t m o d i f i c a t i o n . i i . S p a t i a l a n d a l t i t u d i n a l v a r i a t i o n o f f o g i n B r i t i s h C o l u m b i a . I n any a t t e m p t t o c o n s t r u c t a map o f f o g f r e q u e n c y , some k n o w l e d g e o f t h e f a c t o r s i n f l u e n c i n g t h e d i s t r i b u t i o n o f f o g w o u l d be u s e f u l . C o r r e l a t i o n a n a l y s e s were c a r r i e d o u t on t h e " f o g - day" d a t a , a v a i l a b l e i n t h e f o r m o f mean v a l u e s , 18 f o r 36 stations i n B r i t i s h Columbia. No s i g n i f i c a n t c o r r e l a t i o n (at the 0.5 per cent l e v e l ) was found between fog - day frequency and e i t h e r a l t i t u d e or distance from the sea. Even i f only non - coastal stations are considered, to eliminate the eleven stations recording 'distance from seaiS as zero, s t i l l no s i g n i f i c a n t c o r r e l a t i o n i s found. The r e s u l t s are shown i n Table 3, below. CORRELATION COEFFICIENTS OF FOG-DAY FREQ. with d i s t . from sea. with a l t i t u d e . A l l stations (36) + 0.080* - 0.061* Non - coastal _ 0 < 0 3 0 * _ 0.219* stations (25) * = not s i g i l i f l e a n t at the 0.5 per cent l e v e l . Tab!e 3« C o r r e l a t i o n of fog - day frequency, distance  from sea, and a l t i t u d e i n B r i t i s h Columbia. In an attempt to see i f t h i s i n s i g n i f i c a n t c o r r e l a t i o n was true even with more defensible data, means of the number of hours of fog with v i s i b i l i t y of 5/8ths mile or l e s s (as i n t e r n a t i o n a l l y defined), and also w i t h t v i s i b i l i t y of \ mile or l e s s , were compiled f o r 13 stations i n southern B r i t i s h Columbia. The period used was the f i v e years 1957 - 1 9 6 l i n c l u s i v e , since t h i s was the l a s t complete f i v e year period f o r which the data were" iavailable i n published form. The "General Summaries of Hourly Weather Observations i n Canada" (D.O.T., annually), from which the data were extracted, ceased p u b l i c a t i o n i n 1961. One of the s t a t i o n records used was that f o r 19 Old Glory Mountain, near Rossland, B.C., at an a l t i t u d e of 7700 f e e t , several thousand feet above the next highest s t a t i o n i n B.C. Since t h i s s t a t i o n i s somewhat exceptionally high, c o r r e l a t i o n analyses were c a r r i e d out both i n c l u d i n g i t and excluding i t , against a l t i t u d e . Results are shown i n Table; 4, below. •JL. HOURLY DATA: Co r r e l a t i o n c o e f f i c i e n t : F og 1/ A l t i t u d e Thick f o g 2 / A l t i t u d e With Old Glory Mt. + 0 . 8 1 7 7 + 0.8064 Without Old Glory Mt. + 0 . 3 8 9 9 * + 0.3450* 2. FOG - DAY DATA, FOR COMPARISON: Cor r e l a t i o n c o e f f i c i e n t : 1 2 Fog / A l t i t u d e Thick fog / A l t i t u d e With Old Glory Mt. • Without Old Glory Mt. * = not s i g n i f i c a n t at the 0.5 per cent l e v e l . 1 = v i s i b i l i t y recorded as; 5/8ths mile or l e s s . 2 = v i s i b i l i t y recorded as % mile or l e s s . Table; 4. C o r r e l a t i o n of fog and a l t i t u d e f o r 13 B.C. sta t i o n s . The scatter diagram f o r hours of fog against elevation i s shown i n F i g . 2, overleaf. I t can be seen that the hourly data produce higher c o r r e l a t i o n s than the fog - day data. The production of a s i g n i f i c a n t c o r r e l a t i o n c o e f f i c i e n t by the; i n c l u s i o n of an extra s t a t i o n , Old Glory Mountain, i s s l i g h t l y dubious i n t u i t i v e l y , but. i s s t a t i s t i c a l l y - 0.0624* „ , „ ... Data not a v a i l a b l e . + 0.1538* 2 0 x 8000 7000 6000 5000 Uj U. 0 U j —j 4000 3000 2000 1000 otto 0. 500 WOO 1500 2000 2500 3000 HOURS OF FOG y PER YEAR F i g . 2. . Hours of fog with elevation, southern B\ C. 21 permissable; the trend i n the scatte r of f o g / a l t i t u d e pl o t s at low. frequencies and a l t i t u d e s i s resolved by the i n c l u s i o n of t h i s both high frequency and high a l t i t u d e s t a t i o n . I t may thus be t e n t a t i v e l y stated that fog duration, as measured by hourly frequency, increases with height i n southern B r i t i s h Columbia. Without Old Glory Mountain, however, the poor c o r r e l a t i o n of hourly fog frequency with height means that more data would be required before techniques of s p a t i a l i n t e r p o l a t i o n could be c o r r e c t l y applied to construct a fog frequency map. 22 THE METEOROLOGICAL FACTORS ASSOCIATED WITH FOG. I t i s i m p r a c t i c a l i n the present, context to carry out a comprehensive analysis of the meteorological parameters associated with the occurrence of fog on any widespread scale. I t i s considered, however, that i t would be u s e f u l to analyse i n d e t a i l the 'factors promoting the formation of fog, and those hindering i t s formation, f o r a single : s t a t i o n . Although t h i s s t a t i o n may not be completely t y p i c a l , i f such a s t a t i o n e x i s t s at a l l , t h i s should provide general i n s i g h t s into the factors a f f e c t i n g the frequency with which fog forms. I t i s also u s e f u l to analyse the meteorological f a c t o r s which accompany fog before attempting to reverse the a n a l y s i s , with the selected "best" v a r i a b l e s , to p r e d i c t the occurrence or non - occurrence of fog. Analysis? of fog at Vancouver I n t e r n a t i o n a l A i r p o r t . Mainly because of ease of a v a i l a b i l i t y of data, and also because the fog gauge described i n Ch. k could be located there, Vancouver Intern a t i o n a l A i r p o r t was chosen f o r the d e t a i l e d fog a n a l y s i s . Monthly c l i m a t o l o g i c a l summary sheets, issued f o r t h i s s t a t i o n by the Canada Department of Transport, were u t i l i z e d f o r t h i s study.. (D.O.T., monthly). The use of summary data makes analysis of a long period more f e a s i b l e than i t would be i f the d a i l y surface records had to be used. By reference to a subjective one -sentence desdription of an i n d i v i d u a l day's weather, the occurrence or non - occurrence of fog, and i t s approximate duration, could be found. I f a long period of data was analysed, such as that f o r which c l i m a t i c normals are derived (30 years), 23 the problem of non - s t a t i o n a r i t y of the data would probably become acute, since much recent evidence has been found f o r the a l t e r a t i o n of c l i m a t i c parameters with the advance of urbanization, and the development of the Greater Vancouver area has been considerable i n the l a s t 30 years. Thus i t was f e l t advisable to reduce the period considered to 5 years, and since concern i s here with what i s happening at the present time, the l a s t period of 5 consecutive; years (1965 - 1969 i n c l u s i v e ) was used. For the purpose of the present part of the study, considering the a s s o c i a t i o n of meteorological parameters with fog, without attempting to predict, a f i v e year period i s considered adequate. Fog was analysed at Vancouver Intern a t i o n a l A i r p o r t on the basis of three categories: " a l l day fogs", "morning fogs", (A.M. fog) and 'evening fogs", (P.M. fog). The Department of Transport defines A.M. fog as any fog which occurred during the period 0000 hours to 1200 hours, but not elsewhere; P.M. fog means any fog that occurred between the hours 1200 and 2400, but not elsewhere; a l l day fog r e f e r s to fog that occurred during a minimum o f a l l the daylight hours. The errors that have been introduced by the use of these durational d i v i s i o n s are not considered l i k e l y to be of s i g n i f i c a n c e f o r the present: purpose, though the l i m i t a t i o n s imposed by the data must be borne i n mind . The use of e a s i l y a v a i l a b l e data outweighs any minor disadvantages. In the f i v e year period, 120 A.M. fogs were reported, 16 a l l day fogs, and 11 P.M. fogs. Time of year. I t i s u s e f u l to look f i r s t at the time of year i n which fogs have been reported, since the periods of 24 maximum frequencies presumably represent those times when the causal factors most frequently i n t e r s e c t , or the d i s s i p a t i o n f a c t o r s are infrequent. A l l but four of .the 120 days on which morning fog was reported f e l l w i thin the period August to March i n c l u s i v e . These eight months have thus been given the most att e n t i o n . The frequency of days with morning fog, by months, i s shown i n F i g . J,, p. 25. Days recorded as having fog a l l day are concentrated into the months of November and October. (Fi g . 4, p. 26.) I f i t i s hypothesized that the majority of morning fogs are l i k e l y to be r a d i a t i o n fogs, whereas the majority of fogs p e r s i s t i n g a l l day are advection fogs, i t i s i n t e r e s t i n g to compare the data f o r V i c t o r i a I n t e r n a t i o n a l A i r p o r t with the Vancouver data. During the same period of f i v e years, V i c t o r i a recorded a s i m i l a r number of a l l day fogs; ( 2 l ) , but considerably fewer morning fogs. (?0) Evidence w i l l be presented l a t e r (p. 38 ) showing that wind speeds of l e s s than aboutr 5 mph are required f o r the formation of r a d i a t i o n fog. Thus i t may be concluded that, provided other fact o r s remain r e l a t i v e l y unchanged, the l e s s e r number of morning fogs recorded at V i c t o r i a i s due to the higher wind speeds normally recorded there, (see p. 44) Fog and temperature. , The volume of water required to saturate or nearly saturate the a i r increases as the temperature i s increased. I f other conditions remained equal, fog frequency would thus be expected to decrease with increase i n temperature, but i n f a c t higher temperatures are i n d i c a t i v e of higher l e v e l s of a v a i l a b l e energy, and 25 F i g . 3 . Mean annual frequency of A.M. f o g . V a n c o u v e r International A i r p o r t , 1965 - I 9 6 9 . 2b F i g . h . T o t a l days with a l l day fogt bY month. Vancouver Internat i o n a l A i r p o r t , 1 9 6 5 - 1 9 6 9 . *as defined; see p. 23 2? consequently greater evaporation may also occur. I t should be mentioned that i t is; normal f o r a v e r t i c a l i n v e r s i o n of the temperature- lapse rate to be associated with fog. This i s always true with r a d i a t i o n fogs, but "the same i s true, i n most cases, f o r advection fogs."(Pettersen, 1939, p. 18). In h i l l fog, as> defined here (see p. 6 ) , t h i s does not have to be the case, as; the fog top i s at the point where s t a b i l i t y i s reached. The temperatures at which fogs c h a r a c t e r i s t i c a l l y form w i l l of course vary with the l o c a t i o n of i n d i v i d u a l s t a t i o n s . However, where fogs form at temperatures below about - 30°C., they are composed p r i n c i p a l l y of i c e c r y s t a l s , and as such they are probably unusable by vegetation. Fogs,at temperatures above about - 30 ° C , remain as l i q u i d droplets because of t h e i r minute i n d i v i d u a l droplet s i z e s . These fogs, which form' 95 per cent of the world's fogs (Myers, 1 9 6 8 ) , are termed "warm" fogs i f i t i s required to d i s t i n g u i s h them from i c e fogs. Mean temperatures during fogs are reduced, since the t o t a l s o l a r r a d i a t i o n input is; reduced. For instance, at Vancouver Intern a t i o n a l A i r p o r t , the mean temperature on days recording a l l day fog i n November was kl.l °F. as compared with 45.3 ° F. on days; without fog. (From a sample of 5 per month per year f o r the 5 years; differences s i g n i f i c a n t at the 0.05 per cent l e v e l . ) I t should be noted that the minimum temperatures are not n e c e s s a r i l y lower when the mean temperature i s lower, f o r the temperature v a r i a t i o n i s within a smaller range (see F i g . 5, p. 2 9 ) . When the fog formed i n the morning only, the minimum temperature f o r the day as a whole may be decreased. This i s due to the loss of heat by the 28 ground surface, cooling the a i r beneath i t s dewpoint, and the l e s s e r advection of warm a i r to the s i t e due to the lower windspeeds recorded during fog. (see p.43) The reduction of minimum- temperatures f o r the day when morning fog was recorded i s shown i n Table 4 , below. Month Temperature min. ( P) Temperature min, ( F) Days recording A.M. fog. Days not recording fog. August 5 3 . 1 7 5 4 . 5 6 * Sept. 46 .88 4 9 . 9 2 Oct. 3 9 . 9 0 4 4 . 9 6 Nov. 3 3 . 5 3 4 0 . 0 8 Dec. 2 9 . 4 4 35.04 Jan. 3 0 . 0 8 3 0 . 2 0 * Feb. 2 8 . 6 0 3 3 . 6 0 Mean "" 3 8 . 1 6 4 0 . 8 3 ; * = not s i g n i f i c a n t at 0 . 0 5 per cent l e v e l . For t e s t of s i g n i f i c a n c e , see Appendix I. Table 4. Minimum temperatures: days with morning fog and days  without fog, Vancouver International A i r p o r t ,  1965 - 1 9 6 9 . D a i l y temperature range i n fogs that ex i s t a l l day i s reduced, compared with the temperature range on non - foggy days. This i s accounted f o r by the fac t that the top of a cover of fog r e f l e c t s a high percentage of the incoming s o l a r r a d i a t i o n , and at the same time the fog droplets absorb and re-transmit much of the long -wave r a d i a t i o n from the ground. Means of temperature ranges; f o r fog and non - fog conditions are shown i n F i g . 5 , P» 29 Q 1 1 I I I I I -I 1 A S O N 0 J F M MONTH A-A - Days with a.m. fog B-B : Days without fog C-C - Days with all day fog w. ; :— ~ F i g . 5 . Mean d a i l y temperature ranges. * Vancouver International A i r p o r t , 1965 - 1969. * "mean temperature range"as used here i s the differe n c e between mean maximum and mean minimum temperature f o r the month concerned. 3 0 I t can be seen that the mean d a i l y temperature i s lowest during days when fog xvas recorded a l l day. The mean d a i l y temperature range i s highest at Vancouver International A i r p o r t on days when morning fog was recorded. This i s due to the reduction of temperature by r a d i a t i o n a l cooling, with subsequent formation of fog i n the early morning, and also the clearness of the skies (allowing uninterrupted inputs of solar r a d i a t i o n ) , which, a f t e r "burning o f f " the fog, increases the a i r temperature. An i n t e r e s t i n g comparison with the Vancouver data may be made with the following table (Table 5 ), taken from data c o l l e c t e d by Grubb and Whitmore ( 1 9 6 6 ) , i n the t r o p i c a l montane f o r e s t of Ecuador. Temperatures i n •c. Fog • ( - bound days = 7 ) Fog - free days ( = 14 ) I t n e t r e above Temp. max. 19.4 2 5 . 6 ground i n cl e a r i n g Temp. Temp. min. mean 1 3 . 0 1 6 . 2 1 1 . 5 18 . 6 D a i l y range 6.4 14 . 1 1 metre above ,\v • :Temp. max. 1 6 . 3 1 9 . 3 ground i n forest- Temp. min. 1 3 . 0 1 2 . 1 undergrowth Temp. mean 1 4 . 7 1 5 . 7 • - -D a i l y range 3 . 3 7 . 2 Table 5 . Temperature c h a r a c t e r i s t i c s on fog days and non - fog days i n the montane fo r e s t of Ecuador. (From Grubb and Whitmore, 1 9 6 6 ) I t can be seen that the d a i l y temperature range i s halved when fog occurred. Minimum temperatures were 31 not. reduced, but the mean temperature was reduced. The f a c t t h a t a r e d u c t i o n i n the minimum temperature does n o t n e c e s s a r i l y accompany r e d u c t i o n i n mean temperatures, s i n c e the temperature range i s a l s o reduced, has been p r e v i o u s l y noted, (p. 27) Fog and r e l a t i v e h u m i d i t y . R e l a t i v e h u m i d i t y approaches 100 per cent as the dewpoint temperature i s approached, and f o g forms. A l t h o u g h n o r m a l l y r e l a t i v e h u m i d i t y w i l l be 100 per cent when the f o g forms, e a r l i e r c o n d e n s a t i o n i n d u c e d by a s u r p l u s o f c o n d e n s a t i o n n u c l e i may a l l o w i t to be lower, (see p. 7) At Vancouver I n t e r n a t i o n a l A i r p o r t , the r e l a t i v e h u m i d i t y i s g i v e n i n . t h e summary sheets f o u r times a day, at 0600, 1200, 1800 and 2400 G.M.T. (2200, 0400 , 1000, and 1600 P.S.T.) T a b l e 6, below, shows r e l a t i v e h u m i d i t i e s on days w i t h f o g a l l day, as compared w i t h days w i t h o u t fog:. P.S.T. 0400 1000 1600 2200  f o g no f o g f o g no f o g f o g no f o g f o g no f o g Oct. 95.50 89.44 96.00 81.72 92.25 75.52 95.25 91.76 Nov. 98.44 85.92 98.11 83.52 97.44 80.44 9 8 . l l 87.60 Dec. 100.00 90.40 100.00 85.96 100.00 83.8O 100.00 84.76 Jan. 98.OO 82.10 99.00. 83.35 93.50 83.55 97.50 83.75 A J 1 . . . 97.75 8.7.22 97.81 83.65 95.81 80.68 97.44 87.14 obs. A l l d i f f e r e n c e s s i g n i f i c a n t to the 0.05 per cent l e v e l , (see Appendix I.) T a b l e 6. R e l a t i v e h u m i d i t y (per cent) w i t h and without f o g a l l day, Vancouver I n t e r n a t i o n a l A i r p o r t , 1965 - 69 32 I t i s clear' that the r e l a t i v e humidity stays closer to saturation' a l l the time during fog, as might be expected. Decrease of r e l a t i v e humidity during daylight: hours i s c h a r a c t e r i s t i c of most days, with or without fog, due to the increase i n temperature, and the consequent increase i n the water holding capacity of the a i r . This decrease i s apparent from Table 6. (p. 31)• S i m i l a r differences may be; noted i n the relative; humidities, recorded on days with morning fog (Table 8, overleaf.) Prom the' readings at 1000 hours, PST, onward, however, the differences between foggy and non - foggy days becomes- less; as most of the morning fogs: are at l e a s t i n the process of being d i s s i p a t e d by the sun by that hour. The increase of r e l a t i v e humidity with fog i s to be expected anywhere, since i t i s always necessary f o r the dewpoint temperature to be approached or reached to give fog. This increase has been shown by Grubb and Whitmore (1966)for a t r o p i c a l montane r a i n f o r e s t i n Ecuador. The r e l a t i v e humidity of this, environment i s normally high, but i s even higher i n fog. They showed an increase i n the duration of periods with a r e l a t i v e humidity of greater than 95 per cent. (Table 7, below.) Si t e Means f o r fog - bound Means f o r fog - free; ' ' days ( = 7) days (= Ik) 1 metre above g r o u n d i n 19.5 hours; 13.5 hours c l e a r i n g 1 metre above— ground-in-forest 23.0 hours 16.0 hours und e rgrowth. Table 7. Durations of high r e l a t i v e humidities with and without fog, i n montane f o r e s t i n Ecuador. (Adapted from Grubb and Whitmore, I 9 6 6 , pp. 311 & 312) 5£/T. OCT. A/Ol/. free. # 1 . 1 - 0 6 i E . « V f l T l O A i S P.S.T. Fig Mo Fog ^5 to.'tv Wo fog witk F o 3 Ulrk No * 5 Fog Fog Fog No Fo3 /too 71.0 63. 6 75.8 70. 8 83.4 75. 5 83.9 80. 4 89.7 83.8 86.6 83. 5 82.2 78. * 6 78.0 66. 9 81.1 75.2 n o o 88.3 85. 2 91.5 85. 6 91.9 9 1 . » 6 92.7 87. 6 96.3 84. 8 94. 8 83. 7 92.3 88. 4 93.7 82. 7 92.4 86.3 0 4 0 0 95.2 90. 2 97.6 91. 0 96.1 89. 4 98.0 85. 9 96.9 9 0.4 94.6 82. 1 96.0 89. 6 95.2 87. 7 96.8 88.4 I O O O _ e / fobo 80.0 7>. 8 84.7 79. 2 85.0 8 1 . # 7 91.2 83. 5 95.8 85.9 96.2 83. 3 89.3 82. 9 83.2 74. 5 87.8 80.7 67.7 63. # 6 74.9 70. * 8 80.6 75. 5 85.9 80. 4 88.2 83.8 90.0 83. 5 79.4 78. * 7 75.0 66. 8 79.6 75.2 noo 81.5 85. 2 89.1 85. 6 91.7 9 1 . 7 91.0 87. 6 94.1 84. 7 95.8 83. 7 91.3 88. 4 92.0 82. 7 90.4 86.3 * = d i f f e r e n c e n o t s i g n i f i c a n t ! a t 0 . 0 5 p e r c e n t l e v e l . ; T a b l e 8. R e l a t i v e h u m i d i t y ( p e r c e n t ) b e f o r e , d u r i n g , and a f t e r t h e f o g d a y s , b y m o n t h s , o c c u r r e n c e o f A . M . , w i t h c o m p a r i s o n o f n o n -I 9 6 5 - 1 9 6 9 , V a n c o u v e r I n t e r n a t i o n a l A i r p o r t . 34 Hours of sunshine:. Bright sunshine i s notably reduced when fog occurs at Vancouver International A i r p o r t . (Table 9, below.) Oct. Nov. Dec. Jan. A l l obs. Days-with fog 2 2 Q o.28 0.0 0.1 a l l day 0.72 Days without 4 # 2 4 1 > 6 g 6 k g 2 # 2 8 fog. A l l d ifferences s i g n i f i c a n t at the 0.05 per cent l e v e l , (see Appendix. I) Table; 9. Mean number of hours of bright sunshine with and without fog, Vancouver International A i r p o r t , 1965 - 1969. The study of Grubb and Whitmore ( 1 9 6 6 ) , previously noted, found a s i m i l a r r e s u l t from the t r o p i c a l montane f o r e s t of Ecuador. Mean sunshine duration i n a c l e a r i n g was 0.7 hours on foggy days, but 5.5 hours on fog free days. This is: s i g n i f i c a n t i n that the probable energy a v a i l a b i l i t y f o r evapotranspiration is> reduced on foggy days, a f a c t o r increasing the p o s s i b i l i t y of water economy by the vegetation, aside from any considerations of actual water gain from fog. (see Ch. 5) Net r a d i a t i o n i s a fun c t i o n of sunshine. A considerable reduction of the net r a d i a t i o n i n fog has been observed by Yevfimov ( 1 9 5 1 ) , Krasikov ( 1 9 4 8 ) , and S h i f r i n and Bogdanova (1955) S h i f r i n ( l 9 5 l ) computed an equation f o r t h i s reduction i n net r a d i a t i o n , of the form: 35 P 0 = F Q ( 1 - eT< a + r ) ) where;: F Q = the net r a d i a t i o n of a c l e a r sky. and a + r = c o e f f i c i e n t s of absorption and r e f l e c t i v i t y c a l c u l a t e d f o r the whole fog thickness. The e f f e c t of sunshine as a fog control i s of relevance. As w i l l be shown l a t e r , the maximum p r o b a b i l i t y of fog i s . j u s t before sunrise, when the sun has had no chance to d i s s i p a t e ("burn o f f " ) the fog by evaporation from the top surface of the fog and to create turbulence that might d i s s i p a t e i t . This was also found by Buma ( i 9 6 0 ) , i n an analysis of fog at Leeuwarden, i n the Netherlands. In London, England, Davis ( l 9 5 l ) found that the maximum frequency of fog was two to three hours a f t e r sunrise, a delay a t t r i b u t e d to p o l l u t i o n e f f e c t s a r i s i n g from smoke f i r e s . I t has already been hypothesized that most of the fog recorded at Vancouver Intern a t i o n a l A i r p o r t i s r a d i a t i o n fog.. I f t h i s i s so, then the hours of sunshine recorded on that day might be expected to be increased from that on days without fog, since c l e a r skies during the nmght of r a d i a t i o n fog formation are necessary to allow enough r a d i a t i o n a l cooling to occur. An examination of Table 10, overleaf, shows that this; i s indeed the case, and i t might reasonably be concluded that the hypothesis presented above i s co r r e c t . Most authors concur with the above r e s u l t , but George; (1951) states as; well that one of the main conditions f o r the formation of r a d i a t i o n fog is; that "the; a i r has been under a cloud cover ... during the day previous to i t s formation." (George, 1951, P« 1184) He i s alone i n t h i s conclusion, however. (Myers,1968) 36 Month. Day preceding A.M. Day with A.M. Day without, fog;. fog. and not preceding A.M. fog. 1 2 3 Aug. 8 . 1 5 0 * - 9 . 0 3 0 7 . 7 2 4 Sept. 6 . 8 2 9 6 . 9 1 0 6 . 7 5 6 Oct. 5 . 0 0 5 * 6.080 4 . 2 4 0 Nov. 4 . 7 7 2 4 . 0 8 0 1 . 6 8 4 Dec. I.75O* 2 . 3 8 0 1.644 Jan. 1.14.0* 1 . 9 4 0 1 . 3 7 5 Feb. 5 . 2 8 6 7 . 0 6 0 2 . 5 3 6 Mar. 6 . 9 7 5 7 . 0 7 0 4 . 7 4 0 Apr. 1 . 7 5 0 1 0 . 0 0 0 5 . 4 2 0 * = not a s i g n i f i c a n t d i f f e r e n c e at the 0 . 0 5 per cent l e v e l . (Col. 1 and 2 compared to Col. 3 « ) Table> 1 0 . Hours of sunshine associated with morning fog occurrence, Vancouver Interna t i o n a l A i r p o r t , 1 9 6 5 - 1 9 6 9 . (See text f o r explanation.) 37 George's statement i s not supported by the data of hours of sunshine; ( i n d i c a t i n g l i t t l e cloud cover) on days previous to morning fog formation at Vancouver. (see c o l . 1 i n Table- 10, p. J,6) In f a c t , the amount: of sunshine i s higher (but i n s i g n i f i c a n t l y so i n four months), and thus cloud cover may be- taken as l e s s , on days preceding morning fog, as compared to the days not preceding fog formation. Fog and wind. The i n t e n s i t y of turbulence generally increases with the wind speed. Thus i t might be expected that moderate wind speeds would create enough turbulence to enable the fog tro thicken above i t s s i t e of formation and a t t a i n a height i n the order- of several hundred f e e t . Fogs that e x i s t when there are- strong winds are u s u a l l y very deep, because they f i l l the e n t i r e l a y e r below an i n v e r s i o n , which i s i t s e l f normally r a i s e d by turbulent mixing. Lack of any mixing due to turbulence on calm days w i l l not allow the formation of any great thickness of r a d i a t i o n fog, and may prevent active inland movement of advection fogs. On the other hand, once; the wind speed increases too much, the absolute value of t h i s speed being dependent on the s t a b i l i t y of the a i r , the fog may be d i s s i p a t e d , or be r a i s e d to a l a y e r of stratus cloud. Strong winds are i n many s i t u a t i o n s l i k e l y to be one of the more s i g n i f i c a n t controls on fog formation, and p r e d i c t i o n of the wind speed w i l l often allow p r e d i c t i o n of a s i t u a t i o n where i t ; i s possible f o r fog to form i f the required increase i n r e l a t i v e humidity to nearly .100 per cent also occurs. Fog w i l l tend to be dispersed by wind unless there 38 i s a constant supply of further fog. For the reason that stratus cloud at higher' l e v e l s tends to be more stable than fog on the ground beneath an i n v e r s i o n (which i s e a s i l y disturbed by wind unless, i t i s very marked), the strongest wind recorded during fog i s often on high ground. This i s w i t h i n h i l l fog, as defined here, (see p. 6.) For instance, Nagel (1956) found an average wind speed of 13 m. sec""''. during the times that the "Table Cloth" covered Table Mountain, South A f r i c a . During an experiment by the writer, (see p. 66 ) v i s i b i l i t y at 500 feet a l t i t u d e on Beachy Head, a coastal headland i n Sussex, England, remained at about 30 yards with a WSW .wind greater than 11 m. sec ^. With fogs formed at lower l e v e l s , wind i s u s u a l l y an important c o n t r o l . I t i s c l e a r that the i n s i t u conditions of the formation of r a d i a t i o n fog make i t more s e n s i t i v e to wind; s t a b i l i t y , through turbulence, may e a s i l y be disturbed. However, the upper l i m i t of windspeed at which fbg can e x i s t i s greater over the sea than over land. This i s p a r t l y due; to the increased f r i c t i o n over land, which causes increased v e r t i c a l mixing. (Taylor, 1917) Nevertheless, advection fog appears to have a modal occurrence c o i n c i d i n g with comparatively low windspeeds. F i g . 6, p. 39 , shows a graph constructed from data given by Taylor (1917) of the frequency of c e r t a i n windspeeds i n advection fog over the sea o f f Newfoundland. Considerable reductions i n both the average hourly wind run and the maximum (one minute) windspeed during a l l day fogs were found at Vancouver Intern a t i o n a l Airport.(Tables 11 and 12, p. 40.) The average wind run and the average maximum 'windspeed f o r a l l observations of a l l day fog at V i c t o r i a I n t e r n a t i o n a l A i r p o r t , (21 obs. ) during the same f i v e year 39 sot O 1 2 3 4 5 6 7 8 WI ND SPEED -BEAUFORT SCALE F i g . 6 . Frequency of fog at d i f f e r e n t windspeeds, over sea o f f Newfoundland. ( a f t e r Taylor, 1917.) Beaufort Scale 0 = Less than 1 mph. Beaufort Scale 7 = More than 32; l e s s than 38 mph. 4 o Fog days f o g f r e e days O c t o b e r 4.82 MPH 7 . 3 3 MPI-I November 3 . 0 6 6 . 2 9 December 6 . 5 0 8 . 8 5 J a n u a r y 4 . 6 5 8 , 3 7 A l l obs. (Oct - J a n o n l y ) 3 . 9 1 7 . 6 8 A l l d i f f e r e n c e s s i g n i f i c a n t a t t h e 0 . 0 5 p e r c e n t l e v e l . T a b l e . 1 1 . Average H o u r l y Wind Run a t V a n c o u v e r I n t e r n a t i o n a l A i r p o r t , 1 9 6 5 - 1 9 6 9 . c Fog days f o g f r e e days O c t o b e r 9 . 0 0 MPH 1 3 . 9 2 MPH November 7 . 0 0 i l . 7 2 ^ December 7 . 0 0 1 5 . 8 8 J a n u a r y . 9 . 0 0 1 6 . 4 0 A l l obs. (Oct - J a n o n l y ) 7 . 7 5 : 1 4 . 3 8 A l l d i f f e r e n c e s s i g n i f i c a n t a t the 0 . 0 5 p"er c e n t l e v e l . Table 12. Mean maximum (one minute) windspeeds at Vancouver I n t e r n a t i o n a l A i r p o r t , 1 9 6 5 - 1 9 6 9 . 4 i p e r i o d ' 1965 - 1969 were s l i g h t l y h i g h e r t h a n t h o s e a t V a n c o u v e r , b e i n g 5.99 MPH and 11.90 MPH r e s p e c t i v e l y . T h i s s u g g e s t s t h a t a l a r g e r p r o p o r t i o n o f t h e f o g s a r e a d v e c t i o n f o g s , •which may e x i s t a t t h e s e w i n d s p e e d s . F o r t h e f o r m a t i o n o f r a d i a t i o n f o g , i t ha s a l r e a d y b e e n n o t e d ( p . 38) t h a t w i n d speeds h a v e t o be r e l a t i v e l y l o w , t o a v o i d t o o much t u r b u l e n c e , w h i c h w o u l d t e n d t o d i s s i p a t e t h e f o g . I t ha s a l s o b e e n n o t e d t h a t c o m p l e t e ca lm; i s n o t s u i t a b l e f o r f o g f o r m a t i o n , s i n c e a c e r t a i n amount o f t u r b u l e n c e i s r e q u i r e d t o t r a n s p o r t fog ; f o r m e d a t t h e g r o u n d s u r f a c e u p w a r d s , and r e p l a c e t h e g r o u n d l a y e r w i t h more a i r f o r c o o l i n g . A s t u d y o f t h e c h a r a c t e r i s t i c xirindspeeds a s s o c i a t e d w i t h t h e f o r m a t i o n o f m o r n i n g f o g ( t h e m a j o r i t y assumed t o be r a d i a t i o n f o g s ; see- p . 35) » a t V a n c o u v e r I n t e r n a t i o n a l A i r p o r t showed a v e r y s i g n i f i c a n t d i f f e r e n c e i n t h e w i n d s p e e d s on d a y s p r e c e d i n g t h e f o r m a t i o n o f f o g as compared w i t h t h e d a y s n o t p r e c e d i n g f o g f o r m a t i o n . F i g s . 7 and 8, on the= n e x t two p a g e s , show t h e mean w i n d s p e e d s i n b o t h c a s e s f r o m 1800 PST on t h e day p r e v i o u s t o 1200 PST t h e n e x t d a y . The g r e a t d i f f e r e n c e . ' i n w i n d s p e e d s , and t h e a p p a r e n t e a r l y s t a r t t o t h e t r e n d t o l o w e r w i n d speeds when f o g f o r m s t h e n e x t m o r n i n g , i s t a k e n a d v a n t a g e o f l a t e r ( p . 47) t o p r e d i c t t h e o c c u r r e n c e o f m o r n i n g f o g . I n a n e g a t i v e s e n s e , T a y l o r (1917) showed how w i n d s p e e d s t h e e v e n i n g b e f o r e c o u l d be u s e d t o p r e d i c t t h e " n o n - o c c u r r e n c e " o f f o g . H i s t a b l e , g i v e n i n T a b l e 13, p . 44, was u s e d f o r ' t h i s " n e g a t i v e \ f o r e c a s t i n g . " A f o g - f r e e m o r n i n g was p r e d i c t e d a t Kew, E n g l a n d , f r o m where; t h e r e s u l t s a r e d e r i v e d , o n e v e r y o c c a s i o n when t h e w i n d s p e e d a t 2000 GMT was g r e a t e r t h a n 5.5 MPH, and o n l y two i n c o r r e c t p r e d i c t i o n s were made i n t h e f i v e y e a r ' p e r i o d . F i g . 3 « Mean wind speeds p r e c e d i n g and d u r i n g morning f o g s , by months, Vancouver I n t e r n a t i o n a l A i r p o r t , 1965 - 1969-kk Wind v e l o c i t y MPH * 1 6 0 0 1 8 0 0 2 0 0 0 2 2 0 0 2 4 0 0 GMT 0 - 3 . 3 2k 35 5 0 5 8 6 2 3 . 3 ; - 5 . 5 2 3 2 0 1 8 1 0 5 5 . 5 - 9 . 2 1 6 1 3 1 2 3 9 . 2 - 1 3 . 6 7 2 1 1 1 T o t a l number 1 o f cas e s = 7 0 * These c l a s s i n t e r v a l s c o i n c i d e w i t h the B e a u f o r t s c a l e u n i t s . T a b l e 1 3 . F r e q u e n c i e s o f windspeeds p r e c e d i n g f o r m a t i o n  o f n i g h t fogs; a t Kew, E n g l a n d , 1 9 0 0 - 1 9 0 5 . ( A f t e r T a y l o r , 1 9 1 7 . ) A t V i c t o r i a I n t e r n a t i o n a l A i r p o r t , many o f t h e fogs; r e c o r d e d as m o r n i n g f o g s a r e p r o b a b l y a d v e c t i o n f o g s , s i n c e t h i n a d v e c t i o n f o g s a r e known t o be f r e q u e n t a t t h i s s t a t i o n . ( P i n c o c k and T u r n e r , 1 9 5 6 ) Some o f the f o g s r e c o r d e d o n l y d m r i n g t h e A . M . h o u r s a r e r a d i a t i o n a l i n o r i g i n , b u t t h e r e c o r d o f mo r n i n g f o g e s s e n t i a l l y c o n t a i n s b o t h t y p e s . Thus i n t h e g r a p h o f h o u r l y windspeeds f o g V i c t o r i a , s h o r n o v e r l e a f , ( F i g . 9 - ) > a l t h o u g h windspeeds a r e l e s s d u r i n g and p r e c e d i n g f o g , as compared v / i t h p e r i o d s n o t p r e c e d i n g f o g , t h e y v a r y about: a h i g h e r mean v a l u e . Wind d i r e c t i o n . B r i e f m e n t i o n s h o u l d be made o f w i n d d i r e c t i o n s d u r i n g f o g . L i k e t e m p e r a t u r e s , t h e r e w i l l be no c h a r a c t e r i s t i c d i r e c t i o n f o r a l l s t a t i o n s ; t h e p r e v a i l i n g 12n 5 ^ DflfS W l T H o l / r P o C r MO 12 18 19 2 0 21 2 2 2 3 2 4 1 2 3 "4 5 6 7 8 9 10 11 1 2 ° h o u r F i g . c j . Mean wind speeds p r e c e d i n g and not p r e c e d i n g .morning f o g , V i c t o r i a I n t e r n a t i o n a l A i r p o r t , 1965 - 1969. 46 wind d i r e c t i o n depending s o l e l y on l o c a l conditions. The p r e v a i l i n g wind d i r e c t i o n at Vancouver i s east, and t h i s also applies during fog. (56.25 per cent of the t o t a l observations.) Whilst c h a r a c t e r i s t i c wind d i r e c t i o n s during fog may be of value at some sta t i o n s , e s p e c i a l l y x^rhere a causal r e l a t i o n s h i p i s expected, the lack of gen e r a l i t y of a p p l i c a t i o n l i m i t s t h e i r use. (Woodward, 1 9 4 l ; Pincock and Turner, 1956.) 47 FOG PREDICTION : A MODEL. I t i s of value to attempt to predict the occurrence of fog, since successful p r e d i c t i o n may allow,? extrapolation of fog frequency data beyond that a c t u a l l y a v a i l a b l e . I t i s evident that f o r fog to occur and p e r s i s t , c e r t a i n combinations of c l i m a t i c f a c t o r s must occur. Some of these factors have previously been mentioned; i t i s , however, important to d i s t i n g u i s h between cause and  e f f e c t . Fogs can be formed i n several d i f f e r e n t ways, and an i d e a l model would include a l l the possible approaches to fog formation, but such a model would be very complex. A fog p r e d i c t i o n model is; presented here which applies, to Vancouver Intern a t i o n a l A i r p o r t , and i s derived from 12 years; of data, from 1958 to 1969 i n c l u s i v e , i n i t i a l l y using only the l a s t 5 of those years. This; 12: year data period is; the maximum a v a i l a b l e , i n accessible fomv, since the times of the r e l a t i v e humidity and temperature readings on which the model depends were a l t e r e d on 1 s t June, 1957• This period i s only 3 years short of the 15 year period recommended by Panofsky and B r i e r (1968) f o r t h i s type of usage. The p r o b a b i l i t y of fog by hours has; been • calc u l a t e d from a 20 year summary published by the Department of Transport. (D.O.T., 1970) Although t h i s data includes cloud c e i l i n g s of lower than 200 f e e t i n a d d i t i o n to fogs with v i s i b i l i t y of 0.5 mile- or l e s s , i t allows the deduction of c e r t a i n conclusions, (see F i g . 10, overleaf) Two periods of maximal fog p r o b a b i l i t y are evident, and.if the; hypothesis that the majority of fog at Vancouver I n t e r n a t i o n a l A i r p o r t i s r a d i a t i o n fog i s accepted (p. 2:4 and 35) , the analysis may be based on the expectation of r a d i a t i o n fog. I t may thus be hypothesized HOUR F i g . 10 . E m p i r i c a l p r o b a b i l i t y o f f o g w i t h v i s i b i l i t y l e s s  t h a n "0.5 m i l e o r c e i l i n g o f l e s s t h a n 200 f e e t , "by  h o u r s , V a n c o u v e r I n t e r n a t i o n a l A i r p o r t . 20 y e a r  r e c o r d 1950 - 1969. T h i s " g r a p h r e f e r s t o t h e p e r i o d S e p t . - J a n . o n l y . S o u r c e : V . I . A . r e c o r d s , D e p t . o f T r a n s p o r t , 1970. h9 that the two periods of maximal fog p r o b a b i l i t y , from 0300 to 0500 and from 0600 to 0830 PST, are the r e s u l t s r e s p e c t i v e l y of the f i r s t f a l l i n g of the temperature to near the dewpoint temperature, and to the period just a f t e r sunrise when the sun (unhindered normally by cloud; see p. 35) heats the ground surface enough to cause turbulent mixing, so that a t h i n l a y e r of ground fog not n e c e s s a r i l y recorded (observations are taken at eye l e v e l ) thickens; and deepens;. I t i a evident that the; degree of saturation of the a i r , and the windspeed, w i l l be the major c l i m a t o l o g i c a l f a c t o r s c h a r a c t e r i s t i c a l l y associated, at a c e r t a i n value, with fog formation. Diagrammatically, these ideas may be shown: Pre s ent Temperature cooling Dewpoint T = T, E d fog s t a r t s to form. further cooling' mist phase . Sunrise: uneven -> FOG heating; turbulemce; thickening. Thus i t may be stated that fog (F) i s a function of several v a r i a b l e s : F = f- T d(RH., T.) E n c r i t . where: n RH. E c r i t w the dewpoint temperature, i t s e l f a function o i n i t i a l r e l a t i v e humidity, and i n i t i a l temperature. a i r temperature c r i t i c a l value of the differ e n c e between the dewpoint and the actual a i r temperature, windspeed. 50 The c r i t i c a l value of n, or at l e a s t an acceptable- estimate w i l l have to be found by an examination of the data. I t i s hoped that the main causal factors of morning fog formation at Vancouver In t e r n a t i o n a l A i r p o r t have now been i d e n t i f i e d , and so they w i l l now be used i n an attempt to pr e d i c t the occurrence of fog. The model constructed here u t i l i z e s ; the recorded parameters of temperature, r e l a t i v e humidity, and windspeed. The f i r s t two are used to c a l c u l a t e the dewpoint temperature at the time concerned ( ^ (RH^ ., T^) ) windspeed i s added as an important control f a c t o r , (see p. 37 ) In the f i v e year period of data i n i t i a l l y considered, 1965 ~ 19^9, there were 120 days with morning fog; recorded during the eight month period considered, August to March i n c l u s i v e . Also, a stratified.random sample 200 days without, fog was used (5 pei* month per year) to c a l c u l a t e "non - fog" p r o b a b i l i t i e s ; the days preceding these were of course used. Relative humidity and temperature were noted f o r 1600 and 2200 PST, and windspeed at 2200 PST. To f i n d the a s s o c i a t i o n of r e l a t i v e humidity and temperature to dewpoint temperature, a multiple regression equation was set up, from values extracted from a chart given by Pettersen ( 1 9 3 9 ) . The equation took the form: y = o . l 9 9 7 x J L + o . 9 7 5 0 x 2 - 19.2786 where: y = dewpoint temperature ( 0 G) x^= r e l a t i v e humidity (per cent) x 0= temperature ( C) 51 which had a multiple c o r r e l a t i o n c o e f f i c i e n t of 2 , , r = O.966? s i g n i f i c a n t at the 0.001 per cent l e v e l . The standard error of estimate was 0.^995 0 C . Thus, f o r each day before morning fog and each sample day not before morning fog i n the f i r s t f i v e year sample, the dewpoint was calcula t e d from observed values of r e l a t i v e humidity and temperature, at 1600 and 2200 PST. Since temperatures were recorded i n degrees; Fahrenheit at Vancouver, a conversion of the•temperature to t h i s scale (° C) was c a r r i e d out p r i o r to using the above formula. The: c a l c u l a t e d values of the dewpoint were then converted back to Fahrenheit f o r comparison with the recorded actual a i r temperature. The frequency, i n class i n t e r v a l s of one degree Fahrenheit, of the difference between the actual temperature and the dewpoint temperature, was pl o t t e d i n four tables, f o r cases with fog the next day, and those without, f o r 1600 and 2200 PST. (see Tables; Ik and 15, p. 52 and 535.) Another frequency table, again f o r both fog and non - fog cases, was drawn up f o r windspeeds, taken at 2200 PST. (see Table 16, p. 5k.) From observation of these s i x tables, i t was s u b j e c t i v e l y decided to define the following three parameters f o r use i n the p r e d i c t i v e model, the f i r s t two being the decided values f o r the c r i t i c a l value of n. ( p. k9 - 50) v = At l 6 0 0 PST, the differ e n c e between the actual 1 0 a i r temperature and the dewpoint LESS THAN 5 F. y = At 2200 PST, the dif f e r e n c e between the actual a i r temperature and the dewpoint LESS THAN 5 'F. z = At 2200 PST, the windspeed LESS THAN: OR EQUAL TO 5 MPH. 52 a)/ Preceding Fog. -. / . • It 0-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-: LO 10-11 11-12 12 + Aug _ — — t 1 - - - - 1 - - 3 1 Sep - 2 - . 1 - 4 2 4 5 3 7 2 2 Oct 2 - 1 3 4 2 2 3 2 - - - 1 Nov 1 2 4 4 1 1 2 2 2 - • -• • -Dec 1 2 l 4 l - - . - - - - -Jan - 1 2 - - 2 r . - - - - -Feb - 2 1 8 .4 2 1 1 1 .. -Mar i - - 1 - 1 1 . -. - 1 • - ' -T o t a l 4 7 8 17 14 . 11 11 11 11 4 8 5 4 b) Not preceding fog. i Aug - - 2 - - l 3 1 - ': 4 14 Sep - 2 l 1 - 2 3 2 6 ' 1 • 7 Oct 2 - • 2 l - 4 5 3 1 7 3 - - , 4 Nov 1 1 3 4 1 4 2 3 1 1 1 2 l Dec 5 2 1 2 4 3 1 1 2 1 1 -Jan 4 2 1 2 1 3 2 3 - 1 - 1 Feb _ 2 3 1 3 1 3 3 1 4 1 1 2 Mar 2 1 - 1 - 1 3 1 1 3 1 11 T o t a l 12 11 11 11 12 13 13 20 14 12 : 16 10 40 Table 14 • Frequencies, by months, o f days when the d i f f e r e n c e between the a c t u a l temperature and the dewpoint was o f the magnitude i n d i c a t e d , l 6 0 0 PST. Vancouver I n t e r n a t i o n a l A i r p o r t , 1965 - 1969« 5 3 a) P r e c e d i n g F o g 0 - 1 1 -2 2 - 3 3-4 4-5 5 - 6 6 - 7 7 - 8 8 - 9 9 - 1 0 1 0 - 1 1 1 1 - 1 2 1 2 + A u g — 1 2 1 1 _ l _ — _ _ _ S e p 1 0 9 4 4 2 2 1 1 -O c t 6 3 7 1 1 - . 1 , 1 : -N o v 11 2 3 1 - 1 - I -D e c 8 1 - - - - - _ . -J a n 2 3 . - - - - - - - - -• F e b 8 6 3 1 - - 2 -M a r 1 3 - — . - .— — — — — - '— — T o t a l 46 28 1 9 8 4 3 5 3 - . - , b ) N o t p r e c e d i n g f o g . ( s a m p l e s i z e = 2 0 0 ) A u g 2 2 2 4 3 5 4 - 2 - - 1 • -S e p 3 1 1 4 6 4 I 2 1 1 - - -O c t 5 5 1 0 3 1 1 - - -N o v 5 5 2 5 2 2 1 - 2 - 1 - -D e c 5 2 5 2 1 2 1 3 1 2 - - 1 J a n 2 1 2 1 5 1 2 2 - 1 1 ; -F e b 7 3 4 2 3 2 2 2 M a r 2 3 1 4 2 3 6 , 4 • — . — — — 3 T o t a l 31 2 2 27 2 5 2 3 2 0 17 1 3 - 6 4 2 2 4 T a b l e 15 . F r e q u e n c i e s , b y m o n t h s , of d a y s when t h e d i f f e r e n c e b e t w e e n t h e a c t u a l t e m p e r a t u r e a n d t h e d e w p o i n t was - o f t h e m a g n i t u d e i n d i c a t e d , 2200 P S T . , V a n c o u v e r I n t e r n a t i o n a l A i r p o r t , 1 9 6 5 - 1 9 6 9 . 54 a) Preceding fog. MPH 0 1 2 3 4 5 6 7 8: 9 n o : : 11 \ 12 13 14+ Aug 4 2 - 1 - 1 • - - - - - - - -Sep 13 4 5 6 . 1 1 - 1 1 - 1 - ' - - -Oct 4 - 3 2 4 2 1 1 2 - • -. - - 1 Nov 6 3 - 3 3 2 1 - • 1 - - -• Dec - 2 3 - 2 1 _ - - 1 • - - - - - . Jan 2 - 1 2 Feb 3 2 6 2 '4 2 1 - - 1 - ; - - -Mar 1 - - - .- 1 1 — — — 1 — T o t a l 3 3 13 18 16 14 10 4 3 3 2 - - - 1 . b) Not preceding fog. (sample si z e = 200) Aug 3 1 1 1 6 2 2 5 ' - 1 2 1 • - -Sep 1 2 3 ,3. 1 7 1 1 3 - - 2 - - l Oct 2 1 2 ' 3 1 3 3 3 - 1 1 1 1 - 3 Nov 4 - • - 3 1 2 3 2 l 1 - 1 3 4 Dec 2 3 1 l 1 - 1 3 2 1 - 5 1 4 Jan. 1 1 -. - - 2 1 2 4 2 1 2 1 3 Feb 3 2 3 2 3 2 3 1 2 1 1 - -' 2 Mar 3 . - .. 2 3 5 1 2 .ft 2 1 — mm 1 1 3 T o t a l 19 10 12 16-, 18 19 13 22 11 9 5 6 12 3 20 T a b l e 16. . F r e q u e n c i e s , • b y m o n t h s , o f d a y s when t h e w i n d s p e e d v/as o f t h e m a g n i t u d e i n d i c a t e d , 2200 P S T . V a n c o u v e r I n t e r n a t i o n a l A i r p o r t , 1965 - 1969. 55 These parameters, v, y, and z, w i l l be used i n the following discussion. The p r o b a b i l i t y of fog the next morning may be ca l c u l a t e d from any ONE of these parameters i n the following manner: Consider two sets, one the occurrence of fog, the other the occurrence of a c e r t a i n parameter, x. There w i l l be an i n t e r s e c t i o n of the two sets, which w i l l have a c e r t a i n p r o b a b i l i t y , p ( x A f ) The p r o b a b i l i t y of a s s o c i a t i o n of a c e r t a i n parameter, x, given that fog (f) e x i s t s , i s thus p(x/f) = p(x7Yf) P(f) where: p(f) = the p r o b a b i l i t y of fog independent of x; and p(x) = the p r o b a b i l i t y of x independent of fog. I t i s then possible to reverse the an a l y s i s , according to the law of condit i o n a l p r o b a b i l i t y , (see Mosteller, et a l . , 1961, p. 8 5 . ) Then p(x f) = p ( x / f ) . p ( f ) and p ( x / f ) . p ( f ) = p(f/x).p(x) Thus p(f/x) = p ( x / f ) . p ( f ) ( 3) _ 56 or n(fn x).n(f) n ( x ) 2 (4) where n i s the number of occurrences of the parameters. ' The- p r o b a b i l i t y of fog remains a constant throughout t h i s analysis of the f i v e year period of data: p(f) = n(f) ( 5 ) n(f+nf) where: n ( f ) i s the t o t a l number of foggy days, and n(n+nf) i s the sample space, the t o t a l number of days; between August and March i n f i v e years, allowing f o r the additi o n of February 29th i n I968. Thus: p(f) = 120 = 0.0994 1216 The p r o b a b i l i t y of fog given a parameter within a s p e c i f i e d range was calcula t e d from equation (.3), and the r e s u l t s are shown, i n Table; 17, below. Dewpoint di f f e r e n c e Dewpoint diffe r e n c e Windspeed at 2200 at 1600 PST at 2200 PST PST Range p(f) Range p(f) Range P(f) 0 - Oo99 0.0341 0 - 0.99 0.0669 0 mph 0.0711 1 - 1.99 0.0493 1 - 1.99 0.0650 1 mph 0.0655 2 - 2.99 0.0525 2 - 2.99 0.0517 2 mph 0.0683 3 - 3.99 4 - 4.99 0.0324 0.0632 3 4 - 3.99 - 4.99 0.0333 0.0224 3 mph 4 mph 5 mph 0.0598 0.0541 0.0448 TOTALS v =0.2315 y = 0.2393 z = O.3636 Table 17. Fog p r o b a b i l i t i e s i n s p e c i f i e d parameter ranges. 57 These p r o b a b i l i t i e s were also cal c u l a t e d f o r t h i s period with another seven years added. (1958 - 1964, i n c l . ) t o t a l l i n g twelve years i n a l l . The f i n a l single parameter c o n d i t i o n a l p r o b a b i l i t i e s , then, were: p(f/v) = 0.4336 p(f/y) = 0.2552 p ( f / z ) = 0.4595 with standard errors of estimate f o r the c o n d i t i o n a l fog p r o b a b i l i t i e s , c a l c u l a t e d by considering i n d i v i d u a l years of 0.068, 0.007, and 0.039, r e s p e c t i v e l y . These p r o b a b i l i t i e s are themselves i n t e r e s t i n g and are i n some part u s e f u l . However, the p r e d i c t i v e power of the; model may be increased by combining the parameters. The p r o b a b i l i t y of fog given the i n t e r s e c t i o n of a l l three parameters, p ( f / v n y / l z ) , i s not equal to the sum of the p r o b a b i l i t i e s of fog occurring when one i s present, since some of the set overlaps would be double counted. Reference should be made here to Pig. 11, overleaf, which shows a set t h e o r e t i c a l model i l l u s t r a t i n g fog and "non - fog" i n t e r s e c t i o n s with the defined parameters; v, y, and z . I t may be stated that p ( f / v / i y n z ) = p ( f A v n y n z) p ( v A y A z) Now, p ( f r t v A y A z ) = n ( f A v A y A z) ^ \ n(f+nf) where: n ( f A v A y A z ) i s the number of cases of i n t e r s e c t i o n 58 FOG NON-FOG f A V f A y A v nf A V nfny fAVAy z nfAVAy z > c f AyAZ nf Ay A z frt z nf A Z 11 • Set Model of fog and non-fog intersections with the parameters v, y, and z, (see text for explanation.) \ 1 Total sample space » 2919 days; fog « 314 days; p(f) » 0.1076. . 7 • -59 of fog with v, y, and z; and n(f+nf) i s the sample space, as i n equation (5). Also p(vn y/l z) = n ( v / l y / l z ) n(f+nf) so that, s u b s t i t u t i n g i n equation (6), and c a n c e l l i n g , p ( f / v n y n z ) = n ( f r t v r t y n z ) ^ ) n(v/1 y n z) ( i t should be noted that the set (vn y A z) includes the set ( f n v n y n z ) as a nesting subset. See F i g . 11, p. 58.) Analysis of the f i v e year data period 1965 - 19^9 for Vancouver, f o r August to March i n c l u s i v e , y i e l d s p(f/vA y n z) = = 0.703 giv i n g a con d i t i o n a l fog prediction, to a 70.3 per cent p r o b a b i l i t y l e v e l . For the months of October to January i n c l u s i v e , only, the p r o b a b i l i t y p(f/v/i y n z) increases to 0.778. The s t a b i l i t y of these p r o b a b i l i t i e s f o r any i n d i v i d u a l year may be tested by adding a further period of data to the ana l y s i s , and t e s t i n g f o r any trends. The seven year period 1958 - 1964 w i l l be used f o r t h i s . The t o t a l number of occurrences of morning fog i n the months of August to March i n c l u s i v e i n t h i s seven year period was 194 days, out of a maximum possible of 1703;. (Allowing f o r two leap years i n t h i s period.) The p r o b a b i l i t y of fog f o r the .7 year period, then, was 0.113;9. The p r o b a b i l i t y of fog f o r the t o t a l 12 years was thus O.IO76, representing 3l4 days out of a maximum 60 possible' of 2919. Dealing now with a 12 year period of data, the p r o b a b i l i t i e s of fog given the defined parameters v, y, and z,. may be computed, using equation (9) . The average p r o b a b i l i t i e s , f o r a cumulative number of years, from the expression p ( f / v / l y n z ) are given i n Table 18, p. 6 l . The standard error of estimate of the f i n a l p r o b a b i l i t y , 0.7485, cal c u l a t e d from i n d i v i d u a l p r o b a b i l i t i e s f o r singl e years, i s 0.08266. The p o s s i b i l i t y that a trend e x i s t s i n the p r o b a b i l i t y of fog f o r i n d i v i d u a l years may'be tested using the Mann Test. (Mann, 1945; Tintner, 1952, p. 214 - 15.) This i s a non - parametric t e s t f o r trend. The; technique i s described f u l l y i n Appendix I I , p. 15b . Applied to the present data, f o r the 12 years 1958 - I969, the t e s t y i e l d e d a rank c o r r e l a t i o n c o e f f i c i e n t of 'X = + 0.9394, which, according to a formula given by Kendall (1955), means that there i s l e s s than a 0.0001 per cent p r o b a b i l i t y that there i s no trend. (See Appendix II) The magnitude of t h i s trend i s not investigated further here, as a very long time series of p r o b a b i l i t i e s would be required for' t h i s . Predictions of fog p r o b a b i l i t i e s f a r i n advance of the f i n a l year (1969) considered here, however, would f i r s t have to assess the magnitude of t h i s trend, or compute the fog p r o b a b i l i t i e s f o r the intervening period. The p r o b a b i l i t i e s ; computed here appear l i k e l y to apply at l e a s t f o r several years. Despite the good p r e d i c t i v e power of the combined parameter ( v A y n z ) , i t only accounts f o r some 42.8 per cent of a l l the fog cases, and i t would be more acceptable i f this;; amount were increased. I t has been found that t h i s may be done; by using the parameter (yn z) , that i s , both the 2200 PST parameters. In t h i s way, 84.3 per cent of the fog may be accounted f o r whilst maintaining a good l e v e l of p r e d i c t a b i l i t y . As may be seen from Table 19> 6 1 1 2 y Year added n ( f n v n y n z) n ( v n y n z) 0 0 1 1 = p ( f / v / i y O Z ) c o l 2 added and averaged. 1 9 6 9 8 1 2 O . 6 6 6 7 1 9 6 8 1 0 1 4 0 . 6 9 2 3 1 9 6 7 7 1 1 O . 6 7 6 0 1 9 6 6 8 1 2 0 . 6 7 3 5 1 9 6 5 1 2 1 5 0 . 7 0 0 3 1 9 6 4 9 1 0 0 . 7 2 9 7 . 1 9 6 3 1 0 1 4 0 . 7 2 7 2 1 9 6 2 : 15 17 0 . 7 5 2 3 1 9 6 1 1 1 1 5 0 . 7 5 0 0 I 9 6 0 1 7 2 3 0 . 7 4 8 2 . 1 9 5 9 7 1 0 0 . 7 4 5 0 1 9 5 8 14 18 0 . 7 4 8 5 T a b l e IS. Average p r o b a b i l i t y o f morning f o g : t h r e e v a r i a b l e s . Vancouver I n t e r n a t i o n a l A i r p o r t , 1958 - I969• (see t e x t f o r e x p l a n a t i o n . ) 6 2 p. 6 3 , the p r o b a b i l i t y of fog given ( y f l z ) , i s 0 . 6 6 9 5 * averaged over the 1 2 year period; that i s , as equation ( 9 ) : p ( f / y n z) = n ( f n y n z) = 2 2 7 = 0 . 6 6 9 5 n ( y n z ) The trend i n the p r o b a b i l i t i e s of fog f o r i n d i v i d u a l years f o r this- model were also investigated using the Mann Test (op. c i t . ; see Appendix II) The rank c o r r e l a t i o n c o e f f i c i e n t was here f = + O . 8 7 8 8 , so that here the p r o b a b i l i t y of no trend (Kendall, 1 9 5 5 ) was also extremely small. (Again l e s s than 0 . 0 0 0 1 per cent.) The high precentage of fog cases accounted f o r by t h i s model suggests that the s l i g h t l o s s of p r e d i c t i v e power i s worthwhile; i t also suggests that the degree to which the dewpoint i s approached and the c o n t r o l l i n g influence of windspeed are very r e a l f a c t o r s i n fog formation at Vancouver Intern a t i o n a l A i r p o r t . The remainder of cases ( 1 5 . 7 per cent) are l i k e l y to be caused by other mechanisms; one of these may be advection. At l e a s t then, i n the case of r a d i a t i o n fog, i f the major causal and c o n t r o l l i n g factors have been found, i t may pnove possible to extend t h i s model to cover stations at which fog has not long been recorded, but only the c l i m a t i c parameters of temperature, r e l a t i v e humidity, and windspeed. A p r e d i c t i v e model, f o r fog p r e d i c t i o n i n areas wb;ere c l i m a t o l o g i c a l data are sparse, may be invaluable i n estimating the fog frequency, and thus the p o t e n t i a l p o s s i b i l i t i e s of water addit i o n to vegetation. 6 3 1 2 3 Year added n ( f rs y n z) n(y n z) c o l c o l added 1 — = p ( f / y f l z) 2 and averaged. I969 9 13 0.6923 1968 2-5 35 0.7084 1967 17 26 0.6885 1966 18 17 0.6819 1965 19 2k 0.7053 1964 17 26 O.6956 1963 23; 35 0.6879 1962 26 30 0.6649 1961 20 28 O.6700 i 9 6 0 Jk 46 0.6823 1959 19 26 0.6557 195 8 29n 36 0.6695 Table 1 9 . Average p r o b a b i l i t y of morning fog; two v a r i a b l e s . Vancouver I n t e r n a t i o n a l A i r p o r t , 1958 - 1969. (see text f o r explanation.) 6k CHAPTER FOUR FOG INTERCEPTION INSTRUMENTATION. The d i f f i c u l t y of d i r e c t l y measuring fog i n t e r c e p t i o n by vegetation (see Ch. 7) has l e d several workers to devise mechanical instrumentation. The idea of these instruments, which substitute an a r t i f i c i a l i n t e r c e p t i n g surface i n place of the vegetal surface, is; to give some idea of the r e l a t i v e p o t e n t i a l s of various; s i t e s to gain water from fog. A l l the instruments devised s u f f e r from the d i s a b i l i t y that they cannot be d i r e c t l y c a l i b r a t e d with a s p e c i f i c vegetation type, since t h e i r i n t e r c e p t i n g surfaces do not, and are frequently not intended to, reproduce the morphology of vegetation. , As early as 1900, Marloth, i n South A f r i c a , was measuring water contribution on Table Mountain by p l a c i n g reeds i n bundles over raingauges and comparing the amounts caught with uncovered gauges (Marloth, 190k, 1 9 0 7 ) . Fog i n t e r c e p t i o n by screens;. For a number of years the Atomic Energy Commission on Mt. Washington Observatory has maintained a screen device p r i n c i p a l l y f o r studies of riming, but which also i n t e r c e p t s fog i n the l i q u i d state. Twenty r i g i d 3mm thick bars i n a frame? about two feet by one foot have been known to c o l l e c t about a quart of water i n two hours;. The f i r s t use of a v e r t i c a l wire mesh i n s p e c i f i c fog and cloud water i n t e r c e p t i o n studies was by Twomey (1956, 1957) i n Tasmania, A u s t r a l i a . On 65 Mt. Wellington, Tasmania, a 4,160 f t . peak, Twomey set up a "cloud c o l l e c t i n g screen" which he constructed using wire mesh attached to an angle frame. An 8 inch raingauge was placed under the mesh, and a control gauge was located i n the open a few yards away. The gauges were read at i n t e r v a l s over a 10 day period, during which the mountain summit was covered almost continuously with stratocumulus clouds. Over the 10 day period the screened gauge c o l l e c t e d 43.99 inches, wh i l s t the open gauge recorded only 4.075 inches. Thus the o v e r a l l r a t i o of the volumes of water c o l l e c t e d by the screened and unscreened gauges r e s p e c t i v e l y was 10.4; the r a t i o s of i n d i v i d u a l readings ranged from 2.6 to above 200. Twomey states that " i t i s s i g n i f i c a n t that low values of t h i s r a t i o were associated with higher r a i n f a l l rates." (Twomey, 1956, p. 1 2 l ) Twomey calcula t e d that, on the basis of estimated wind v e l o c i t i e s , the maximum value of the r a t i o as a r e s u l t of raindrop c o l l e c t i o n by the mesh would be about 4.0, which i s l e s s than the o v e r a l l r a t i o and f a r less; than the maximum r a t i o values obtained, v i z . 100, i n l\ hours i n a 5 m.sec" . estimated wind; 114, i n the same time and windspeed, and 224, i n 4 hours with a 5 m.sec-"''. wind. Twomey thus concludes: " i t seems c e r t a i n , therefore, that the major part of the water received by the screened gauge origi n a t e d as cloud water wlieh was intercepted by the wire mesh. " (Twomey, 1956, p. 122) A s i m i l a r apparatus was u t i l i z e d i n Hawaii, by Carlson ( 1 9 6 1 ) , and Ekern ( 1 9 6 4 ) , but here the screen was pivoted and kept orientated into the wind by a vane. Thus t h i s i s not s t r i c t l y comparable with Twomey's apparatus. 66 A comparable experiment was c a r r i e d out i n December, 1967, and January, I 9 6 8 , by the writer, on Beachy Head, near Eastbourne, Sussex, England. The experiment was designed to test the l i k e l i h o o d of fog water p r e c i p i t a t i o n at lower l e v e l s , ( s i t e was at 400 f t . a l t i t u d e , ) and i n England, where i t had apparently never before been attempted. Two s t e e l wire screens were constructed, each 3- f t . square, and supported on t h i n painted hardwood frames above two standard B r i t i s h 5 inch raingauges; the bottom; of the mesh was not attached to the frame but kept taut by supports on the ground, so as to f a c i l i t a t e the runoff of water. The screened gauges were placed about 10 feet apart with an unscreened control gauge approximately halfway between the two. One screen was orientated north - south, and the other east - west. The gauges were read at i n t e r v a l s depending on the conditions between December 1 7 t h , 1967 and January 6 t h , 1968. An abstract of the r e s u l t s i s given on p. 67 , overleaf. In a 6 hour period from 1500 to 2100 on December 22nd, I 9 6 7 , gauge ( l ) y i e l d e d 2 5 9 . 5 times that of the open gauge, and gauge (3) y i e l d e d 337.6 times that of the open gauge. Gale force winds and l i g h t d r i z z l e could have given a s i m i l a r r e s u l t , but as i t was known that actual conditions were heavy fog, with a maximum estimated wind speed of 10 m.sec i t i s considered that t h i s was not l i k e l y to be the case i n t h i s instance 1. Not so spectacular, but perhaps the most s i g n i f i c a n t r e s u l t of a l l , was the r a t i o 7.8 times, given at 2200 hours on December 1 9 t h , 1967» when fog covered parts of Beachy Head, and there was l i t t l e wind to influence the r e s u l t s . Over the e n t i r e period the screened fog gauges each caught about four times that recorded by the open gauge. This compares with about 67 PREVIOUS AMOUMT CAUGHT AMOUNT CAU&HT RATIOS E*T. MIM0 GrAUOE (i. O.Ol) &Au&e (« o.oi) % ( . S*c '.) Di4Ec.no.) 13: 23.5 23.5 3.0 7.834 7.834 calm 6 173.0 i4 o . o 3.3' 52.420 42.420 5 - 6 wsw 6 98.0 101.3 0.3 259.500 337.600 8 -10 ¥S¥ 24 61.0 84.0 9.0 6.700 9.300 5 - 6 WSW 17 4o.O 53.0 12.0 3.300 4.4oo 2 N 23! 50.0 45.0 19.0 2.600 2.300 2 N 30. 8.0 8.0 . 1.5 5.300 5.300 2 W 16.5 84.0 76.0 16.0 5.250 4.750 3i w 8 89.0 71.0 10.0 8.900 7.100 10 -11 w TOTALS 1024.0 1009.8 260.5 3.930 3.880 Table. 20. Abstract of r e s u l t s of screened gauge exposures  on Beachy Head, England, Dec. 196? - Jan. I 9 6 8 . 68 10 times the open gauge i n Twomey's (1956, 1957) r e s u l t s ; i t must be remembered that he was dealing with conditions of continuous cloud cover, whereas many days during the three weeks of t h i s experiment were t o t a l l y c l e a r . Although i n d i c a t i n g that considerable amounts of water may be added, vto. the ground through fog and low cloud, no attempt w;as made to measure the amounts of water caught by the downland chalk scrub of the area. However, the importance of t h i s probable a d d i t i o n of water i s suggested by the f a c t that the Penman and Thornthwaite evapotranspiration formulae, (see Penman, 1963) when cal c u l a t e d f o r t h i s area show a t h e o r e t i c a l need f o r i r r i g a t i o n i n nine years out of every ten. That i r r i g a t i o n i s not practiced even i n .those types of a g r i c u l t u r a l enterprizes where i t i s known to be p r o f i t a b l e , strongly suggests an a d d i t i o n a l water source not accounted f o r by the equations, which u t i l i z e the p r e c i p i t a t i o n data from normal raingauges. Local knowledge of the Sussex downland s i t u a t i o n tends to support the idea of a d d i t i o n a l water' from fogs. The f i l l i n g of the so - c a l l e d "dew - ponds" has never been s a t i s f a c t o r i l y explained, and these have been observed to have t a l l e r vegetation growing round them. The combining of cereal crops i n the summer i s often impeded u n t i l 10.00 or 11.00 i n the morning due to wettness of the crop. This i s sometimes the r e s u l t of heavy dew, but i s more often the r e s u l t of i night-time fogs which d i s s i p a t e r a p i d l y when warmed by the sun. The a d d i t i o n of a small amount of water may be c r i t i c a l i n the water economy of crops here; i f nothing e l s e , excessive evapotranspiration may be delayed u n t i l the l a t e morning. Thus the screen fog water i n t e r c e p t i o n technique i s here j u s t i f i a b l e 69 i n that i t ind i c a t e s an unaccounted water input. The actual importance of t h i s water i s perhaps better assessed by the use of a balance type of instrument. Balance: instruments. •„• Many of the dew gauges now on the market u t i l i z e a balance mechanism to co n t i n u a l l y weigh any water added to a f l a t surface by dew. -An adaptation of t h i s idea follows the technique devised by H i r s t (1954) i n co n t i n u a l l y weighing an actual plant shoot. An i n i t i a l l y t u r g i d shoot i s kept constantly supplied with water from the mid-point of the balance beam; t h i s makes up t r a n s p i r a t i o n l o s s . Growth i s too slow to a f f e c t d a i l y records, and d a i l y changes from photosynthesis or r e s p i r a t i o n are too s l i g h t f o r the beam s e n s i t i v i t y . The balance i s placed i n the required p o s i t i o n , which may be i n the centre of a growing crop ofi the same plant. The main problem with t h i s type of instrument i s that wind may t o t a l l y upset the records i f the windspeed i s too high, despite the damping of the o s c i l l a t i o n with a vane immersed i n o i l . However, wind speeds tend to be low during the occurrence of fog (see Ch. 3) so that the c r i t i c a l periods of water ad d i t i o n may be properly recorded. A modification of t h i s type of instrument replaces the plant with a c y l i n d r i c a l screen f o r ho r i z o n t a l fog i n t e r c e p t i o n ; nevertheless t h i s s t i l l s u f f e r s from the drawback'of wind e f f e c t s . The same applies i f the apparatus i s placed on an e l e c t r o n i c transducer., which converts downward pressure to an e l e c t r i c a l current; several measuring devices of t h i s type; have been~designed, but the probable d i s t u r b i n g 70 e f f e c t s of wind meant that they were u n l i k e l y to operate s u c c e s s f u l l y , and so they were not b u i l t . Fog i n t e r c e p t i o n by c y l i n d r i c a l - mesh modified raingauges. i The most widely used type of gauge fo r the measurement; of fog i n t e r c e p t i o n i s the raingauge modified by the pl a c i n g of a c y l i n d r i c a l wire gauze d i r e c t l y above the gauge o r i f i c e . The p r i n c i p l e i s that water runs from the gauze and drips into the gauge, where i t i s recorded. Since the i n t e r c e p t i n g device i s a cylinder,: catch i s independent of wind d i r e c t i o n . The f i r s t use of a gauge of t h i s type appears to be by De Forest (1923)» i n i n t e r c e p t i o n studies i n Maryland, U.S.A. He used a narrow c o l l a r of wire n e t t i n g on top of a non - recording raingauge. Dieckmann (l93l)» i n Germany, used a 35 cm. high gauze c y l i n d e r i n s e r t e d into a raingauge. Grunow. (1952) followed Dieckmann'1 s work but made the height of the , gauze c y l i n d e r equal to twice i t s diameter, on the p r i n c i p l e that the e f f e c t i v e i n t e r c e p t i n g area, taken as height x diameter, was equal to the catching area of the raingauge. This enabled him to compare the r e s u l t s more c l o s e l y with the amounts recorded i n a normal open raingauge. Since Grunow's work, a number of workers have u t i l i z e d the c y l i n d r i c a l gauze technique, i n various studies. Nagel (1956, 1962) used a s i m i l a r gauge to measure water gained from Table Mountain's "Table Cloth," i n South A f r i c a , and from South-West A f r i c a ' s c o a s t a l fogs. Baumgartner (1957) u t i l i z e d i t i n a study of the v e r t i c a l v a r i a t i o n s of fog p r e c i p i t a t i o n i n Bavaria; K i r i g i n (1959) used i t fo± a study of the e f f e c t s of c o n t i n e n t a l i t y and slope on the amounts of 71 fog p r e c i p i t a t i o n i n Yugoslavia; Bauer (1963) f o r i n v e s t i g a t i o n s of v e r t i c a l fog p r e c i p i t a t i o n v a r i a t i o n s i n the Hunsruck and E i f e l Mountains of Germany. Kummerow (1962a, 1962b), working with a Grunow type fog i n t e r c e p t o r on the north Chilean coast, recorded i n one year an extra 1700 mm of water reaching the ground as fog p r e c i p i t a t i o n . Comparisons with amounts recorded i n gauges under trees showed that the gauges caught an average amount s i m i l a r to that dripping from the trees. Vogelmann et a l . ( 1 9 6 8 ) , used a double c o i l of aluminum wire screen f o r t h e i r study i n the Green Mountains of Vermont, U.S.A., and found that catch increased s i g n i f i c a n t l y with elevation, even compared to open raingauge catches. Most recently, the U n i v e r s i t y of ThesSalonika, Greece, has i n i t i a t e d a project to discover how water may be drained from fog and cloud on Mt. Olympus; s i m i l a r mesh cylinders are being used f o r t h i s (Kyriazopoulos, I 9 6 8 ) . The writer investigated the use of c y l i n d r i c a l gauze screened gauges i n a preliminary experiment, again on the South Downs of Sussex, England, during July, I 9 6 9 , when f i v e non-recording gauges, screened as shown i n Plate 1, p. 72, were exposed f o r 14 days. I t was here that the problem! of water reten t i o n on the mesh was f i r s t noticed by the present author. Although water c o l l e c t e d i n the screened gauges whi l s t none c o l l e c t e d i n the open gauges, i t was evident that not as much was c o l l e c t e d as could have been i f a l l the water intercepted by the gauze flowed from i t . Since i t was known that the c o l l e c t i o n e f f i c i e n c y of a s o l i d c y l i n d e r can be calculated with some accuracy (Langmuir, 1 9 6 l ) , the idea of using a c y l i n d e r composed of v e r t i c a l wires only seemed to have merit. Knowing 72 P l a t e 1. Gauge f i t t e d w i t h mesh m o d i f i e r , Beachy H e a d , E n g l a n d , J u l y , 1969. 7 3 i n d i v i d u a l wire diameters and length, t h e i r t h e o r e t i c a l c o l l e c t i o n e f f i c i e n c i e s could be summed. Albrecht ( l 9 3 l ) showed that impaction occurs on to c y l i n d r i c a l wires when a c e r t a i n c r i t i c a l value of the expression rv c i s reached. Here r = radius of droplets v = wind v e l o c i t y and c = c y l i n d e r radius. With t h i s im mind, a gauge modifier was constructed from a s e r i e s of v e r t i c a l wires, each of diameter 0.008 inch, with 16 wires, l 4 inches high, per 1 inch of c y l i n d e r circumference. (Plate 2 , p.74 ) In t h i s manner, i t would have been possible to f a i r l y accurately c a l c u l a t e the c o l l e c t i o n e f f i c i e n c y of the apparatus under varying conditions. Once b u i l t , however, preliminary tests showed that although t h i s type of gaug modification captured fog p a r t i c l e s , i t d i d not do so very r a p i d l y . In a fog at the U n i v e r s i t y of B r i t i s h Columbia, the gauge was exposed from 1000 hours on 3<lst October, I969 to 1000 hours on j-st November, 1969. During t h i s time, approximately 0.06 inches of water was c o l l e c t e d ; during the same period a normal raingauge at U.B.C. Climate s t a t i o n , c o l l e c t e d only 0.01 inch. However, the c o l l e c t i o n of an a d d i t i o n a l 0.05 inches of water i n a 24 hour period of thick fog was not considered a f a s t enough response rate i n view of the f a c t that the water was to be measured i n a t i p p i n g bucket recording raingauge where one t i p of the bucket represented 0.01 inch of water. Thus the advantages of t h i s type of gauge, the r e l a t i v e l y small i n t e r r u p t i o n of the a i r f l o w , the p o s s i b i l i t y of c a l c u l a t i n g an accurate t h e o r e t i c a l 74 P l a t e 2 . V e r t i c a l w i r e gauge m o d i f i e r as o r i g i n a l l y -c o n s t r u c t e d . 75 c o l l e c t i o n e f f i c i e n c y , and the l e s s e r water retention on the mesh part of the gauge, were disregarded i n favour of a net - mesh type c o l l e c t o r , as used i n other studies. This type modifies the a i r f l o w to a l a r g e r extent than the more open prototype, but i s f a r more e f f i c i e n t as a c o l l e c t o r , i n terms of the absolute amount of water c o l l e c t e d . Construction and exposure of a fog gauge at Vancouver In t e r n a t i o n a l A i r p o r t . The mesh type fog catcher a c t u a l l y constructed and used i s shown i n p o s i t i o n on a 0.01 inch t i p p i n g bucket recording 8 inch raingauge i n Plates J and k , (p. 76 and 77 ). This gauge was exposedofor a t r i a l period i n the meteorological enclosure at Vancouver In t e r n a t i o n a l Airport, from 2 6 t h November, 196<?> to 1 s t March, 1970. The frame of the fog i n t e r c e p t i n g device was constructed of aluminum s t r i p and covered with aluminum mesh, with wires of 0.153 inch diameter, with 18 meshes to the inch. The height of the device was Ik inches, and the diameter 7.5 inches, so that i t f i t t e d just i n s i d e of an 8 inch gauge o r i f i c e . This gave an e f f e c t i v e v e r t i c a l i n t e r c e p t i n g area of 102.5 square: inches. This compares with an area of 50.3 square inches which the gauge would have presented h o r i z o n t a l l y i f unmodified. Thus the c o l l e c t i n g area was e f f e c t i v e l y doubled. The h o r i z o n t a l and v e r t i c a l water c o l l e c t i n g surfaces are quite comparable i n fog, due to the h o r i z o n t a l nature of the fog p r e c i p i t a t i o n , (see p. 80 ) Records from the normal recording raingauge, i n s t a l l e d about 6 feet from the fog gauge, were used as a c o n t r o l . This was a 10 inch diameter 7 6 P l a t e 3 . Fog gauge, Vancouver I n t e r n a t i o n a l A i r p o r t , w i n t e r 1969 - 1970. 77 P l a t e k. D e t a i l o f f o g c a t c h i n g d e v i c e , f o g gauge, Vancouver I n t e r n a t i o n a l A i r p o r t , w i n t e r 1969 - 1970. 78 o r i f i c e gauge, g i v i n g a h o r i z o n t a l c o l l e c t i n g area of 78.5 square inches. During r a i n , the fog catcher also c o l l e c t s water from the r a i n , and since the t o t a l r a i n f a l l catching area of a fog gauge i s that of the raingauge o r i f i c e and the gauze c y l i n d e r together, the fog gauge w i l l c o l l e c t more water from r a i n than an open gauge, except during very l i g h t winds. Nagel (1962) has shown that the wind has to be strong enough to angle the f a l l i n g r a i n to 8° from the v e r t i c a l to catch rainwater. He noticed that the r a t i o of catch of r a i n by the gauze to that of the open gauge did not increase g r e a t l y after, a windspeed of about 14.25 m.sec . (approx. 25 mph); some c a l c u l a t i o n s with the present gauge indicated a s i m i l a r asymptote to t h i s r a t i o at about 8.6 m.sec (approx. 15 mph) I t i s evident that most of the r a i n that can be angled into the gauge i s being so by the time t h i s windspeed i s reached. The problems associated with the measurement of fog p r e c i p i t a t i o n during r a i n are considerable;. During the J: month period that the fog gauge was exposed, no r a i n was ever observed to f a l l during fog. In f a c t , this; i s normally the case at low l e v e l s t a t i o n s , and i t i s only i n higher h i l l and mountaintop s i t u a t i o n s that the problem of r a i n catch simultaneously with fog catch becomes a r e a l one. (A. Bleasdale, 1968, personal communication.) At a l l other times i t has been the usual p r a c t i c e to deduct the amount caught i n a normal open gauge from the t o t a l caught i n the screened gauge. (Nagel, 1956) There are errors i n t h i s p r a c t i c e , however, since during r a i n the v e r t i c a l area of the, fog catching device i s not s t r i c t l y comparable with the h o r i z o n t a l area of the raingauge o r i f i c e . Thus, during the period of gauge exposure here, 79 only those s i t u a t i o n s where fog existed without, r a i n at the same time have had to be considered. Nagel (1956) has shown how^  e a s i l y fog droplets can be induced to to flow h o r i z o n t a l l y . (Pig. 12 , p. 80 ) I t i s evident that beyond a windspeed of about 2 m.sec l i t t l e fog w i l l enter a h o r i z o n t a l o r i f i c e , but w i l l a l l be intercepted by the v e r t i c a l gauze. Since windspeeds of greater than 1 m. see" 1 -. (2 mph) have been shown (p. 37 ) u s u a l l y to be necessary during fog, i n order to prevent the fog droplets from f a l l i n g out of the a i r , l i t t l e fog i s p r e c i p i t a t e d into the open control gauge; that a i very small amount i s recorded i s accounted f o r by the downward turbulent catch from eddies over the raingauge i t s e l f , i n a s i m i l a r manner to the small amounts deli v e r e d d i r e c t l y to the ground surface by turbulent impaction. (Kuroiwa and K i n o s i t a , 1953) The1 amount captured by a c y l i n d r i c a l gauze may be ca l c u l a t e d approximately from an equation, modified from Albrecht (1931) and Matsumura ( 1 9 5 3 ) . Let E denote the "capturing c o e f f i c i e n t " of the fog catching c y l i n d e r , A the e f f e c t i v e s e c t i o n a l area of the wire screen, i n t h i s case 1 0 2 . 5 square inches, v the wind v e l o c i t y , and w the fog water contents. Then the amount of fog water F deposited i n time t i s represented by the equation F = EwvAt In the present case there i s a measured amount of fog water f a l l i n g into the gauge (D); t h i s amount w i l l equal (F - R) , where R i s the water retained on the meshes. Thus the amount: recorded i n the gauge may be represented by D = (EwvAt) - R 80 0 i 0 5 10 15 20 WIND SPEED (M.SEC'.) Fog droplet diameter 10 ^  40-50 /u-100 /x. F i g . " 12 . A n g l e o f f a l l o f f o g a n d r a i n d r o p s i n v a r y i n g w i n d s p e e d s . ( A d a p t e d f r o m N a g e l , 1956, p. 456.) 81 and the fog water content by w = (D+R) EvAt I t has already been mentioned (p. 14) that inc r e a s i n g amounts of fog p r e c i p i t a t i o n are dependent on the weather s i t u a t i o n ; not: only does the v e l o c i t y of the depositing a i r current influence intercepted water amount, but also the o r i g i n of the a i r mass a f f e c t i n g the area. The heaviest deposits are l i k e l y to occur where maritime warm a i r masses from temperate or subtropical zones are predominant. The deposits are small i f the a i r masses originate i n polar or a r c t i c zones. (Grunow, 1959) This v a r i a b l e a f f e c t s the value of E i n the previous formulae, i n that i t a f f e c t s the drop size d i s t r i b u t i o n of the fog, the modal drop diameters being smaller i n the a i r masses of polar and a r c t i c o r i g i n . Some r e s u l t s from a fog gauge i n s t a l l e d at Vancouver In t e r n a t i o n a l A i r p o r t . I t became obvious a f t e r a short period that the fog gauge i n s t a l l e d at Vancouver International A i r p o r t (Plates 3 and 4 , p. 76 and 77 ) would only be e f f e c t i v e i n recording a d d i t i o n a l water i n t e r c e p t i o n during long period fogs, when a threshold amount of water was reached, allowing water to flow from the gauze into the gauge o r i f i c e . This threshold may be as much as 0.1 inch of water, and i s accounted f o r by water caught on the gauze but retained on i t due to surface tension. This i s the main problem with any sort of device that requires the water to flow 82 from a gauze before i t i s measured, despite the fac t that water that was deposited on f o l i a g e of plants may be of use to those plants even i f i t i s not a s u f f i c i e n t quantity to d r i p , (see Ch. 6) Thus amounts smaller than approximately the 0,1 inch threshold value escape the record, and the water- evaporates. The amount of water held on the gauge can be seen from Plate k , p. 77. The effect on the recorded p r e c i p i t a t i o n even during fogs that exceed the threshold value i s that there i s a time l a g between deposition and recording. I t was also found that droplets deposited on the gauge gauze froze when the ^ temperature dropped to 3 2 0 F. , and took some time to melt again when the temperature rose, since the energy required f o r the la t e n t heat of melting must f i r s t be s a t i s f i e d . The e f f e c t of these two lags; was to make the data very hard to analyse. Regression techniques could not properly deal with the d i f f e r e n t i a l time lags; also, the water deposited was recorded i n 0.01 inch amounts (that' required to t i p the gauge bucket). Consequently, the fog record, at the time scale considered, was d i s c r e t e ; that i s , an event was e i t h e r recorded or i t was not. This added to the d i f f i c u l t i e s of any s t a t i s t i c a l a n a l y s i s . Nevertheless, c e r t a i n inferences may be drawn by reference to some of the graphs. (Figs. 13 and 14 , p. 85 and 86 ) F i r s t l y , i t i s evident that the temperature within the fog must be higher than 32° F.; otherwise any water deposited on the gauze w i l l freeze and escape being recorded. Sometimes t h i s water i s recorded many hours l a t e r , perhaps a f t e r the fog i t s e l f has been d i s s i p a t e d by the sun or wind. For instance, the fog that covered much of the lower part of the Fraser Delta on the night of 3 1 s t December, 1969 - 1 s t January, 1970 83 (see p. 1^3) , under f r e e z i n g conditions, deposited some water on thee gauze of the gauge. This was not, recorded at a l l u n t i l 1000 hours on the morning of 1 s t January, 1970; by which time the v i s i b i l i t y had r i s e n to 3 miles. Between 0900 and 1000, the temperature rose from 31° to 35° P. I t also appears that l i t t l e deposition of water occurs unless the a i r has been f u l l y saturated f o r at l e a s t 5 hours; v i s i b i l i t y also has to have been zero f o r t h i s time. This i s the time l a g of the instrument. V i s i b i l i t y i s p a r t l y a function of the l i q u i d water content of the fog. V i s i b i l i t y i s recorded as zero i f v i s i b i l i t y i s l e s s than 220 f e e t . (70 metres) Thus, according to Trabert's Law (Trabert, 1 9 0 1 ) , which states the r e l a t i o n s h i p between v i s i b i l i t y and l i q u i d water content of fogs, w = grc v where: w* = l i q u i d water content (gm. m .) r = mean drop radius c = a constant, approximately 3*0 v = v i s i b i l i t y the l i q u i d water content of fog w i l l have to be greater _3 than 1.29 gm. m before any water w i l l be deposited i n the gauge. This c a l c u l a t i o n i s based on a mean droplet diameter of 30yu. , which microscopic studies have shown to be a good estimate. Wind speed and run also appear to have to be above about 2 mph ( l ra. sec""1.) f o r water to be deposited i n the gauge. The as s o c i a t i o n of the higher maximums of wind speed ( > 5 mph) with l a r g e r amounts of water f a l l i n g into the gauge i s probably due to the dislod g i n g of droplets deposited on the gauze, allowing 8k them to drop d i r e c t l y into the gauge o r i f i c e . I t i s possible that some water could be l o s t to the ground outside the gauge i n t h i s manner, but i s much more l i k e l y to occur on the leeward piece of gauze. I t is- evident that i n a lowland temperate s i t u a t i o n , fog gauges of the c y l i n d r i c a l gauze type are l i k e l y to be l e s s e f f i c i e n t than balance type gauges. Although the l a t t e r are f a r more expensive to construct,, and s t i l l s u f f e r from c e r t a i n disadvantages (see p. 69)., they would probably y i e l d r e s u l t s amenable to analysis by normal s t a t i s t i c a l procedures, and thus of greater i n t e r p r e t i v e value. 85 36 3 3 32 33 33 33 36 35 ^34 .... 35 36 RiTtf lTlV£ HUM/DlTY ( f e K * i i - J 1 0 0 1 0 0 e.4 1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 =5 i i : f o g d r i p 1 \: Z t -<x 5 -Q +-o i: O * I U . 1 - i 1 o ' F i g . 13. Climatic parameters and fog d r i p . ( I ) Vancouver In t e r n a t i o n a l A i r p o r t . 86 DEWPOINT T E M P E R A T u A E (•(=) 3 6 36 35 34 33 35 35 33 32 31 33 31 REuftTWE. HUMIDITY CP*f ""^ 100 100 100 100 100 100 100 1 do 100 97 100 9^1 0 0 0 0 0 0 0 0 0 0 0 0 a *_ u 9 io o s ^ « o 5 o 4-° 1 1 o fog drip FREEZING F i g . Ik . Climatic parameters; and fog d r i p . ( I I ) Vancouver In t e r n a t i o n a l A i r p o r t . 87 CHAPTER FIVE. DIRECT ABSORPTION OF FOG WATER BY THE AERIAL PARTS OF PLANTS. 1 The a e r i a l parts of some plants are capable of d i r e c t l y absorbing, and u t i l i z i n g , water deposited from fogged a i r . However, the conditions under which t h i s "negative t r a n s p i r a t i o n " may occur, and i t s importance, are not f u l l y understood, and some of the experimental evidence i s contradictory. The factors a f f e c t i n g the entry of atmospheric water i n t o , and i t s passage through a plant are the surface c h a r a c t e r i s t i c s of the leaves, the resistance to movement within the plant, and the gradient of water p o t e n t i a l across the s o i l - plant - atmosphere system. Numerous studies have shown that the rate of water uptake w i l l depend p r i m a r i l y on the p o t e n t i a l gradient developed, and also the resistance to flow. (Bonner, 1959; Edlefsen, 19 kl; P h i l i p , 1958; van den Honert, 1948.) In t h i s , i t i s no d i f f e r e n t from normal (root) absorption of water.. THEORETICAL CONSIDERATIONS. In theory, water may be absorbed by any part of the plant that i s at l e a s t s l i g h t l y permeable i f s u f f i c i e n t d i f f u s i o n pressure d e f i c i t 1 (Meyer, 19^5) i s allowed to develop, or, i n more modern terms, the chemical p o t e n t i a l 1. D i f f u s i o n pressure d e f i c i t DPD = OP - TP , where OP i s osmotic p o t e n t i a l , and TP i s turgor pressure. This equation equals (2) overleaf. 88 i s h i g h e n o u g h . The w a t e r p o t e n t i a l o f t h e p l a n t c e l l v a c u o l a r s a p ( T*V ) u n d e r i s o t h e r m a l c o n d i t i o n s i s d e t e r m i n e d b y t h e c o n c e n t r a t i o n o f s o l u t e s i n t h e v a c u o l e ; t h a t i s , i t s 3 o s m o t i c p o t e n t i a l , m a t r i c p o t e n t i a l , and t h e p r e s s u r e e x e r t e d b y t h e c e l l w a l l . T h u s t h e e q u a t i o n may be w r i t t e n K * YS + *<« + ^ (i<) where ^ , ^ , a n d Vfc r e p r e s e n t t h e c o n t r i b u t i o n s ; made b y s o l u t e s , t h e m a t r i x , and t u r g o r p r e s s u r e r e s p e c t i v e l y . ^ a n d a r e n e g a t i v e amounts;, w h i l e Vfc- i s g e n e r a l l y p o s i t i v e . tym i s c o n v e n t i o n a l l y assumed t o be n e g l i g i b l e , so t h a t t h e e q u a t i o n r e d u c e s t o % " + ft (2) W a t e r w i l l be t a k e n u p b y a c e l l a s l o n g a s V/ i s more n e g a t i v e , t h a t i s ; , l o w e r , t h a n t h e p o t e n t i a l o f t h e w a t e r on t h e o u t s i d e o f t h e c e l l (Ve ) • U p t a k e o f w a t e r w i l l d i l u t e t h e v a c u o l a r s a p , a n d jf% w i l l become more p o s i t i v e , t h a t i s , i t w i l l i n c r e a s e . However, s i n c e t h e c e l l w a l l i s s w o l l e n o u t w a r d s b y t h e i n c r e a s e i n c o n t e n t o f t h e c e l l , ^fe w i l l a l s o i n c r e a s e . W a t e r u p t a k e w i l l s;top when % = Ve. . I f f o r t h e moment i t c a n be a ssumed t h a t t h e w a t e r on t h e o u t s i d e o f t h e c e l l i s p u r e (pH = 7) so t h a t i t s p o t e n t i a l w i l l be z e r o ( = 0 ) , t h e n a t t h e p o i n t o f e q u i l i b r i u m fv = 0; t h e c e l l i s t e r m e d 2. C h e m i c a l p o t e n t i a l e x p r e s s e s t h e same p r o p e r t y o f a s y s t e m as DPD, a n d t h e two a r e n u m e r i c a l l y e q u a l , b u t o p p o s i t e i n s i g n : V = -DPD. 3. The a t t r a c t i o n ; b e t w e e n w a t e r m o l e c u l e s and t h e m a t r i x i w i t h w h i c h t h e y a r e i n c o n t a c t . 89 f u l l y t u r g i d . Under such conditions, osmotic p o t e n t i a l w i l l equal turgor pressure, or % = - ft ( 3 ) I f the- external water contains solutes, so that , then equilibrium w i l l be established at below the maximum volume of the c e l l , when i t i s not f u l l y t u r g i d . Here h - - (h) However, i n a plant under water stress i n , f o r instance, a coastal desert area, where the atmosphere i s at or approaching saturation due to impinging advection fog, the t o t a l plant system w i l l be i n a s i t u a t i o n with two external water solutions i n contact with the outermost c e l l walls. One of these i s i n the s o i l , and the other i n the atmosphere. Evaporation f o r a long period from the surface layers of the s o i l , before the onset of a foggy period, w i l l be p a r t l y responsible f o r the water d e f i c i t i n the s o i l and w i l l have increased the concentration of s a l t s i n the remaining s o i l water ava i l a b l e to the plant. Thus the water p o t e n t i a l i n contact with the root system w i l l be 'more negative' than the p o t e n t i a l of the water vapour i n contact with the a e r i a l part of the plant, and that already caught or condensed out on vegetal surfaces. Thus YU-I < Va» (5) where V'sml is; the p o t e n t i a l of the s o i l water i n contact with the root system; and is; the p o t e n t i a l of the a e r i a l water, (water on the vegetal surfaces;, and water vapour i n the zone immediately above the leaves.) 90 But where HUi < O (60 and water absorbed by the t o t a l plant system w i l l tend to be from the source with the l e a s t d i f f u s i o n t r a n s f e r resistance, that i s , from the fogged or saturated a i r . Several German workers have mentioned the fa c t that l e a f surface water (they were concerned with dew) i s r e l a t i v e l y free of s a l t s , unlike'water taken up by the roots, and they considered that much of the importance of t h i s water source was i n that i t provided a supply of pure water otherwise unobtainable. (Arens, 1934; H i l t n e r , 1930, 1932; Lausberg, 1935; Z a t t l e r , 1932.) Although many coastal fogs have been found to contain sea s a l t s , derived from condensation n u c l e i , the s a l t concentration of deposited fog water i s nevertheless f a r l e s s than that to be found i n the s o i l , p a r t i c u l a r l y i n a r i d areas. I t should be noted that water a c t u a l l y deposited from fogged a i r i s probably more l i k e l y to be absorbed than water vapour, since no phase change need occur. This has; been confirmed experimentally by Jensen et a l . ( l 9 6 l ) , who observed that resistance to water flov* through l e a f tissue was l e s s when a d i r e c t water - l e a f i n t e r f a c e was substituted f o r an atmosphere i n t e r f a c e , even when the r e l a t i v e humidity of the impinging a i r approached 100 per cent. This; was also found by Hohn (1951) , and Janes, (1954) . (see p. 108) Whatever the main route of water entry into leaves, i t may always be expected that the i n i t i a l rate of uptake w i l l be i n v e r s e l y r e l a t e d to l e a f water content 9 1 and the water p o t e n t i a l of the l e a f . (Slatyer, 1967) This was found true experimentally by Krause ( 1 9 3 5 ) , and by Slatyer's experiments with Pinus echinata. (Slatyer, 1956.) PHYSIOLOGICAL CONSIDERATIONS: PATHWAYS OF WATER ENTRY INTO AERIAL PLANT ORGANS. 1. THE STOMATA. Since most of the water derived from t r a n s p i r a t i o n passes through the stomata, i t may be expected that any water vapour re-absorbed from fogged a i r under conditions of moisture stress would also u t i l i z e t h i s pathway. However, the fac t i s often overlooked that most stomates on the majority of plant species are on the lower surface of the l e a f . Thus the net amount of intake from; water (seen from the above to be the most e f f i c i e n t absorption form) i s l i k e l y to be small owing to the dislodging of i n c i p i e n t water droplets by gravity, aided by movement of the plant by shaking. Nevertheless, i t seems that water vapour can be absorbed i n t o the l e a f v i a the stomata, and there are often just a few stomates on the upper l e a f surface, i n any case. Normally, the stomata would be closed during the night, since at l e a s t one of the functions of the stomata i s to allow the absorption of carbon dioxide f o r photosynthesis, which cannot take place without l i g h t energy. The fa c t that the r e q u i s i t e v;apour pressure gradient would u s u a l l y only develop during the night, when the stomata are closed, i s often considered evidence that i f water i s absorbed, the amounts would be very small. However, i t i s known that some plants, under 92 very unfavourable water conditions, such that the leaves s t a r t to lose t h e i r turgor, are able to close t h e i r stomata p a r t i a l l y , or even completely, f o r a time i n the middle of the day. This may be because photosynthesis i s reduced and the concentration of carbon dioxide i n the i n t e r c e l l u l a r spaces r i s e s ; the cause i s uncertain, but the phenomenon has been observed. ( S u t c l i f f e , I 9 6 8 ) The l i k e l y c o r o l l a r y of t h i s i s the important one from the present point of view. This i s t r a n s i t o r y opening during the night, which may be adding to the economization of water by the plant i f i t allows absorption of water from fogged a i r , or from dew. The patterns.of stomatal opening and closure vary considerably from species to species. Most cereals hardly ever, open t h e i r stomata at night, and many of them can remain closed during most of the day. ( S u t c l i f f e , 1968) In contrast, there are some plants, such as potato and onion, i n which the stomata are co n t i n u a l l y open under normal moisture conditions, except f o r a few hours immediately a f t e r sunset. Midday closure does not take place i n these plants u n t i l they are badly w i l t e d , and even under conditions of extreme water stress opening may occur during the normal closure period. The pattern of stomatal behaviour i s quite different: i n many succulents and other xerophytes; here the stomata commonly open at night and close by day, perhaps because the need to conserve water i n order to . survive i s stronger than the need to photosynthesize r a p i d l y i n order to grow. Some studies have in d i c a t e d that the vapour pressure at the surfaces of the i n t e r n a l mesophyll c e l l s under the stomata i s l i k e l y to be greater than 0.95 (100 to 95 per cent external to i n t e r n a l r e l a t i v e humidities) i n most non - t r a n s p i r i n g plants, even when wilted, which 1 93 would mean that the gradient and hence the amount of reverse d i f f u s i o n of water would be small. The vapour pressure has been observed o c c a s i o n a l l y to be l e s s than 0.80, but only i n the laboratory, under c a r e f u l l y c o n t r o l l e d conditions (Stone et a l . , 1950; Whiteman and R o l l e r , 1 9 6 4 ) . T u r r e l l (19^7) argued that C i t r u s stomata are not penetrated by l i q u i d water at a l l , although some water vapour may penetrate, as may some o i l s . He studied f r e s h l y k i l l e d stomates of several v a r i e t i e s of orange and lemon, and suggested that the resinous stomatal plugs and c u t i n i z e d stomatal chambers e f f e c t i v e l y prevented the stomates from f i l l i n g with water. An explanation may be provided from the work of Ebeling ( 1 9 3 9 ) . The contact angle (see p. 103) between d i s t i l l e d water and waxed c a p i l l a r y glass tubes was found to be large, and the height of the water r i s e i n the tubes, zero; whereas i n clean, unwaxed glass tubes, the contact angle was found to be small, and the r i s e of water, more than 2 em. This showed that where contact angles are large, surfaces are small. The c i t r u s stomatal plug consists of r e s i n , thought to be formed by polymerization and low order oxidation of ethereal o i l s produced by the plant. In t h i s , the citrus; r e s i n i s s i m i l a r to most plant r e s i n s . The ethereal o i l limonene i s found i n c i t r u s , and T u r r e l l reasonably concluded that the c i t r u s r e s i n i s a polymer of t h i s substance. The s t r u c t u r a l formula shows that t h i s substance i s r e l a t i v e l y non - polar, and thus the contact angle of water with i t would be large (see s t r u c t u r a l formula o v e r l e a f ) . T u r r e l l i n t h i s way concluded that no l i q u i d water would penetrate c i t r u s stomata. However, the argument seems to be based on c i r c u m s t a n t i a l evidence, which rather points to the l i k e l y slow rate of water absorption rather than t o t a l 9k exclusion. Much more s i g n i f i c a n t as f a r as the adherence of water droplets i s concerned i s the f a c t , already mentioned, that i n common*, with many other plants citrus; leaves have t h e i r stomata on the lower l e a f surface, and thus g r a v i t y i s working to dislodge the droplets of water. S t r u c t u r a l formula  of Limonene. (After T u r r e l l , 19^7.) The s i g n i f i c a n c e of contact angles of water droplets adhering to l e a f surfaces; x ^ i l l be further discussed, (see p. 103) Rhythmic variations; of tissu e tensions within leaves under normal conditions have been observed by some workers, and Fogg (19^ 7 ) provides i n d i r e c t evidence f o r t h i s . Such v a r i a t i o n s may well have "important1 influences on stomatal movement, e s p e c i a l l y since i t has been observed that the stomata of w i l t i n g leaves open f i r s t more widely, and l a t e r close. ( S u t c l i f f e , 1968) The experiments of Laidlaw and Knight (1916) i l l u s t r a t e d t h i s , with Phaseolus v u l g a r i s , Eupatorium  adenophorum, and Maranta coccinea, var. floribunda. Stomata on leaves detached and allowed to w i l t opened temporarily before c l o s i n g . Since a con t r o l detached under water, and kept supplied with water, d i d not a l t e r 95 i t s stomata, Laidlaw and Knight a t t r i b u t e d stomatal opening on the wil t e d l e a f to water s t r e s s . They considered that t h i s was probably due to the guard c e l l s r e t a i n i n g t h e i r turgor longer than the other epidermal c e l l s . This stomatal opening on w i l t e d leaves was also shown, e a r l i e r s t i l l , by Darwin and Pertz. (1911) To explain the phenomenon, Fogg (1947) put forward the hypothesis-, following Martens ( 1 9 3 4 ) , of d i f f e r e n t i a l rates of contraction of s u p e r f i c i a l and underlying t i s s u e s . The d i f f e r e n t i a l rates were noticed i n t r y i n g to explain v a r i a t i o n i n water droplet contact angles on r a p i d l y w i l t i n g leaves. This behaviour of the stomata may be a t t r i b u t e d to preliminary s t r e t c h i n g of the whole epidermis, followed by release of the tension. I f guard c e l l s of the stomata are able to r e t a i n t h e i r turgor longer than the c e l l s underneath, whatever the reason, the replenishment of water by reversed d i f f u s i o n through the stomata from fogged a i r or water droplets caught from fogged 1 a i r , may mean that the l e a f mesophyll c e l l s may be able to p a r t i a l l y regain t h e i r turgor before the stomata reclose. Despite the obscurity of the r e l a t i v e importance of the stomata as the point of i n i t i a l water absorption, i t i s cle a r (Slatyer, 1.960?,) that the absorption of nutrie n t s and i n s e c t i c i d e s frequently occurs i n t h i s manner. (Cook and Boynton, 1952) The l a t t e r found that the uptake of urea nutrient by Mcintosh apples was through the stomata, and t h i s has been observed with i n s e c t i c i d a l o i l s , on various plants. (Ginsburg, 1930; Kelley, 1930, Knight et a l . , 1929; Rohrbaugh, 1934.) 96 2 . THE CUTICLE AND THE EPIDERMIS. The properties of the c u t i c l e also profoundly influence t r a n s p i r a t i o n , so that they would also be expected to influence any reverse flow;. The influence of the c u t i c l e i s p a r t i c u l a r l y noticeable when the stomata are closed. In shade plants, such as ferns, where the c u t i c l e i s t h i n , as much as 30 per cent of the t o t a l transpired water loss; i s thought to take place as c u t i c u l a r t r a n s p i r a t i o n . ( S u t c l i f f e , 1968) In desert s u c c u l e n t s b y way of contrast, l o s s through the c u t i c l e i s probably n e g l i g i b l e ; t h i s presumably means that absorption of water through the c u t i c l e w i l l also be small, unless the plant i s s p e c i a l l y adapted. Slatyer ( 1 9 6 7 ) , however, following Gessner, ( 1 9 5 6 ) , and r e f e r r i n g to dew deposit on leaves, pointed out that since dew i s pri m a r i l y y o f nocturnal occurrence, and since the stomata of most species normally appear to be closed f o r most of the night, i t seems probable that water uptake occurs p r i m a r i l y through the c u t i c l e . The e f f e c t of moisture stress on the behaviour of the stomata i s apparently overlooked. The r e l a t i v e s i g n i f i c a n c e of water absorption through the stomata w i l l i n any case tend to be reduced i n c e r t a i n plant species with r e l a t i v e l y high permeability to l i q u i d flow. Vaadia and Waisel (1963) showed that the entry of t r i t r i a t e d water (HTO - la b e l l e d ) was f a s t e r into the leaves of sunflower plants (Helianthus annuus var. advance), than i n t o Aleppo Pine (Pinus halepensis M i l l . ) , under the same conditions.. They a t t r i b u t e d t h i s to the thicke r c u t i c l e of the l a t t e r . In A u s t r a l i a , Wood (1925) compared water absorption by branches cut from Eucalyptus corynocalyx, S t e r c u l i a  d i v e r s i f o l i a , Acacia decussata, and A t r i p l e x vesicarium. 97 To avoid the p o s s i b i l i t y that sprayed moisture might be retained i n the c a p i l l a r y spaces between the v e s i c l e s . or h a i r s which cover the epidermis and thus cause an apparent absorption on weighing, he placed the branches i n a nearly saturated atmosphere. His r e s u l t s showed n e g l i g i b l e absorption by the leaves of Eucalyptus  cornocalyx, S t e r c u l i a d i v e r s i f o l i a , and Acacia decussata, but, there was s i g n i f i c a n t absorption by A t r i p l e x vesicarium. This was a t t r i b u t e d to the f a c t that the three former species have c u t i n i z e d leaves, whereas the leaves of the l a t t e r are uncutinized. This suggests that l e a f c u t i c l e i s normally impervious to water. Eisenzopf ( 1 9 5 2 ) , i n experiments to induce negative water transport with several coniferous species, observed a peak rate of absorption a f t e r . 9 0 minutes immersion i n water, a f t e r which the rate of absorption decreased r a p i d l y . He suggested that the increased rate f o r the f i r s t 90 minutes- was the r e s u l t of an increase i n the permeability of the c u t i c l e , i t s e l f caused by increased c u t i c l e hydration, although he admitted that a s i m i l a r r e s u l t could have been obtained i f the stomata had opened. S l a t y e r ( 1 9 6 7 ) , however, considers that "a more probable explanation" was the simple one that the gradient of water p o t e n t i a l had been progressively reduced. This does not account f o r the observed d i s t i n c t break i n the absorption curve a f t e r 90 minutes. The comparatively slow rates of c u t i c u l a r and epidermal absorption are suggested by the experiments of Slatyer ( 1 9 5 6 ) , with Pinus echinata. His r e s u l t s i n d i c a t e d that the steepest part of the gradient of increa s i n g water p o t e n t i a l was at the l e a f surface, whether under conditions favouring negative or p o s i t i v e water transport. Since the steepest part of the gradient might be expected to be at the root surface 98 during negative transport, the f a c t t h i s i s not so shows that, with Pinus echinata at l e a s t , the c u t i n i z e d epidermis of a l e a f i s much l e s s permeable to water than the suberized epidermis of a root. Slatyer ( i 9 6 0 ) concluded that " i n general, and with the present state of knowledge, i t appears that most of the water absorbed by leaves i s through the c u t i c l e . " (p. 369) He notes, however, that i f t h i s i s so, a marked increase i n c u t i c l e permeability must occur on wetting, i n order to explain the paradox of high c u t i c u l a r - r e s i s t a n c e to water transport during t r a n s p i r a t i o n . Slatyer regarded the evidence as "inconclusive." Absorption through the c u t i c l e almost c e r t a i n l y does; occur, i n some species, and i n some s i t u a t i o n s , but i n general i t seems that c u t i c l a r resistance i s so high that the t o t a l amount absorbed i n a short period i s u s u a l l y i n s u f f i c i e n t to do more than p a r t i a l l y allow, the l e a f tissue to regain t u r g i d i t y . Nevertheless, t h i s could be important where the plant i s i n badly stressed conditions and i s struggling to survive. 3. SPECIALIZED CELLS FOR WATER INTAKE. Some les s well known modes of water intake, v i a the leaves of plants u t i l i z e s p e c i a l i z e d c e l l s tructures. How generally these s p e c i a l i z e d c e l l s occur i s s t i l l unknown. Spe c i a l i z e d c e l l s f o r water intake have been observed by Zamfirescu ( 1 9 3 1 ) , and Meidner ( 1 9 5 4 ) . F i e l d observations by the l a t t e r , i n Natal, suggested that several species that were adapted to withstand severe s o i l moisture d e f i c i t s were able to absorb water from dew deposited on t h e i r leaves. I t was noted that 9 9 \ c£^*- CONTENT F i g . 1 5 . Longitudinal section of a s p e c i a l i z e d  epidermal c e l l of a l e a f of  Chaetacme a r i s t a t a , (After Meidner. 1 9 5 4 . p. 424.) 100 water disappeared most r a p i d l y from the leaves of Chaetacme a r i s t a t a . Microscopic examination revealed, and subsequent experimentation confirmed, the presence of s p e c i a l i z e d epidermal c e l l s with denser contents than normal (see F i g . 15, p.99). Meidner found 34 s p e c i a l i z e d c e l l s per square millimetre i n the upper epidermis, and 12 per square millimetre i n the lower epidermis. Since absorption was found to be s i g n i f i c a n t l y greater on the upper epidermis than through the lower epidermis;, i t was concluded that absorption was at l e a s t aided by these s p e c i a l i z e d c e l l s . This conclusion was confirmed by evidence from s e l e c t i v e v i t a l s t a i n i n g techniques. Meidner also noted that water was only absorbed i n the l i q u i d state, not from water vapour. Thus t h i s process may be s i g n i f i c a n t with some species; i n absorbing water from fog that has coalesced on the leaves; i t s possible a p p l i c a t i o n would at f i r s t be r e s t r i c t e d to a search f o r species that possess these s p e c i a l i z e d c e l l s . The development of these species might be worthwhile, but, of course, a l l the other f a c t o r s , such as l e a f configuration, surface features, and economic use, would also have to be considered concurrently. One species with an obvious commercial value that i s known to possess an apparently s p e c i a l i z e d l e a f structure i s the apple, Malus pumila. Roberts et a l . (1948) found that the c u t i n of the epidermis of apple leaves was i n discontinuous lamellae p a r a l l e l to the outer epidermal w a l l . Peetinaceous substances, known to have great powers of water absorption, were found to occur i n intermittent layers i n the outer epidermal walls;, interspersed with these c u t i n lamellae. These substances apparently formed a continuous pathway from the layers i n the c u t i c l e through the a n t i c l i n a l walls of the epidermal c e l l s to the c e l l walls of the vein extensions and 101 ,'P/vrnww EPlDEKHIS I VEiM SftEETS F i g , 16. Upper l e a f surface structure of Mcintosh Apple.(Adapted from microphotographs given  by Roberts et a l . , 1948.) 102 bundle sheaths surrounding the l a r g e r veins of the leaves. This- was considered to provide a possible pathway f o r water intake through the c u t i c l e to the l i v i n g c e l l s surrounding the vascular t i s s u e s , (see F i g . 16 ,- p. 101 ) A s l i g h t l y e a r l i e r study by the same authors (Palmiter et a l . , 1946) showed that solutes did move along t h i s pathway; s i m i l a r l y Steubing (1949) observed t h i s movement of water using fluorescent dyes. PHYSICAL CONSIDERATIONS. 1. THE EFFECT OF TEMPERATURE. Temperature can have an e f f e c t on the rate of water absorption i n the normal (positive) way, through i t s e f f e c t on t r a n s p i r a t i o n rates. F i r s t recognized by Delf ( 1 9 1 6 ) , the slowing of water uptake with decrease i n temperature was l a t e r confirmed by Clements and Martin ( 1 9 3 4 ) , with sunflower, and by Arnt (193?) with cotton plants'. Reduction of water intake i s l e s s i n species native to cool environments than i n species which normally grow i n warm ones (Kramer, 1942), This has been attributed- to the combined e f f e c t s of decreased permeability of the root membranes and the increased v i s c o s i t y of the water with temperature reduction. Kramer ( l 9 4 o ) observed that water flow through root systems increased as temperature increased to 35*C., the highest temperature studied. Experiments c a r r i e d out by Jensen and Taylor ( 1 9 6 1 ) , on sunflower (He 1 ianthus a n n u u s a n d tomato (Lycopersicon eseulenturn M i l l . ) i n d i c a t e that the slowing of absorption with decreased temperature also applies to reversed water intake through the leaves under conditions of moisture 103 s t r e s s . I t may be c o n s i d e r e d , h o w e v e r , t h a t w a t e r s t r e s s i s more l i k e l y t o d e v e l o p n a t u r a l l y u n d e r h o t c o n d i t i o n s t h a n u n d e r c o l d , and t h u s t o r e p l e n i s h t h e d e f i c i t b y a b s o r p t i o n w i l l a l s o be s l i g h t l y e a s i e r . 2. THE WETTABILITY OF THE C U T I C L E . I f t h e m a i n s o u r c e o f w a t e r a b s o r b e d i s f r o m w a t e r a c t u a l l y h e l d as d r o p l e t s on t h e l e a f s u r f a c e , as i s n o r m a l l y t h e ca se f r o m f o g p r e c i p i t a t i o n ( a n d as i t w o u l d be w i t h d e w ) , an i m p o r t a n t p o i n t t h a t may be o v e r l o o k e d i s t h e w e t t a b i l i t y o f t h e c u t i c l e . The a b i l i t y o f a l i q u i d t o wet a s u r f a c e i s a f u n c t i o n o f i t s c o n t a c t a n g l e on t h e s u r f a c e ; t h i s i n t u r n depends on t h e s u r f a c e t e n s i o n o f t h e l i q u i d and t h e t y p e o f s u r f a c e . S u r f a c e f o r c e s a r e known t o be t h e r e s u l t , i n p a r t a t l e a s t , o f t h e s t r u c t u r e s o f t h e m o l e c u l e s m a k i n g up t h e s u r f a c e ( W h e l a n d , 1 9 4 4 ) . F o g g (1947) p o i n t e d o u t t h a t t h e a r e a o f c o n t a c t b e t w e e n a l e a f s u r f a c e and t h e w a t e r o r s o l u t i o n d e p o s i t e d on i t i s a n i m p o r t a n t f a c t o r d e t e r m i n i n g t h e amount o f d i s s o l v e d s u b s t a n c e s e n t e r i n g , o r l e a v i n g , t h e l e a f . A d v a n c i n g c o n t a c t a n g l e was u s e d as a measure o f t h e e x t e n t t o xtfhlch w e t t i n g t a k e s p l a c e on t h e l e a v e s o f S i n a p i s a r v e n s i s , T r i t i c u m v u l g a r e , and some o t h e r p l a n t s ( F o g g , 1944, 1 9 4 7 ) . R e s u l t s showed t h a t t h e c o n t a c t a n g l e i n c r e a s e d , and t h u s t h e d r o p l e t s became e a s i e r t o d i s l o d g e , as t h e age o f t h e l e a f i n c r e a s e d , and a l s o as t h e w a t e r c o n t e n t o f t h e l e a f d e c r e a s e d . T h i s may be s i g n i f i c a n t i n i n f l u e n c i n g t h e amount o f w a t e r t h a t a w i l t i n g a n d t h u s s t r e s s e d p l a n t i s a b l e t o a b s o r b f r o m f o g d r o p l e t s d e p o s i t e d and c o a l e s c e d o n i t s l e a v e s . W a t e r a b s o r p t i o n may a l l o w t h e l e a v e s o f some s p e c i e s t o r e m a i n t u r g i d enough t o h a l t t h e i n c r e a s e i n 104 water droplet contact angle. Ho\ir,ever, the steady r i s e i n the value of the contact angle of water on w i l t i n g leaves may i t s e l f be due to increased wrinkling of the l e a f surface as the tissues contract on l o s i n g water. This wrinkling probably involves the epidermis as a whole. (Fogg, 19^7) The c u t i c l e may pucker independently of the underlying c e l l walls, as Martens (1934) has shown with Tradescantia v i r g i n i c a . In t h i s case the scale of wrinkling i s l e s s but the e f f e c t on the contact angle w i l l be s i m i l a r . Fogg (1947) overlooks the f a c t that the wrinkling of the l e a f surface i t s e l f , although i t increases the contact angle of the water' droplets, may not increase the l i k e l i h o o d that they w i l l be dislodged, since the wrinkles themselves w i l l tend to r e t a i n water. Contact angle i s known to be reduced by the a d d i t i o n of wetting agents ; (Ebeling, 1939). Thus i t may be expected that the a d d i t i o n of such materials would s i g n i f i c a n t l y improve absorption p o t e n t i a l , at l e a s t on some types of leaves', (Slatyer, 1967). The addition of detergents to f o l i a r sprays has- been found to increase nutrient uptake, (Guest and Chapman, 1949; Cook and Boynton, 1952) , and although no studies dealing d i r e c t l y with water absorption have apparently been c a r r i e d out, a s i m i l a r r e s u l t may l o g i c a l l y be expected. However, the work of Boynton ( 1 9 5 4 ) , on the f o l i a r a p p l i c a t i o n of nutrients for 1 plant use, has i n d i c a t e d that nutrient uptake can occur even when the l e a f i s apparently dry, so i t i s evident that there must be other absorptive mechanisms that are operative besides those i n v o l v i n g the intake of water. Even so, some of t h i s work i s of value i n aiding the understanding of the behaviour of water i n contact with a l e a f surface. 1 0 5 ABSORPTION AND TRANSLOCATION : EXPERIMENTAL EVIDENCE. Many experiments have demonstrated that water impinging on the l e a f surfaces of plants i s absorbed, whatever the process; l e s s evidence i s a v a i l a b l e to show that the absorbed water i s transported away from the leaves. One of the e a r l i e s t s c i e n t i f i c experiments was that of Lloyd ( 1 9 0 5 ) , c a r r i e d out i n the Arizona Desert on the o c t i l l o , Fouqueria splendens. In the summer, th i s shrub u s u a l l y loses i t s leaves but i s able to form new. ones i n a matter of days when water i s a v a i l a b l e . Lloyd wetted the upper part of the stem of one of these shrubs by wrapping i t i n cotton gauze, dipping the end into a water r e s e r v o i r . New. leaves were then found to develop on that p o r t i o n of the stem, showing that the water was absorbed. Marloth ( 1 9 0 8 ) , i n South A f r i c a , conducted a C d e t a i l e d p h y s i o l o g i c a l study of the a p i c a l h a i r s of succulent leaves and stems of Mesembrlanthemum densum and M. barbaturn, and stated that these h a i r s were "admirably suited" to water absorption, but he d i d not state how. Although Marloth d i d not demonstrate absorption with these species, he l a t e r studied several species of Lila c e a e and Amaryllidaceae, and a f t e r also reporting t h e i r a p i c a l h a i r s suitable f o r water absorption, did demonstrate some absorption of water. (Marloth, 1 9 2 6 ) Lack of a v a i l a b l e water because that i n the s o i l was frozen has not generally been considered as a condition causing moisture s t r e s s . I t was, however, mentioned i n research by Gates 1.1914) . He c a r r i e d out a seri e s of water balance measurements with several evergreen shrubs and trees i n Michigan. During the winter, he found that a cut branch took: up moisture on a f r o s t y 106 night equal to three or four times; that l o s t by t r a n s p i r a t i o n from the; same branch on a cold winter day. The actual cause of the moisture here is. unimportant, whether i t was fog or dew, f o r both would wet the branch and thus be p o t e n t i a l l y absorbable. Absorption has been demonstrated more recently by the used of dyes or l a b e l l e d water. Vaadia and Waisel (1963) used t r i t r i a t e d water (HTO la b e l l e d ) to trace water absorption and transport i n Helianthus annuus and Pinus halepensis M i l l . They found that transport away from the l e a f was u s u a l l y slow, and i n some cases did not occur at a l l . Gindel (1966) used various dyes, such as fuchsin, and reported that 'reverse t r a n s p i r a t i o n ' was a c t u a l l y seen to take place, by the gradual colouring of the l e a f , and the stem, of Zea mays and P. halepensis M i l l . They had been exposed i n a state of water stress; i n the f i e l d at Rehovot, I s r a e l . Gindel was mainly concerned with dew absorption; nevertheless, the technique has merit fo r use i n studies of fog water absorption. During the 1950' s a series of experiments on a e r i a l water absorption were c a r r i e d out, which are generally considered to be the "classics'' i n the f i e l d . As such, they are worth further consideration. Probably the most well - known experiments are those of Breazeale, McGeorge and Breazeale (1950, 1 9 5 l ) , and Breazeale and McGeorge. (1953a, 1953b) These experiments demonstrated, remarkably w e l l , both absorption and negative transport through the whole atmosphere -plant - s o i l - system. With tomato and corn plants, they demonstrated that water could be absorbed from an a r t i f i c i a l l y fogged atmosphere and exuded from the roots, whether the plants were i n s o i l or not. The conclusions a r r i v e d at by Breazeale et a l . (1950) were s t a r t l i n g ; they found that 107 " A tomato plant can absorb water from a saturated atmosphere, transport i t to the roots, and b u i l d up the, s o i l moisture to or above f i e l d capacity." (p. h±9.) which would appear contrary to the t h e o r e t i c a l evidence presented previously, (p. 87) They also observed: " Tomato plants w i l l grow to maturity, flower, and set f r u i t with no other source of water than that absorbed through the leaves; from a fog or an atmosphere of 100 per cent r e l a t i v e humidity." (p. 4 l 9 . ) which would obviously be of great s i g n i f i c a n c e i n a r i d areas with frequent fogs. However, under natural conditions, the atmosphere i s u n l i k e l y to stay :constantly fogged, so that t o t a l growth may require some a d d i t i o n a l water source. Nevertheless, t h i s might be minimal. Haines (1952, 1953) enlarged upon the empty f l a s k experiment of Breazeale et a l . , and obtained a s i m i l a r accumulation of xvater i n the f l a s k with e i t h e r a tomato plant or a cotton wick. The l a t t e r f a c t , absorption through a cotton wick, "appeared f i n a l l y to dispense with the necessity of invoking active secretion by the roots into saturated or p a r t i a l l y saturated a i r , 1 1 Haines, 1952, p. 97.) to explain the r e s u l t s obtained by Breazeale and Haines himself under c o n t r o l l e d conditions of temperature and thus r e l a t i v e humidity. This also applied with sealed f l a s k s allowing f l u c t u a t i o n s of pressure r e l a t i v e to that of the surrounding atmosphere. In f a c t , i f the temperature of the f l a s k s into which the roots were sealed was kept.constant, i n wet shingle, no water accumulated. Haines l a t e r concluded: " Although the leaves (of tomato plants) are undoubtedly able to absorb water f o r t h e i r own turgor from 108 f o g g e d a i r ... t h e y do n o t by a c t i v e s e c r e t i o n o r any o t h e r p r o c e s s , f o r c e w a t e r i n t o the stem." ( H a i n e s , 1953, p. 1 0 7 ) . A r e v e r s e movement o f w a t e r was o b t a i n e d i n s e e d l i n g s o f Pinus> p o n d e r o s a by Stone e t a l . (1956) a g a i n o n l y when t h e t e m p e r a t u r e was a l l o w e d t o f l u c t u a t e and t h e r e was a r e s u l t a n t d r o p i n t h e r e l a t i v e h u m i d i t y o f th e a i r s u r r o u n d i n g the r o o t s i n t h e f l a s k . These e x p e r i m e n t s s u g g e s t t h a t o t h e r f a c t o r s , n o t u n d e r s t o o d a t p r e s e n t , may be a t work. I t might be p o i n t e d o u t , however, t h a t under n a t u r a l c o n d i t i o n s t h e s o i l o f a r i d a r e a s i n p a r t i c u l a r may f l u c t u a t e c o n s i d e r a b l y f r o m n i g h t t o day; t h u s f a c t o r s a l l o w i n g a b s o r p t i o n o f w a t e r w i t h t e m p e r a t u r e f l u c t u a t i o n w i l l be f a v o u r e d . Due t o t h e d i f f i c u l t y o f m e a s u r i n g s m a l l i n c r e m e n t s o f s o i l m o i s t u r e i n t h e f i e l d , none have a p p a r e n t l y y e t been made. There seems t o be a need f o r t h e use o f s m a l l l y s i m e t e r s f o r f i e l d measurement i n t h e i n v e s t i g a t i o n o f t h i s p r o b l e m . S p e c i e s o t h e r t h a n tomato o r c o r n must a l s o be i n v e s t i g a t e d ; t h e y may y i e l d t o t a l l y d i f f e r e n t r e s u l t s . Hohn (1951) and Janes (1954) f o u n d t h a t p l a n t s a b s o r b e d more l i q u i d w a t e r than' w a t e r v a p o u r , n e a r t h e immediate l e a f s u r f a c e . W h i l e d e m o n s t r a t i n g a b s o r p t i o n , t h e y b o t h f o u n d t h a t e v a p o r a t i o n o c c u r r e d a t t h e same t i m e i n t o an atmosphere a l r e a d y s a t u r a t e d . Stone and P o w e l l s (1955) r e p o r t e d t h a t s e e d l i n g s o f P i n u s p o n d e r o s a c o u l d r e d u c e t h e s o i l b elow the permanent w i l t i n g p o i n t f o r t h e sunfloxver ( H e l i a n t h u s  a n n u u s ) . A t t h i s h i g h m o i s t u r e s t r e s s , a r t i f i c i a l dew p r o d u c e d from a s p r a y p r o l o n g e d t h e l i f e o f t h e s e e d l i n g s u s e d f o r up t o one month. The u t i l i z a t i o n o f f o g g e d a i r f o r p r o l o n g e d s u r v i v a l u nder w a t e r s t r e s s may be i m p o r t a n t i n terms o f a b i l i t y o f some 109 species to tolerate; a dry season, such as that of southern C a l i f o r n i a . There may be some unrealized s i g n i f i c a n c e inherent i n the p r a c t i c e of p l a n t i n g coastal sandy areas with conifers i f t h i s i s c h a r a c t e r i s t i c of the genus as a whole. I t might be noted that P. ponderosa i s one species able to t o l e r a t e a comparatively dry climate, and over i t s ent i r e d i s t r i b u t i o n i n western North America, i s subject to long periods of drought. Stone (1957) has suggested that i t s distribution may be r e l a t e d to frequent dewfall, and the subsequent a b i l i t y to u t i l i z e i t . S u r v i v a l , however, i s a d i f f e r e n t matter from growth, and the production of utilizable> biomass. Janes (1954) , i n an attempt (which f a i l e d ) to repeat the experiments of Breazeale et a l . , (1950, 1951) found that growth of tomato plants sealed i n dry s o i l and exposed to fogged a i r , was slower than s i m i l a r plants grown i n moist s o i l , despite the f a c t that both appeared equally t u r g i d . A possible explanation advanced by Janes, was that: the s l i g h t moisture stress i n the plants growing i n dry s o i l prevented c e l l d i v i s i o n or enlargement. Also, the uptake of nutrients from the s o i l was prevented i n the plant exposed to fog, since any water transport was i n the reverse d i r e c t i o n . A l l the tomato plants i n Janes' experiment, which l a s t e d 18 days?., w i l t e d immediately they were removed from the fog. Several authors have noted that growth i s slowed or stopped by moisture s t r e s s . (Wadleigh and Ayers, 1945; Wadleigh et a l . , 1946; Wadleigh and Gauch, 1948.) Thut and Loomis (1944) found i n c i t r u s trees that as moisture stress increases, a turgor d e f i c i t a r i s e s within the tree before the f i r s t v i s i b l e sign of w i l t i n g appears; growth was checked by water d e f i c i t s w ithin the plant, induced by incre a s i n g moisture s t r e s s . 110 Absorption of fog water i n s u f f i c i e n t q u a n t i t i e s to permit the plant to regain it s ' t u r g i d i t y , as appears to be accepted by most authors even i f the main pathway i s disputed, may well be s i g n i f i c a n t i n helping to minimize the reduction i n plant growth that would otherwise occur during a dry period. . A number of these experiments have demonstrated simple absorption without proving t r a n s l o c a t i o n from the leaves to other parts of the plant, although transport i s often implied. For instance, B r i e r l e y (1934) did not observe any increase i n s o i l moisture when wilt e d raspberry plants were exposed to a water spray, but he found that i f one of a p a i r of joined raspberry canes was sprayed, the leaves o f both canes became turg i d again. This was taken , reasonably enough, to mean that the water applied to the w i l t e d leaves on one cane must have moved down that cane and then up the other ( B r i e r l e y , 1936). AERIAL ABSORPTION IN DOUGLAS FIR. Plants possessing a thick c u t i c l e are l i k e l y to be the most r e s i s t a n t to water absorption through t h e i r a e r i a l parts, (see p. 96 ) One species with a thick c u t i c l e i s Douglas F i r (Pseudotsuga meniesii (Mirb.) Franco.). I f absorption could be demonstrated i n t h i s species, the r e s u l t s may well be applicable to other species with thinner c u t i c l e s . Accordingly, four Douglas F i r seedlings were c o l l e c t e d from a f o r e s t edge, potted i n natural f o r e s t s o i l , and kept under c o n t r o l l e d conditions f o r eight weeks. Temperatures during t h i s period ranged from extremes of 50° F to 80°F , but were mainly i n the range ?0°F - 80°F. At f i r s t , the seedlings were kept well watered, I l l then-, a f t e r about three weeks, the plants were v i s i b l y beginning to w i l t . For 2k hours previous to the s t a r t of the experiment, a l l of the plants were kept continuously at 8 5 0 F (± 3°F) to encourage a d i f f u s i o n d e f i c i t in. the a e r i a l parts of the; plant. One plant died i n t h i s process and was discarded. Two plants were selected f o r the f i r s t experiment. One was about 10-| inches high, and the other about 6 inches, and w i l l be r e f e r r e d to as specimens A and B r e s p e c t i v e l y . This height r e f e r s to the a e r i a l part of the plant, (see Plate 5 , p.113) The s o i l around the roots of these two plants was not disturbed except f o r the taking; of a sample to determine' the moisture cont ent. The s o i l moisture content of specimen A was found to be high, despite the apparent dryness of the s o i l . Moisture content was 22.91 per cent. The s o i l of specimen B was much d r i e r , being 9*09 per cent. The plants were sealed i n t o watertight; containers with p l a s t i c cement, polythene, and f i n e wire. They were then exposed to the f i n e spray of the fog simulator, described i n d e t a i l on p. 131. The temperature fo r the whole period of these experiments was 65° F. (- 1° P) i n s i d e the simulator, and the r e l a t i v e humidity was not allowed to f a l l below 98 per cent. A f t e r 2k hours, the specimens were removed, dried, and t h e i r weights checked. The smaller plant (B) showed an unexpectedly r a p i d weight increase, so the experiment was terminated on t h i s plant. ( i t should be mentioned at t h i s stage that a rigorous inspection was c a r r i e d out f o r leaks; none were found.) The s o i l moisture content of B was again determined, and was found to be 16.93 per cent, an apparent increase of 7.84 per cent. The l a r g e r plant (A) had not at t h i s point increased i n weight, so i t was replaced 112 i n t o the fog spray f o r a further 72 hours, making a t o t a l exposure time of 96 hours. At the end of t h i s time, the experiment was terminated, the plant and container inspected f o r leaks (none were found), and the s o i l moisture content determined. This was 20.8 per cent, a decrease of 2.1 per cent. I t appears from these two experiments that Douglas F i r i s capable of absorbing water a e r i a l l y and t r a n s f e r r i n g i t to the s o i l only i f the moisture stress i s high. The1 reasons f o r the loss of weight from specimen A are probably a t t r i b u t a b l e to the lack of s o i l moisture stress at the s t a r t of the experiment. A t h i r d plant, specimen C, shown i n Plate 6, overleaf, was exposed with i t s roots i n an Erlenmeyer f l a s k without s o i l f o r 60 hours. The i m p r a c t i c a b i l i t y of weighing the plant by i t s e l f a f t e r exposure to the spray, due to the adherence of some of the p l a s t i c cement to the stem, meant that the ent i r e f l a s k and plant were weighed. However, no water deposit was v i s i b l e i n s i d e the f l a s k a f t e r exposure, so i t may be assumed that any weight increase was due to water retained i n the plant i t s e l f . This assumption was considered v a l i d , since the weight of the f l a s k and se a l i n g material remained constant. Thus; i t was possible to c a l c u l a t e the net increase of weight i n the plant. This y i e l d e d a weight increase of 40.1 per cent. I t i s l i k e l y (see p. 107) that the seedling acted as a wick, but the f a c t that the water was apparently able to pass through the thick c u t i c l e possessed by Douglas F i r i s of i n t e r e s t , and suggests that a thick c u t i c l e , by i t s e l f , does not prevent a e r i a l absorption of water. (These experiments arose out of c u r i o u s i t y a r i s i n g from the w r i t i n g of the present chapter, and the r e s u l t s are included only from the point of view of 114 i n t e r e s t and relevance to the present theme; they are not intended to be d e f i n i t i v e . I t i s f u l l y appreciated that s t a t i s t i c a l s i g n i f i c a n c e requires experimental r e p e t i t i o n . ) '-. ' i INTRA - PLANT WATER FLOW. Some studies; have s p e c i f i c a l l y considered the movement of water i n a negative d i r e c t i o n within plant systems, themselves. The term ' i n t r a -' as used here r e f e r s to t r a n s f e r of water' within plant systems, as opposed to into plants from; the atmosphere. Bormann (1957) found that moisture was transfe r r e d between tomato plants whose roots were associated i n sand or loam, or through the roots of an intervening plant which i t s e l f had access only to transf e r r e d water. This water moved, i n quantities; s u f f i c i e n t to delay the onset of w i l t i n g i n r e l a t i v e l y large plants. Bormann suggested that t r a n s f e r occurs only a f t e r a c r i t i c a l value of s o i l moisture tension i s achieved. An i n t e r e s t i n g point i s ra i s e d by Bormann's work. I f water can be absorbed by the a e r i a l parts of one species of plant, growing i n a s s o c i a t i o n with another species not capable of absorption (perhaps i n a semi - a r i d environment, or one with a dry season), i t i s s p e c u l a t i v e l y possible f o r the non - absorbing species to survive by tra n s f e r of water across the root systems. The pressures set up by root systems that would encourage the absorption and negative transport of water, have been considered by several authors. Hagan (19^9) investigated t h i s problem and found that there were rhythmic v a r i a t i o n s i n absorption of water in t o detopped root systems. He used the term "negative exudation" to describe the phenomenon of water absorption into the: roots from the stem. Sunflower was 115 placed i n s o i l which was reduced to the permanent w i l t i n g point. The plant was then detopped under water, and connected to a measurable water supply. Water intake by the cut stumps, or negative exudation, proceeded i n i t i a l l y at a high rate, and approached zero a f t e r approximately one week. This negative exudation was found to be d i s t i n c t l y p e r i o d i c . Since the experiments were ca r r i e d out under' c o n t r o l l e d isothermal and i s o t r o p i c conditions, the periodic f l u c t u a t i o n s must be considered autonomic, but the reasons f o r the phenomenon are not c l e a r . ' Grossenbacher' ( 1 9 3 9 ) , r e f e r r i n g to p o s i t i v e exudation, suggested that autonomic cycles of root growth may be responsible f o r the c y c l i c r e g u l a t i o n of the rate of water absorption from the s o i l . Skoog et a l . , (1938) considered that p e r i o d i c i t y may r e s u l t from changes i n root permeability, but found that the maxima f o r cycles of p o s i t i v e and negative exudation were twelve hours out of phase. From t h i s they concluded that d i u r n a l cycles i n root permeability could not be responsible f o r the observed cycles unless u n i d i r e c t i o n a l permeability was involved. The maxima of negative exudation occur at night. This suggests that the l i k e l i h o o d of absorption of fog water or dew i s enhanced at night, and t h i s increased p o t e n t i a l f o r absorption may coincide with t r a n s i t o r y opening of stomata at night i n a severely water stressed plant. ( S u t c l i f f e , I 9 6 8 ; see p.. 92 ). CONCLUSION. On balance, the evidence i s that fog water deposited on a e r i a l parts of plants, e s p e c i a l l y the leaves, can be u t i l i z e d . Some sort of turgor l o s s , or water stre s s , appears to be necessary f o r absorption. Turgor 116 may be restored i n plants not under s o i l moisture stress i f they have been exposed to wind or other d e s s i c a t i n g force f o r a period long enough to cause a minor d e f i c i t of water i n the a e r i a l p o rtion of a plant. Usually,, however, a s o i l moisture d e f i c i t seems necessary to induce absorption, and c e r t a i n l y any negative transport of the absorbed water. I t has been suggested that surface water may be harmful ( P h i l i p , 1932; Stockers, 1 9 3 3 ) , but i n an a c t i v e l y growing healthy plant, e s p e c i a l l y i n dry areas or i n dry seasons, t h i s i s probably not as important as the b e n e f i c i a l e f f e c t s . I t i s true that diseases of crops are sometimes correlated with the incidence of l e a f surface water, but, again, t h i s i s more l i k e l y to be the case i n c o n s i s t e n t l y humid areas. B e n e f i c i a l e f f e c t s are, of course, the r e s t o r a t i o n of turgor, s u r v i v a l , and possibly allowance of continued growth i n dry periods. Also, there are two l e s s frequently mentioned b e n e f i c i a l e f f e c t s . The f i r s t has been b r i e f l y mentioned: (p.9 0 ) . This i s that the water absorbed through the leaves may provide an important source of r e l a t i v e l y s a l t - free water, which would be unobtainable from any other source. The second b e n e f i c i a l e f f e c t i s the reduction of t r a n s p i r a t i o n by water remaining on the l e a f surface a f t e r sunrise (Jones, 1957). Since i n i t i a l energy w i l l have to be u t i l i z e d to evaporate t h i s , photosynthesis may be c a r r i e d on with l e s s water l o s s , and the water- stress p a r t l y a l l e v i a t e d . Also, the evaporation of the surface water w i l l perform one of the necessary normal functions of t r a n s p i r a t i o n , that of cooling of the l e a f . I t may be argued that i n order to make a r e a l c o n t r i b u t i o n to the water economy of a plant, fog water deposited on l e a f surfaces must permit greater growth 117 or production of biomass, than would otherwise be the case. D i r e c t evidence f o r t h i s i s lacking, although the r e s u l t s of Dudevani (quoted by Gindel, 1966), showing increased growth and production with l e a f surface dew, almost c e r t a i n l y r e f l e c t some d i r e c t evidence of absorbed water. The question w i l l probably never be solved i n the laboratory, as the environmental system can never be exactly r e p l i c a t e d , and i n the f i e l d the e f f e c t s of many other factors have also to be considered. Nevertheless, there seems, with the ever more pressing requirement of i n c r e a s i n g l y e f f i c i e n t food production, a r e a l need f o r much more experimental f i e l d evidence i n coastal a r i d areas, with a view to the u t i l i z a t i o n of t h i s undoubtedly s i g n i f i c a n t p o t e n t i a l water source, water deposited d i r e c t l y from fogs. 118 i CHAPTER SIX FOG DRIP AND FORESTS. The contribution to the water economy of vegetation by i n t e r c e p t i o n of fog water droplets, d r i p tx> the s o i l , and subsequent normal root absorption, w i l l now be considered, The contribution of d r i p from trees i s considered since these present the l a r g e s t p o t e n t i a l i n t e r c e p t i n g surface to the fog, and thus the amount; of water caught would be expected to be the l a r g e s t of a l l types of vegetation :. (see Ch. 7 0 . Probably due to the comparatively small amounts involved, fog d r i p from other types of vegetation have apparently never, been considered. Due to the decreased frequency of water stressed conditions i n humid environments, i t i s probable that fog d r i p from the leaves i n t o the s o i l i s greater than any d i r e c t absorption i n t o the leaves, (see Ch. 5), although no d i r e c t evidence i s a v a i l a b l e . S i g n i f i c a n t wetting of surfaces below trees under foggy conditions has been noticed from the time of G i l b e r t White, i n 17&9, who wrote that "an amazing amount of water may be d i s t i l l e d by one tree i n a night's time, by condensing the vapour which t r i c k l e s down the twigs and boughs so as to make the ground below quite i n a f l o a t , " (White, I789). A century l a t e r , the A u s t r a l i a n n a t u r a l i s t Cox noted s i m i l a r e f f e c t s of trees i n elevated l o c a t i o n s i n New South Wales, Australia., (Cox, 1888). The a d d i t i o n a l p r e c i p i t a t i o n of water to the ground by vegetal surfaces has been termed "occult p r e c i p i t a t i o n " by Descombes (1922/23), and A u b r e v i l l e (19^9), although c e r t a i n writers (such as Kerfoot, 1968) 1 119 have r e s t r i c t e d the term just to dew or rime. Suring (1915) termed the phenomenon "horizontal p r e c i p i t a t i o n " , and Rubner (1932) c a l l e d i t simply "fog p r e c i p i t a t i o n " . The term now used most generally i s that suggested by Kittredge ( 1 9 4 8 ) : "negative interception' 1^ since the water i s p r e c i p i t a t e d i n the same location(under vegetation) where "p o s i t i v e i n t e r c e p t i o n " would normally reduce the water contributed. Here, the term "negative i n t e r c e p t i o n " w i l l be used f o r the catching of water from the fog, and "fog d r i p " f o r the dripping of the water to the s o i l a f t e r i n t e r c e p t i o n . Techniques and r e s u l t s . The most commonly used technique to measure fog d r i p i s simply the pl a c i n g of some form of r a i n gauge under the plant or tree and the comparison of the amount i t c o l l e c t s with the amounts c o l l e c t e d by s i m i l a r gauges i n the open. Oberlander (1956) was impressed by the apparently large amount of negative i n t e r c e p t i o n of \irater from summer sea fogs by trees on the San Francisco Peninsula of C a l i f o r n i a . In an attempt to measure t h i s water source, Oberlander used f i v e t o t a l i z e r gauges (storage gauges) of f i v e g a l l o n capacity, and placed them under various tree, types along a three mile s t r e t c h of the C a h i l l Ridge. The gauges were l e f t i n p o s i t i o n f o r 39 days i n the summer of 1951. The r e s u l t s are shown i n tabular form overleaf. (Table 21 ). Considerable v a r i a t i o n was found i n the amounts of water caught. These r e s u l t s suggest that fog p r e c i p i t a t i o n i s a v a r i a b l e phenomenon, increas i n g with the exposure of the tree, but s t i l l varying considerably as a r e s u l t of the 120 S p e c i e s h e i g h t exposure i n c h e s i n gauge Sequoia sempervirens 200' i n f o r e s t 1.8 L i t h o c a r p u s d e n s i f l o r u s 20' d i r e c t 58.8 Pseudotsuga t a x i f o l i a 125 1 p a r t l y p r o t . by mt. 7.2 Pseudotsuga t a x i f o l i a 125 1 d i r e c t 8.9 Pseudotsuga t a x i f o l i a 125 1 d i r e c t 17.1 T a b l e 21 . Fog d r i p measurements on the San F r a n c i s c o P e n i n s u l a , f i v e week p e r i o d . ( A f t e r Oberlander 1956.) s p a t i a l non - c o n t i g u i t y o f the f o g . The s m a l l amount o f f o g p r e c i p i t a t i o n r e c o r d e d w i t h i n the f o r e s t (Sequoia  sempervirens) i s a r e s u l t d i r e c t l y c o n t r a r y to the Japanese work on f o g p r e c i p i t a t i o n i n the f o r e s t . ( H o r i , 1953; see p.126) The r e s u l t from the gauge under the tan oak L i t h o c a r p u s d e n s i f l o r u s s t r o n g l y suggests some s o r t o f sampling e r r o r a r i s i n g from the p o s i t i o n i n g o f the gauge under the canopy; the 58.8 i n c h accumulation would r e p r e s e n t a h i g h e r p r e c i p i t a t i o n f o r a month than found i n the open f o r the e n t i r e r e g i o n . Oberlander u n f o r t u n a t e l y o m i t t e d to measure the r a i n f a l l i n the open d u r i n g the same p e r i o d ( a l t h o u g h t h i s was p r o b a b l y z e r o ) , and t h i s i t s e l f d e t r a c t s from the v a l i d i t y o f h i s r e s u l t s . The foggy San F r a n c i s c o Bay a r e a a l s o s t i m u l a t e d the biogeographer James J . Parsons to experiment w i t h the 121 f o g d r i p phenomenon. ( P a r s o n s , i 9 6 0 ) He p l a c e d a s t a n d a r d 8 i n c h r a i n g a u g e under and s l i g h t l y t o the l e e o f a 100 f t . h i g h M onterey P i n e , ( P i n u s r a d i a t a ) , a dense - f o l i a g e d , c o n i c a l shaped t r e e , f r om w h i c h th e l o w e s t ' b r a n c h e s had been pruned. ( T h i s was presumably i n an a t t e m p t t o m i n i m i z e any s a m p l i n g e r r o r s s u c h as may have been committed by O b e r l a n d e r . ( 1 9 5 6 , op. c i t . ) D u r i n g the whole summer o f 1955, a t o t a l o f 9.84 i n c h e s o f f o g d r i p was r e c o r d e d , a l t h o u g h no r a i n f e l l i n the open. From " e x t r a p o l a t i o n o f i n c o m p l e t e d a t a , " f o r 1954, 1957» and 1958, P a r s o n s judged t h a t the average a n n u a l f o g d r i p i n t h e summers o f t h e s e f o u r y e a r s was a p p r o x i m a t e l y 10.00 i n c h e s . S i n c e t h i s f i g u r e i s the e q u i v a l e n t o f n e a r l y h a l f the average a n n u a l p r e c i p i t a t i o n f o r t h e Bay a r e a , t h e i m p o r t a n c e o f f o g d r i p f o r t r e e g r o w t h i n t h i s a r e a may w e l l be c o n s i d e r a b l e . A n o t h e r a p p r o a c h t o the measurement o f f o g d r i p has been t r i e d s e v e r a l t i m e s . T h i s c o n s i s t s o f b u i l d i n g a w a t e r s h e d and c o l l e c t o r a round the base o f a t r e e i n o r d e r t o c o l l e c t a l l w a t e r d r i p p i n g f r o m i t , and t h u s m i n i m i z e s a m p l i n g e r r o r s . . C o s t i n and Wimbush (1961) u s e d t h i s p r o c e d u r e i n c o n j u n c t i o n w i t h n o r m a l gauges i n an i n v e s t i g a t i o n o f r a i n and f o g i n t e r c e p t i o n i n the A u s t r a l i a n A l p s . U t i l i z i n g t h i s t e c h n i q u e , t h e y p r o d u c e d e v i d e n c e o f i n c r e a s i n g d r i p w i t h e l e v a t i o n , (see p. 122) I n H a w a i i , E k e r n (1964) u s e d a g a l v a n i z e d r o o f i n g w a t e r s h e d 20 x 36 f t . under A r a u c a r i a e x c e l s a Lamb. R. B r . a t 2750 1 on L a n a i h a l e , on L a n a i , H a w a i i . C o l l e c t i o n o f w a t e r by t h e shed was " i n r e a s o n a b l e a c c o r d " w i t h t h a t by gauges under o t h e r t r e e s . F o r summer p e r i o d s i n 1956 and 1957, the gauges measured 16.19 i n c h e s , t h e w a t e r s h e d r e c o r d e d 15.52 i n c h e s , b u t t h e r a i n f a l l i n 122 these same periods was only 1.16 inches. Average amounts of water c o l l e c t e d i n gauges beneath a tree were 391 inches per year, when 1 4 9 inches was recorded i n the open. The area of the h o r i z o n t a l p r o j e c t i o n of the tree was almost i d e n t i c a l with the 300 square feet area of the v e r t i c a l p r o j e c t i o n of the tree crown. I f i t i s assumed that the tree also c o l l e c t e d a l l the water that would normally have c o l l e c t e d i n the r a i n shadow created i n the lee of the tree, some 50 inches of the annual gain of 80 inches can be assigned as extra r a i n f a l l . A net increase of 30 inches i s thus suggested as the average annual gain of water by negative i n t e r c e p t i o n on the tree. (Ekern, 1964; Carlson, 1961) The e f f e c t of elevation above sea l e v e l . Windspeeds tend to increase with elevation due to the l e s s e r f r i c t i o n a l drag. Several experimenters have confirmed that as a r e s u l t , fog d r i p may also increase i n amount. Siccama (1968) found that at higher elevations i n the Green Mountains of Vermont, U.S.A., gauges under a f o r e s t canopy of Picea rubens, Abies  balsamea and Betula c o r d i f o l i a c o l l e c t e d more water than at lower l e v e l s . Windspeed also increased with height, leading Siccama to a t t r i b u t e the water increase to increased volume of water being blown through the canopy and p r e c i p i t a t e d as fog d r i p . The study by Costin and Wimbush ( 1 9 6 1 ) , mentioned previously, showed that, i n the A u s t r a l i a n Alps, water added by fog d r i p increased with elevation. In catchments between 4,000 and 5»000 feet i n elevation, timbered areas were found to c o l l e c t at l e a s t 1 - 2 inches; of water i n ad d i t i o n to the r a i n f a l l per year, and 2 - 5 inches above 5,000 feet. 123 Isaac (1946) used 27 r a i n gauges i n the Cascade Head Experimental Forest of Oregon to catch fog d r i p , i n coniferous f o r e s t . Gauges were placed i n p a i r s , i n the open and under the f o r e s t canopy. Comparisons of the d i f f e r e n t l o c a tions showed that the water reaching the ground under the trees increased with elevation, and also with decreasing distance from the sea. His data f o r annual p r e c i p i t a t i o n , which, of course, includes r a i n as well as negative i n t e r c e p t i o n from fog, are shown i n Table. 22 , below. Distance from sea. Height Ridge top V a l l e y 5 miles 82.3"(in open) 73.8 "(under tree) 77.6"fin open) 53.0 "(under tree) Less- than 2 miles. 78.6 " ( i n open) 99»l"(under tree) 65 . 9 "(in open) 5 3.1 *(under tree) Table. 22 . Annual r a i n f a l l and negative i n t e r c e p t i o n at selected l o c a t i o n s i n the Cascade Head Experimental Forest, Oregon. (After Isaac, 1946.) Parsons ( i 9 6 0 ) , working i n the Berkeley H i l l s of the San Francisco Bay area, reached the conclusion that fog d r i p was " p e c u l i a r l y a h i l l crest phenomenon." He considered that the amount of the water co n t r i b u t i o n to the ground i s a function of the s i z e , shape, and nature of the trees as well as the wind v e l o c i t y . From 124 experiments^ i n the summer foggy season, he found that the fog d r i p under pine trees (Pinus radiata) was t y p i c a l l y 0.02 to 0.05 inches when the a i r was calm. When there was a westerly wind of 10 - 15 mph, however, he found that the d r i p was much greater, with 0.20 to 0.30 inch.es> often being recorded. He noted that the maximum amounts of d r i p wbrenassociated with the highest wind speeds. The trees on the Berkeley H i l l s of the San Francisco Bay area, where these measurements were taken, are a l l the r e s u l t of a f f o r e s t a t i o n within the present century. There was probably l i t t l e condensation of fog water on the low perennial grasses that existed before the tree planting, but once trees have been a r t i f i c i a l l y established, the fog dr i p they produce might: well help to perpetuate them. The apparent r e l a t i o n s h i p s between fog occurrence and wind speeds have already been noted, i n a s l i g h t l y d i f f e r e n t context. (Ch. 3, p. 37 ) I t perhaps should be mentioned here that although fog water catch may be increased with windspeed, t h i s assumes that the f o r e s t or other plant surfaces are dense enough and continuous enough to prevent the fog (ffroplets d i v e r t i n g around the vegetal surfaces and not impacting on to them:, (see Ch. 7) Costin and Wimbush (1961) found that the strongest winds were not always the most e f f e c t i v e i n causing fog d r i p . They measured d r i p from the Snow Gum, and since t h i s species has a f a i r l y open f o l i a g e , Costin and Wimbush considered that a large proportion of the fog and cloud droplets were blown h o r i z o n t a l l y through, or around, the crowns. Experimental work on t h i s f a c t o r w i l l be ref e r r e d to l a t e r . (Ch. 7.) 125 E f f e c t of elevation of vegetation. i Wind speed i s known also to increase with height above the ground surface, as dislxnet from height above sea l e v e l . This i s to be expected, since f r i c t i o n with the land surface decreases with height. In addition, turbulence created by a rough or uneven ground surface may l o c a l l y cause eddies of wind speeds equalling those at greater heights. However, i f t h i s i s a f a c t o r , most experimental evidence appears not to have shown i t , p o s s i b l y due to the l o c a l i z e d nature of such experiments. Work i n Hokkaido, Japan, by Kuroiwa and K i n o s i t a (1953) has shown that the actual l i q u i d water content of fog decreases near the ground, which they ascribed to turbulent impaction of fog droplets on to the ground, (see Ch. 4.). Many experiments, 'for instance '•  the one described on p. 81, have, however, found that the amounts of water p r e c i p i t a t e d from fog d i r e c t l y into normal raingauges placed on the ground are quite small, r a r e l y exceeding 0.01 inch i n 6 hours. Thus i t seems that the volume of fog water p r e c i p i t a t e d i n this?way may be small. There are undoubtedly some u n i d e n t i f i e d f a c t o r s at work here. Within the f o r e s t Yosida (1953) c a r r i e d out a t h e o r e t i c a l study of the d i s t r i b u t i o n of fog density. On the assumption that the v e r t i c a l motion of a fog p a r t i c l e caused by turbulence of a i r i n the f o r e s t i s the same as a one dimensional "random walk", fog density was found to decrease exponentially downwards towards the ground. The basis of the "random walk" assumption i s not stated, however, despite the f a c t that the e n t i r e v a l i d i t y of the r e s u l t depends on i t s acceptance. In a 'real world' f o r e s t , some of the u n i d e n t i f i e d f a c t o r s a f f e c t i n g fog density with height have been 126 i l l u s t r a t e d by the work of Ekern ( 1 9 6 4 ) , i n Hawaii. Ekern used various types of fog c o l l e c t o r s to measure negative i n t e r c e p t i o n within a stand of Araucaria excelsa Lamb. R.Br, at 2750 feet on Lanaihale Mountain. He found that there was an increase i n negative i n t e r c e p t i o n with elevation greater than that accounted f o r by increase of wind with height. For a period of 35 days i n June and July 1958, the catch at the 30 f t . l e v e l was 17.55 inches, as compared with 4.88 inches at the 6 f t . l e v e l . These difcfferences could r e f l e c t differences i n c o l l e c t i o n e f f i c i e n c i e s of gauges at various heights, but Ekern gives no i n d i c a t i o n of t h i s p o s s i b i l i t y . R a i n f a l l during the same period was 1.88 inches. Windspeed at a height of 30 f t . was approximately double that at 6 f t . This experiment suggests that cloud was s t i l l impacted on tree tops even when water f a i l e d to reach the ground surface. Turbulent impaction on to the ground surface was apparently absent. t Depth of penetration of fog into a f o r e s t . I t has been observed that the fog d r i p from both for e s t s (Oura, 1953) and i s o l a t e d trees (Kittredge, 1948; Ekern, 1964) i s greater on the windward side than on the leeward. I t has been claimed that fog d r i p i s e s s e n t i a l l y an edge e f f e c t , and does not penetrate into most fore s t s ( K i t t r e d g e , 1948). Geiger (1950) c a l l e d fog d r i p "a true t r a n s i t i o n phenomenon which occursonly on the borders of a stand;" he l a t e r conceded that the e f f e c t was not only confined to the f o r e s t edge (Geiger, 1 9 6 5 ) , on the basis of Japanese work (Hori, 1953-). There seems l i t t l e doubt that fog d r i p i s most important on f o r e s t edges, but capture of fog p a r t i c l e s 127 does; o c c u r i n t h e i n t e r i o r o f f o r e s t s . L i n k e (l92l), w o r k i n g i n t h e T a u n u s M o u n t a i n s o f Germany, where t h e r e a r e a b o u t 200 f o g g y d a y s p e r y e a r , f o u n d a n a v e r a g e i n c r e a s e i n gauge c a t c h n e a r t h e f o r e s t edge o f 157 p e r c e n t o f t h a t o f g a u g e s i n t h e open; t h i s was s t i l l a t 123; p e r c e n t i n t h e f o r e s t i n t e r i o r , h o w e v e r . Maximum i n c r e a s e s , ( w h i c h may be s u b j e c t t o s a m p l i n g e r r o r s , ) were 300 a n d 260 p e r c e n t , r e s p e c t i v e l y . The J a p a n e s e work r e f e r r e d t o e a r l i e r (p.120) , p a r t i c u l a r l y t h e work o f O u r a (1953), showed t h a t w h i l s t t h e f r o n t s u r f a c e o f a f o r e s t o f P i c e a g l e h n i was a b o u t t h r e e t i m e s a s e f f e c t i v e i n i n d u c i n g f o g d r i p t h a n t h e t o p s u r f a c e , t h e t r e e c rowns c r e a t e d e n o u g h downward t u r b u l e n t m o t i o n t o a l l o w p e n e t r a t i o n o f f o g i n t o t h e i n t e r i o r o f t h e f o r e s t . I n t h e f o r e s t i n t e r i o r , 0.5 mmi o f f o g d r i p p e r h o u r was m e a s u r e d , when t h e w i n d s p e e d was 4 m. s e c 1 . , and t h e w a t e r c o n t e n t o f t h e -3 f o g 800 mg.m . T h i s was b e t w e e n s i x and t e n t i m e s a s much a s t h a t d e p o s i t e d on t o g r a s s s u r f a c e s u n d e r t h e same c o n d i t i o n s . F u r t h e r J a p a n e s e work ( H u z i o k a e t a l . , 1953) showed t h a t - t h e mean f o g w a t e r c o n t e n t i n t h e f o r e s t c r o w n s c o u l d be as much a s 80 p e r c e n t o f t h a t a b o v e t h e f o r e s t , w h i c h was i t s e l f e q u a l t o t h e f o g w a t e r c o n t e n t o u t s i d e t h e f o r e s t . T u r b u l e n c e a t t h e cr o w n -a t m o s p h e r e i n t e r f a c e was c o n s i d e r e d r e s p o n s i b l e f o r t h i s ; r e s u l t . B o t h o f t h e s e J a p a n e s e r e s u l t s i n d i c a t e t h a t , a t l e a s t i n some s i t u a t i o n s , a l t h o u g h f o g p r e c i p i t a t i o n i s g r e a t e r a t t h e f o r e s t edge t h a n i n t h e f o r e s t i n t e r i o r , i n t h e l a t t e r i t i s s t i l l b y no means i n s i g n i f i c a n t . I t i s c l e a r t h a t i n a s s e s s i n g t h e e f f e c t s on t h e w a t e r b a l a n c e o f a f f o r e s t a t i o n , a t l e a s t some a c c o u n t s h o u l d be t a k e n o f t h e r o l e o f n e g a t i v e i n t e r c e p t i o n a n d 128 fog d r i p . There i s a need too, f o r further research on the importance of fog d r i p on vegetation other than trees, whose vegetal surfaces presented to the fog are comparatively small. In the next chapter, some e f f o r t s at micro - experimentation w i l l be made. 129 CHAPTER SEVEN FORM, WATER CATCH, AND FOG DRIP. Scales of consideration. The morphology of vegetation i s important at three scales; i n i n f l u e n c i n g the amount of water which may be caught and direc t e d to the ground. The f i r s t of these scales i s that of the community: the nature of the edge and top of the group of plants, such as a fo r e s t stand, and the density of the penetrative b a r r i e r exposed to the fog laden a i r . Thus f o r trees at a f o r e s t edge, fo r instance, there may be1 an optimum' spacing depending on the maturity and type of tree. At t h i s scale, the r e f l e c t i v i t y of the vegetation may have some influence; f o r e s t s u s u a l l y have a lower albedo than short vegetation and may create convective updrafts which prevent fog from penetrating into* the canopy. At the same time enough heat w i l l normally be retained within the stand to prevent cooling of the a i r below i t s dewpoint, with the formation of r a d i a t i o n fog. The roughness of the crowns of trees i n a fo r e s t allows enough turbulence to be set up to allow penetration and impaction of fog droplets; on the tree f o l i a g e even within the f o r e s t (see p. 1 2 7 ) . The second scale of i n v e s t i g a t i o n of the fog dr i p problem' as r e l a t e d to vegetal morphology i s that of the i n d i v i d u a l plant. Important here, of course, i s the actual area of a plant presented to the fog, since t h i s is^ by f a r the most s i g n i f i c a n t influence on the water extr a c t i o n amount (see p. l 4 l ) . The surface at 90* 130 to the wind d i r e c t i o n may be expected to show a r e l a t i o n s h i p to the quantity of water extracted. This may be measured by the area of the h o r i z o n t a l p r o j e c t i o n . The fact, that deciduous trees shed t h e i r leaves i n winter means that f o r a large: portion of the year a l e s s e f f e c t i v e c o l l e c t i n g area of f o l i a g e i s presented to any fog;. On these grounds alone conifers may be considered to be more e f f i c i e n t i n d e l i v e r i n g extra water to the ground, at l e a s t on an annual basis;. Another f a c t o r which has to be considered i s the r i g i d i t y of the species involved , since decrease i n r i g i d i t y w i l l be expected to increase shaking with a given wind speed or gusting f a c t o r , and may increase d r i p to the ground. Also worth consideration i s what i s here termed the "multiple i n t e r c e p t i o n .". Water caught by the tree crowns-, which are i n the p o s i t i o n to catch most water from the fog, since the l i q u i d water content of fog increases with height, (see p. 125) has a greater p r o b a b i l i t y of reaching the ground without being re - intercepted and evaporated, the fewer, the intervening branches between crowns and ground. This p o t e n t i a l "multiple i n t e r c e p t i o n " surface could be measured by v e r t i c a l p r o j e c t i o n of the branches and leaves to the ground from a s p e c i f i e d height. The importance of t h i s f a c t o r has been shown experimentally by Oura,(l953) (see: below). At the t h i r d scale of i n v e s t i g a t i o n , the e f f e c t of l e a f shape, o r i e n t a t i o n , and surface c h a r a c t e r i s t i c s , must be considered. At t h i s scale the factors encouraging d r i p are those not favouring r e t e n t i o n f o r possible absorption. The e f f e c t of h a i r s on leaves i n r e t a i n i n g water droplets, and the e f f e c t of the nature of the c u t i c l e , have already been discussed-. (Ch. 5 ) . Needlelike leaves may be an adaptation to "condense water from fog" i n some a r i d and semi - a r i d areas (Went, 1955). 131 Adaptation or not, needlelike leaves, have been observed to be associated with greater d r i p than broad leaved s p e c i e s ; (Went, 1955; Carlson, 1 9 6 l . ) . Oura (1953) found that the most e f f e c t i v e type of f o r e s t f o r "capturing" fog p a r t i c l e s was one grown comparatively sparsely (spacing f a c t o r , scale one), with needlelike leaves, ( l e a f shape f a c t o r , scale three), and with no lower branches ("multiple i n t e r c e p t i o n " f a c t o r y , s c a l e two). THE FOG SIMULATOR. I t i s d i f f i c u l t to study a l l scales of the factors co n t r i b u t i n g to fog d r i p from vegetation simultaneously. Accordingly, one of the l i t t l e studied f a c t o r s was selected f o r more intensive study. This was l e a f shape. In an attempt to discover what l e a f shape was the most e f f i c i e n t i n catching water and d i r e c t i n g i t to the ground, a "fog simulator" was b u i l t i n which leaves of c o n t r o l l e d area and shape could be exposed (see Plate 7, overleaf ). The construction and operation of t h i s apparatus w i l l now be considered. Intended to be a prototype:, the apparatus was constructed to produce a h o r i z o n t a l flow of t i n y droplets of water, of a diameter (20 - 30 y-) comparable to those of natural fogs (see Ch. 2), Some d i v e r s i o n w i l l be made to consider the techniques of producing fog - l i k e diameter water droplets. Commercial fogging nozzles are av a i l a b l e , and have been used by Stone (1957) and Breazeale et a l . (1950, 195l) , i n t h e i r experiments on absorption (see Ch. 5). In these studies the diameter of the water droplets was umimportant, so that noi reference to droplet s i z e was made. Janes (195*0 used a spray nozzle f i x e d so that i t . was s l i g h t l y submerged under water. This; 132 P l a t e 7. The f o g s i m u l a t o r . 1 3 3 was t r i e d in- the present study, but; d i d not produce a s a t i s f a c t o r y r e s u l t f o r the present purpose. The main problem; was to produce a spray from which droplets with a diameter greater than 5 0 had been eliminated. This was eventually achieved, a f t e r some- experimentation. F i r s t l y , the hole i n the spray nozzle, f i t t e d on to the normal piped water supply, was made extremely small, and thuss had to be checked frequently to ensure that i t had not been enlarged by the water f o r c i n g through i t . Although the hole i n the nozzle was made i n aluminum sheet, no problem of hole enlargement was encountered. Secondly, any large droplets were dealt with by making the distance, from where the spray was; produced to where the leaves were exposed (see Plates; 8 and 9 , p. 1 3 5 ) long enough ( 5 ' 6 " approx.) to ensure that a l l the l a r g e r droplets; i n i t i a l l y produced f e l l out under the influence of gra v i t y . A fan was placed behind the spray nozzle to blow the water droplets forward. A resistance was; placed i n the e l e c t r i c i t y supply to slow i t down to the point where the droplets* l a r g e r than about 5 0 /*- diameter could not be c a r r i e d by the a i r current produced (see F i g . 1 2 , p. 8 0 ) , "Wind" speed was; approximately 0 . 5 m. s e c " 1 . ( l mph) at the l e a f . The; size of the droplets; passing or impacting on to the exposed leaves was checked by sampling with microscope s l i d e s coated with a Vaseline and o i l mixture, and viewed \\rith a c a l i b r a t e d eyepiece; samples taken from several natural fogs were compared with the droplets produced by the fog simulator. This sampling technique was that followed and recommended by Fuchs and Petrjanoff ( 1 9 3 7 ) 5 any sampling bias i s towards the l a r g e r droplets. The droplets s t r i k i n g the p o s i t i o n i n which the leaves were exposed were i n no case observed to be greater than 5 0y* i n diameter^ and appeared, a f t e r 134 experimentation,, to have the correct modal frequency of about 25 - 30 /K . Thus i t i s considered that the spray a c t u a l l y reaching the ' l e a f surfaces may be equated with natural fog droplets, since natural fogs have an approximately s i m i l a r droplet s i z e d i s t r i b u t i o n (see Ch. 2 ) . A return a i r duct was included i n the apparatus, so that the a i r , at or almost at, 100 per cent r e l a t i v e humidity, was recycled under the influence of the fan. This was necessary to maintain the r e l a t i v e humidity . near 100 per cent, since the r e l a t i v e humidity i n the laboratory where the apparatus was situated was normally between 40 and 50 per cent. Relative humidity could not be prevented from f l u c t u a t i n g s l i g h t l y , but was always i n excess of 95 per cent; thus evaporation from the " l e a f " surfaces was assumed to be zero. Since the temperature v a r i e d only between 6l° F and 71° E, changes i n v i s c o s i t y could be considered i n s i g n i f i c a n t i n the present study. Flow from the spray nozzle varied from1 38.0 ml. min. 1 to 42.5 ml. min. 1 , unavoidable due to v a r i a t i o n s i n tap water supply.' I t i s stressed that l i t t l e of t h i s amount a c t u a l l y flowed past the l e a f exposure p o s i t i o n , but flowed back to an ou t l e t under the fan; the f l o o r of the apparatus was sloped f o r this- purpose. Experiments. Despite the cru d i t y of the apparatus, i t i s considered that the following r e s u l t s do have at l e a s t some general v a l i d i t y . Aluminum; shapes, representing leaves. (but a l l having the same surface c h a r a c t e r i s t i c s so f a r as the rete n t i o n of water was concerned) were exposed, a l l f o r three hours each, In the fog simulator over one of the three; c o l l e c t i o n beakers i n s e t i n t o the f l o o r . (see plates 8 P l a t e 9. Technique o f a r t i f i c i a l l e a f exposure. 136 and 9, P« 135",). The shapes were suspended on t h i n wire so as not to present any obstruction to a i r and droplet flow other than that of the shape i t s e l f . The b a f f l e placed i n front of the c o l l e c t i o n beakers was to ensure that the water droplets d i d not flow d i r e c t l y into themv-(see Plate 9, p. 135). Any "lee wave" capturing e f f e c t would be equal f o r a l l beakers. Droplet flow over the leaves was observed i n front of a black surface, and found to be g i v i n g a f a i r l y even impaction of droplets a r e a l l y over them. The shapes of the aluminum leaves exposed are shown i n F i g . 17, (p. 137 ). C i r c l e s were also made with i d e n t i c a l areas to the: shapes, v/ith the idea of providing a form of "control shape". The r e s u l t s of the exposure of the c i r c l e s w i l l be considered f i r s t . The water intercepted i n three: hours i s pl o t t e d against c i r c l e areas on a graph i n F i g . 18, p. 138, together with a l e a s t squares regression l i n e . From t h i s graph i t i s apparent that the water intercepted increases i n amount as the area of the catching surface also increases. The r e l a t i v e v a r i a b i l i t y of each set of experiments with an i n d i v i d u a l experiment seems to increase with area up to the la r g e s t area considered, 38.2 sq. cms. (Larger sizes were not exposed since evenness of exposure to the fog spray could not be assured ). In f a c t , the v a r i a b i l i t y of the amounts caught by the c i r c l e of 38.2. sq. cms. (6.39 - 7.70 grams) may be due to increas i n g s p a t i a l d i f ferences i n the flow of water droplets within the simulator. The c o r r e l a t i o n of water intercepted with the c i r c l e areas was r = + 0.9306 , ( s i g n i f i c a n t at the 0.1 per cent l e v e l , ) . I f the amount of water intercepted i s calculated on a "per square centimetre" basis, l e s s order i s evident (see Table 23, <pJ-39). The c o r r e l a t i o n of area with the amount of water intercepted per square centimetre was 137 F i g . 1 7 . Shapes of aluminum 'leaves' used i n experiments. ~ (see text for explanation. ) ' ~" F i g . Iff . Area and d r i p from aluminum c i r c l e s , (see text f o r explanation.) 139 r = + 0.3994 (not s i g n i f i c a n t a t the 0.5 per cent l e v e l s ) . I t may thus t e n t a t i v e l y be s t a t e d t h a t the maximal e f f i c i e n c y i s reached w i t h i n the range 15 to 25 sq. cms.; i t may w i t h more c o n f i d e n c e be s t a t e d t h a t as ar e a i n c r e a s e s , so does the t o t a l amount o f water i n t e r c e p t e d . A r e a o f c i r c l e 38.2 24.5 ( s q . cms) 21.0 17.9 13.2 10.0 5.0 Water e x t r a c t e d 0.18 0.26 per sq. cm. 0.25 0.23 0.14 0.13 0.18 T a b l e 23. Mean amounts o f water (grams) e x t r a c t e d by c i r c l e s The s e t o f n i n e d i f f e r e n t shapes was a l s o exposed i n the f o g s i m u l a t o r f o r t h r e e hour p e r i o d s . The mean amount o f d r i p i n t h a t time (means are d e r i v e d from J to 4 experiments per shape) i s shown I n F i g . 19, p. l 4 o . I t i s immediately e v i d e n t t h a t the shapes have iri; a l l cases e x t r a c t e d more than the c i r c l e s o f c o r r e s p o n d i n g a r e a . C o r r e l a t i o n o f a r e a w i t h amount caught was r = 0.9181 ;, ( s i g n i f i c a n t a t the 0.01 per cent l e v e l ). I n terms o f the amount o f water i n t e r c e p t e d per square centimetre., the shapes 5 and 6 (see T a b l e 24, p. l 4 l , and F i g . 17, p. 137) were the most e f f i c i e n t , 1 e x t r a c t i n g 0.371 and 0.364 grams per square c e n t i m e t r e , r e s p e c t i v e l y . T h i s may be due to the " d r i p t i p " p o s s e s s e d by these l e a v e s , f a c i l i t a t i n g the r u n o f f o f the water once i n t e r c e p t e d from the f o g . Mean amounts o f water e x t r a c t e d per square c e n t i m e t r e (means o f a minimum o f 3> experiments per l e a f ) are shown i n T a b l e . 24, p. 141. The main c o n c l u s i o n s t h a t may be drawn from these experiments are t h a t , a t the windspeed c o n s i d e r e d , g r a m s of w a t e r d r i p p i n g in 3 hrs. 0 =M£HN OF 3oA4 £XP£« lhE*T5 . F i g . 11 • A r e a a n d d r i p f r o m a l u m i n u m l e a f s h a p e s ' , ( s e e t e x t f o r e x p l a n a t i o n . ) Ikl (0.5 n*. sec" . ) : 1. Area has an overriding Influence on the t o t a l amount of water caught. 2. There i s a decline i n the per area ex t r a c t i o n amounts, with sing l e leaves, above about 25 sq. cms. 3. With single leaves, the most e f f i c i e n t shape and size combination i s a. f a i r l y elongated type of l e a f , or one with a d r i p t i p , of about 20 to 2k sq. cms. i n area. Shape number 1 2 3 4 5 6 7 8 9 Amount caught 0.25 0.31 0.33 0.33 0.37 0.36 0.27 0.17 0.17 Table 24. Mean amounts of water (grams) extracted by shapes. These experiments show some Interesting, but as yet inexplicable 1 trends, which require further i n v e s t i g a t i o n . However, the comparatively crude nature of the fog simulator described here, which was i n any case only intended to be' a prototype, suggests that a more sophisticated simulator w i l l be required, so that more control may be exercised over the conditions^ i n which the l e a f may be exposed. I t i s not known whether the comparatively small size of t h i s simulator influenced the r e s u l t s ; although t h i s seems u n l i k e l y , any r e p e t i t i o n of t h i s simulator should be large, to eliminate any possible edge e f f e c t s . Exposure to a range of wind speeds may throw more l i g h t on the problem. The use of d i f f e r e n t i a l windspeeds would require a source of correct size water droplets not dependent on the wind speed; here the wind speed had to be kept constant to ensure that only the 142 correct s i z e droplets reached the l e a f surfaces. However although a large simulator xirould undoubtedly be expensive i t may prove worthwhile i n pointing to the best types of plant morphologies able to e x p l o i t foggy but otherwise a r i d zones, such as the coastal fog deserts of South America. 143 INTERCEPTION PROM NATURAL FOGS. I t i s not d i f f i c u l t to demonstrate that water i s caught by r e a l vegetation from fog, but there seems to be l i t t l e r e l a t i o n s h i p of amount of water caught to weight of plant, the most e a s i l y measurable parameters. On the night of 31. 12. 69. - 1. 1. 70., a thick advection fog covered the southern h a l f of Sea Island, and much of Lulu Island and the mouth of the main arm of the Fraser River, i n Richmond, B.C., extending f o r an unknown- distance out into the Georgia S t r a i t . Vancouver Int e r n a t i o n a l A i r p o r t , on the edge of this; lobe of fog, recorded fog conditions from 2000 PST on 31. 12. 69. to 1100 PST on 1. 1. 70., with v i s i b i l i t y s t i l l at \ mile as l a t e as 0800 PST on 1. 1. 70. Samples of Western Red Cedar (Thuja p l i c a t a Donn.), chosen simply because of a v a i l a b i l i t y , were cut and immediately sealed with Vaseline to prevent any moisture escape from the cut stems. These specimens were then taken to a point near the probable surface centre of the fog, at Steveston Highway Interchange with the Deas Island Freeway (B.C. 499) , approximately -| mile north of the north bank of the main channel of the Fraser River. The specimens were exposed i n various positions (see Table 25, overleaf) f o r one hour, and then sealed i n t o small metal cannisters f o r weighing, about ^ hour l a t e r . This technique i s an adaptation of the one used by C o s t i n and Wimbush (1961) to discover amounts of rime accumulating on trees. They cut small branches and sealed them into p l a s t i c bags, which were l a t e r weighed. A l l the specimens used here appeared to have increased i n weight s l i g h t l y a f t e r exposure. This i s considered s i g n i f i c a n t i n view of the short exposure time (see Table 25, o v e r l e a f ) . s 144 Mode of exposure height of exposure plant weight plant weight before a f t e r (grams) (grams) percentage increase 1 Tied to top of pole 6' 3.93 4.16 5.852 2 Tied on to bush 4' 4.52 4.62 2.655 3 Tied on to sign 2» 2.07 2.14 3.381 k Tied on to bush 5' 3.33 3.43 3.000 Estimated windspeed at s i t e 1.5 Relative humidity: more than 95 mph. Mean temp, per cent. 32°F. Table? 25. Relationships of fog water catch i n one hour to weight i n samples of Thuja p l i c a t a Donn. In an attempt to ensure that t h i s increase iiras not due to water from dew, a metal plate was exposed on the ground surface, and inspected a f t e r one hour. Any dew deposition would be expected to cover the plate uniformly; however, there were only several scattered t i n y droplets on the plate; these were almost c e r t a i n l y , f o r the reasons just stated, fog droplets. At Vancouver In t e r n a t i o n a l Airport, on the night of t h i s experiment, the conditions ivere s i m i l a r to those at the Steveston Highway Interchange; v i s i b i l i t y ranged frbmi l / 8 t o - ' m i l e s , the temperature was also 32°F, and the r e l a t i v e humidity 100 per cent. There was a 2 mph wind. The fog extraction device i n s t a l l e d at the a i r p o r t (see p. 75) caught water from the fog, but t h i s was not recorded u n t i l the next morning, when the temperature rose above freezing, and 0.03 inches was recorded- (see Ch. 4). The comparatively large amounts of water that may 145 be intercepted from fog but then held by the vegetation was: shown by an experiment c a r r i e d out i n fog at the U n i v e r s i t y of B r i t i s h Columbia, i n January, 1970. Small branches of several species were c a r e f u l l y cut and sealed i n t o p l a s t i c bags. (This was the technique, previously mentioned, used by Costin and Wimbush (:196l) to measure rime accumulation ). These were then weighed, the vegetal surfaces; completely dried, and re - weighed, subtracting the weight of the bag. The diff e r e n c e v/as the net water catch, \\rhich may be expressed as; a percentage of the surface dried sample weight. The r e s u l t s are shown> i n Table 26 , below. ! 1 2 3 Species; Height; at which cut surface d r i e d Specimen wt. (grams) Net water catch (grams) 2 I as per cent Sequoidendron gianteum Cunningham!a lanceolata Pinus s i l v e s t r i s var. pumila Pinus r i g i d a Ps eudo t suga menzeii I l e x opaca 5' 5' 5' 5' 6 ' 5' 3 6 . 1 6 12.23 2 0 . 3 3 1 8 . 1 1 21.64 5.54 22.00 21.76 15.33 9.47 10.73 9.27 60.84 77.92 75 .40 52.29 49.58 I 6 7 . 3 2 Table 26. Amounts of water caught from, a natural fog,  U.B.C., January, 1970. The length of time of exposure to the fog i n t h i s experiment i s not known, so that only l i m i t e d conclusions may be drawn from these r e s u l t s . I t i s c l e a r , however, 146 that considerable amounts of water may be held by tree f o l i a g e ; I l e x opaca i s the only broadleaf here ( a l l other species are c o n i f e r s ) , and i t seems l i k e l y that t h i s species normally holds the maximum amount of water f o r an equal weight of f o l i a g e . The l e a f surface of t h i s species i s not covered by any water r e t a i n i n g media, such as t i n y h a i r s , but i s shiny, with a thick c u t i c l e . The f a c t that such a large percentage of water i s held i s i n d i c a t i v e of the comparatively large area exposed to the fog, catching the most water (see p. l 4 l ) , but also of the e f f i c i e n c y of retention, which i s e s p e c i a l l y the case i f the l e a f i s not held v e r t i c a l l y . I t i s apparent that the d i f f i c u l t i e s of measuring fog water additions to varie d vegetal surfaces are great, and this- study concurs with Yosida and Kuroiwa ( 1 9 5 3 ) , when they state: " the d i r e c t measurement of the quantity of fog water captured (by the forest) i s hpoelessly d i f f i c u l t , though i t i s extremely desirable to know i t as. accurately as p o s s i b l e . " (Yosida and Kuroiwa, 1953> p. 2 6 l ) . 147 P R E L I M I N A R Y MODELS OF FOG D R I P . The f a c t o r s at a l l scales i n f l u e n c i n g the amounts of fog d r i p from any vegetation are complex, often i n t e r r e l a t e d , and very d i f f i c u l t to measure. As a r e s u l t , unless the problem i s considerably s i m p l i f i e d , i t i s extremely d i f f i c u l t to construct a model of fog d r i p . At t h i s stage, however, a general statement may be made, recognizing the complexity of the i n f l u e n c i n g f a c t o r s , at the scale of the i n d i v i d u a l . Amount of d r i p at t h i s scale is; at f i r s t a function of water intercepted, minus the amount of water retained, re - evaporated, or absorbed. Thus i t may be stated: D = f ( A , A , s , c, v, d ,; w , r, i , e, / t ) 1 t 1 m g m where: the f i r s t 6 expressions; are i n t e r c e p t i o n f a c t o r s : o A^ = Area of l e a f surfaces exposed to the fog at 90 to the wind d i r e c t i o n . A = Area of stems, trunks. s^ = A shape index, increasing v/ith elongation, and presence of d r i p t i p s . c = A •combing' f a c t o r , dependent on the spacings of leaves;, v = Wind v e l o c i t y . . d = modal droplet diameter of the fog. m the second k expressions are e s s e n t i a l l y d r i p f a c t o r s : w = a measure of wind gusting, and thus shaking, to S dislodge droplets. r = a r e t e n t i o n f a c t o r , dependent on l e a f surface characteristics;. i = multiple i n t e r c e p t i o n , (see p. 130) m e = evaporation (normally 2 0 i n fog). t - time. Ik8 Attempts to construct quantitative models of fog d r i p must take into account as many of these factors as i s f e a s i b l e , commensurate with the accuracy required. Only one known1 attempt has ever been made to a c t u a l l y pnovide an equation to ca l c u l a t e fog d r i p amounts. This was c a r r i e d out by Yosida and Kuroiwa ( 1 9 5 3 ) , as part of the Japanese studies: on "fog - preventing f o r e s t " (see Hori, Ed., 1953). The equation constructed by t h e o r e t i c a l physical techniques by Yosida and Kuroiwa assumes that a conifer tree (the only type considered) i s made up of a number of short c y l i n d e r s . The most d i f f i c u l t problem was the measurement of the e f f e c t i v e f ront area of the tree. I f i t i s assumed that an anemometer placed i n front of the tree measures the wind v e l o c i t y , ( t e c h n i c a l l y a vector quantity), then, i f a i r density i s included, the components of an equation given by Yosida and Kuroiwa for wind force on a coni f e r tree are obtained: f = i ( P • A t . C . v 2 ) where: v A P C t the e f f e c t i v e front area of the tree (m ) wind v e l o c i t y a i r density a drag c o e f f i c i e n t , which w i l l vary s l i g h t l y with the tree s i z e , but for pine needles c l o s e l y approximates the form C = 0.0920 + 0.2330v a calcu l a t e d l e a s t squares regression equation from; a graph given by Yosida and Kurdiiwa- (p« 268). lh9 The same authors have derived the equation f o r the amount of fog d r i p from a conifer, as follows: q = 0.35 .yo.22 . 2* . . (pC)~* . . «f . . f which simpli f i e s , to q =3.12 . 10 3 • • f • A t * • ^ grams se c " 1 , where: p = density of a i r & = density of water // = v i s c o s i t y of a i r r = radius of l e a f f = fog water content per u n i t volume of a i r C = drag c o e f f i c i e n t of c i r c u l a r c y l i n d e r of radius r A = t o t a l e f f e c t i v e front area of a l l the leaves t - . a = mean radius- of fog droplets v = wind v e l o c i t y This appears^ to agree quite well with empirical r e s u l t s derived from a cut specimen of Plcea glehni. I t can be seen from t h i s equation that as area presented to the fog increases, so does in t e r c e p t i o n ; as the fog droplet diameter increases, so does i n t e r c e p t i o n (due to increased momentum); and as l e a f radius (n.b. assumed to be c y l i n d r i c a l ) increases, the amount intercepted decreases, ceteris; paribus, since there i s a greater tendency to flow round the l e a f than to impact on to i t . I t i s c l e a r that the factors i n f l u e n c i n g fog impaction and d r i p from plants need to be measured very accurately, and u n t i l t h i s i s done i t w i l l not be possible to derive accurate estimates of t o t a l water con t r i b u t i o n . Further research may be upon these l i n e s . However, i t 150 must s u f f i c e here to diagrammatically summarize the ways, i n which fog water may come to be u t i l i z e d by plants. ( F i g . 20, p. 1 5 l ) Laboratory and f i e l d research i s needed on nearly a l l the p o t e n t i a l pathways of water use. 151 a i r c h a r a c t e r i s t i c s density u s u a l l y v i s c o s i t y constant fog c h a r a c t e r i s t i c s modal droplet diam. & concentration = ivater content wind v e l o c i t y air, + fog droplets <7 Plant c h a r a c t e r i s t i c s ; t o t a l e f f e c t i v e area presented to fog Evaporation ^  Retention Absorption (water stress) wind gusting and turbulence r i g i d i t y of plant dislodging impaction d i r e c t l y to ground (minor) F O G DRIP Replenish s o i l ^ moisture d e f i c i t — i To groundwater (minor) USE BY PLANT runoff {minor) F i g . 20. Pathways of fog water use by plants. 152 CHAPTER EIGHT CONCLUSION. Fog i s one of the most v a r i a b l e of meteorological phenomena, whether considered s p a t i a l l y or temporally. Since fog i s more s t a t i c and l e s s damaging than some other meteorological events, such as hurricanes, i t has not attracted, at l e a s t u n t i l f a i r l y recently, as much research as might be expected; that research that has, been c a r r i e d out has tended to concentrate on techniques of d i s s i p a t i n g fog, rather .than u t i l i z i n g i t more e f f i c i e n t l y . Fog i n a c i t y rush hour i s a d i f f e r e n t thing from fog i n the desert, and i t i s c l e a r that fog may only be considered a hazard r e l a t i v e to when and where i t occurs, and whom i t a f f e c t s . Control i n the c i t y may mean d i s s i p a t i o n , but control i n the desert may mean the f u n n e l l i n g of fog to areas where i t can be of use. Even i n temperate climates, the use of vegetation to control fog has been extended to i n f l u e n c i n g a i r drainage, and thereby c o n t r o l l i n g the d r i f t of shallow r a d i a t i o n fogs, as i n c e n t r a l Pennsylvania (Myers, 1967) The lessons l e a r n t can be applied i n a r i d areas. Craig (1968) observed that the "garua" (advection fog)of the Peruvian coast wets the surface to the extent of creating runoff, whilst raingauges record no p r e c i p i t a t i o n at a l l . Heavy nylon and wire nets are used In some areas to p r e c i p i t a t e water from fogs, as they are on parts of the Chilean coast. There seems l i t t l e doubt, however, that the f u l l p o t e n t i a l s of water a v a i l a b i l i t y from fog are not at present u t i l i z e d , and there i s scope fo r considerably more work on the types of vegetation most able to use i t . Economic and c u l t u r a l implications 153 also need to be considered. The r e l a t i o n s h i p between the physical requirements f o r the formation of fog and it s ; actual geographical • occurrence has been shown to be complex.; i t Is d i f f i c u l t to make generalizations as to i t s s p a t i a l occurrence. I t has been shown, however, that at present, for- the purposes of assessing the possible additions of water from fog, many errors can be made by taking the s p a t i a l d i s t r i b u t i o n of "fog - days" as equal to the d i s t r i b u t i o n of equal fog durations. The d i f f i c u l t i e s of measuring additions of water by "occult p r e c i p i t a t i o n " has been r e f e r r e d to*several times, both i n the construction of s p e c i a l i z e d instrumentation, and i n c a l c u l a t i n g or measuring catch or d r i p from actual vegetation. The unfortunately inconclusive r e s u l t s from the attempts at laboratory simulation of fog emphasize t h i s d i f f i c u l t y . Despite the many experiments which have shown that water can be absorbed through the a e r i a l parts of plants, and the coincidence of some large areas of the world where moisture stressed conditions; and fog often occur together, this; use of fog water i s often disregarded as i n s i g n i f i c a n t . I t has been shown that fog i s an under - estimated f a c t o r i n the evaluation of the water economy of many parts of the world. This i s p a r t i c u l a r l y the case i n fog - prevalent forested upland areas and i n foggy coastal deserts. In the present undernourished state of much of the world, i t i s c l e a r that any possible opportunity to increase food production, through opening up of water sources not at present u t i l i z e d , must be seized. I t i s contended that i n c e r t a i n parts of the world fog can provide t h i s water source. Research on the ways 154 i n which t h i s l a t e n t source can be tapped should be given high p r i o r i t y . 155 Appendix 1, S i g n i f i c a n c e Test. The s i g n i f i c a n c e of the differences of the means of measured parameters as between foggy and non -foggy days i n a l l eases; i n t h i s thesis; were assessed using the following t e s t s t a t i s t i c . This has, been modified to allow f o r the f a c t that the means of the foggy day parameters are from the entire population and those In, the non - foggy days, are samples;. The t e s t s t a t i s t i c used was where: "x£ = mean of sample (non - fog) = mean of population (fog) •_ 0" = standard deviation of sample n = Number of sample per month N = Population of sample The s i g n i f i c a n c e l e v e l of the c a l c u l a t e d -Z- value may be derived from the standardized normal d i s t r i b u t i o n tables. 156 A p p e n d i x I I . Noi* - p a r a m e t r i c t e s t f o r t r e n d . T he n o n - p a r a m e t r i c t e s t f o r t r e n d u s e d on p. 60 a n d p. 62 was d e r i v e d i n t h e f o l l o w i n g way (Mann, 19^5; T i n t n e r , 1952; K. D e n i k e , 1970, p e r s o n a l c o m m u n i c a t i o n ) : The p r o b a b i l i t i e s f o r t h e 12 y e a r s c o n s i d e r e d were r a n k e d p, » p„ , p , where 1 2 ' n n C 12 i f t h e r e a r e y e a r s w i t h e q u a l r a n k i n g , a n d 12 i f n o t . The number o f r a n k s o f e l e m e n t s l a r g e r t h a n p, ... p , i n t u r n , v/ere t h e n summed o v e r tfhe *± n e n t i r e r a n g e o f n, t o g i v e t h e sum o f t h e p o s i t i v e s c o r e s , P. T h u s a T h e n t h e t o t a l s c o r e , S, was d e r i v e d f r o m S = 2P - N ( N - l ) w h e r e : P i s t h e sum o f t h e p o s i t i v e s c o r e s , a n d N i s t h e number o f o b s e r v a t i o n ( y e a r s ) . The c a l c u l a t i o n o f t h e r a n k c o r r e l a t i o n c o e f f i c i e n t f o l l o w s : T = 2S N ( N - l ) The p r o b a b i l i t y o f a t r e n d NOT e x i s t i n g may t h e n be d e r i v e d f r o m t a b l e s g i v e n b y K e n d a l l (1955, p. 1 7 l ) i f N i s b e t w e e n k and 10. But. i n t h e p r o b l e m on p. 60 and p. 62, N = 12; 157 f o r l a r g e r N, I t i s known (Kendall, 1955) that the d i s t r i b u t i o n of S converges r a p i d l y to normality. Thus f o r N l a r g e r than 10 the quantity S i s d i s t r i b u t e d approximately normally with zero mean and variance: N(N-l)(2N + 5) 18 But the t o t a l score S i s discontinuous. Thus a co r r e c t i o n f o r c o n t i n u i t y may be made (Tintner, 1952) by subtracting 1 f o r p o s i t i v e S and adding 1 f o r negative S. This then allows use of the continuous normal d i s t r i b u t i o n tables f o r the d e r i v a t i o n of the p r o b a b i l i t y of NO trend.!.... e x i s t i n g . 158 BIBLIOGRAPHY Aitken, J . , (1888 - 1892:) Papers on the development and use of the dust counter, i n Knott, C.G., Collected S c i e n t i f i c Papers!, 1923. Camb. , Univ. Press, pp.591. Albrecht, F., ( l 9 3 l ) Theoretische untersuchungen uber die ablagerung von staum aus stromender l u f t und i h r e anwendung auf die theorie der. s t a u b f i l t e r . Phys. Z e i t s c h r . , 32, pp. 4 8 - 5 6 . Arens, K., (1934) Die kutikulare exkretion de laubblattes. Jahrbuch wiss. Botan., 80, pp. 248 - 300. Arnt, C.H., (1937) Water absorption i n the cotton plant as a f f e c t e d by s o i l and water temperatures. A u b r e v i l l e , A., (1949) Climats, forets et d e s e r t i f i c a t i o n de L'Afrique T r o p i c a l e . P a r i s , pp. 351. Bauer, B., (1963) A nexv method of measuring fog p r e c i p i t a t i o n . Mpntes, 19, pp. 323 - 325. Baumgariner, A., (1957) Zur hohenabhangigkeit von regen und nebelniederschlag am1 grossen Falkenstein. I n t e r n a t i o n a l Union. Geod. Geophys., Ass.  S c i . Hydrol., Toronto, 1957, Proceedings, pp. 529 - 534. Bonner, J . , (1959) Water transport. Science, 129, pp. kkj - 450. Bormann, F.H., (1957) Moisture transfer through intertwined root systems. Plant P h y s i o l . , 32, pp. 48 - 55. Boynton, D., (1954) N u t r i t i o n by f o l i a r a p p l i c a t i o n . Ann. Rev. Plant Physiol., 5, pp. 31 - 54. Breazeale, E.L., and McGeorge, W.T., (1953a) Exudation pressure i n roots of tomato plants under humid conditions. S o i l Science, 75» pp. 263 - 298. Breazeale, E.L, and McGeorge, W.T., (1953b) Influences of atmospheric humidity on root growth. S o i l Science, 76, pp. 3 6 l - 365. 159 Breazeale, E.L., McGeorge, W.T., and Breazeale, J.F., (1950) Moisture absorption by plants growing i n an atmosphere of high r e l a t i v e humidity. I Plant Physiol., 25, pp. 413 - 419. Breazeale', E.L. , McGeorge, W.T. , and Breazeale, J.F. , ( l 9 5 l ) Water absorption and t r a n s p i r a t i o n by leaves. S o i l Science, 72, pp. 239 - 244. B r i e r l e y , W.G., (1934) Absorption of water by the f o l i a g e of some' common f r u i t species;. Proc. Am. Soc.  Hort. S c i . , 32, pp. 277 - 283. B r i e r l e y , W.G., (1936) Further studies of the absorption of water by red raspberry f o l i a g e , and some evidence r e l a t i v e to the movement of water within the plant. Proc. Am. Soc. Hort. S c i . , 34, pp. 385 - 388. Buma, T.J., ( i 9 6 0 ) Fog at Leeuwarden, the Netherlands. Meteor. Mag. , 89, pp. 161 - 167. Byers, H.R., (1965a) Elements of cloud physics. Chicago, Univ. Press, pp.191. Byers, H.R., (1965b) General Meteorology. Ch. 20 : Fog. pp. 480 - 510. N.Y., McGraw H i l l , pp. 540. Carlson, N.K., (1961) Fog and lava rock, pines and pineapples. Amer. Forests, 67, pp. 8 - 11 and 58 - 59. Clements, F.E., and Martin, E.V., (1934) E f f e c t of s o i l temperature on t r a n s p i r a t i o n i n Helianthus annuus. Plant Physio1., 9» PP« 619 - 630. Cook, J.A., and Boynton, D., (1952) Some facto r s a f f e c t i n g the absorption of urea by Mcintosh apple leaves. Proc. Am. Soc. Hort. S c i . . 59, pp. 8 2 - 9 0 . Costin, A.B., and Wimbush, D.J., (1961) Studies i n the catchment' hydrology of the A u s t r a l i a n Alps, IV. Interception by trees of r a i n , cloud, and fog. Technical paper, Div. of Plant  Industry, Melbourne, No. 16. Court, A., and Gerston, R.D., (1966) Fog frequency i n the United States. Geogr. Rev., 56, pp. 543 -550. 160 Cox, G.H.,J(1888) In disc u s s i o n of paper by Abbot, W.E., on f o r e s t destruction i n New South Wales. J. Roy. Soc. N.S.W.. 22, pp. 11. Craig, A.K., (1968) Studies i n Marine Desert Ecology. The Paracas Papers, 1, No. 2, F l o r i d a A t l a n t i c U n i v e r s i t y . Darwin, F., and Pertz, D.F.M., ( l 9 1 l ) New method of estimating the aperture of stomata. Proc. Roy. S o c , Ser. B, 84, pp.136 - 154. Davis;, N.E. , ( l 9 5 l ) Fog at London A i r p o r t . Meteor! Mag. , 80, pp. 9 - 20. De Forest, H. , (1923) R a i n f a l l i n t e r c e p t i o n by plants: an experimental note. Ecology, 4, pp. 417 -419. Delf, E.M., (1916) Studies of protoplasmic permeability by measurement of rate of shrinkage of turgi d t i s s u e . Am. Botan., 30, pp. 283 -310. Descombes, P., (1922/23) Forests, r a i n s , and i n v i s i b l e Condensation. Ann. Soc. Met. France, 66, pp. 38 - 46. Dieckmann, A., (1931) Versuch zur niederschlagsraessung aus treibenden nebel. Meteorol. Zeitschr., 158, pp. 400 - 402. Ebeling, W., (1939) The r o l e of surface tension and contact angle i n the performance of spray l i q u i d s . Hi1gardia, 12, pp. 665 - 698. Edlefsen, N.E., ( l 9 4 l ) Some thermodynamic aspects of the use of s o i l moisture by plants. Trans. Am. Geophys. Un., 22, pp. 917 - 926. Eisenzopf, R., (1952) Ionenwirkungen auf die kutikulare wasseraufnahme von koniferen, Phyton, Buenos A i r e s , 4, pp. 149 - 159-Ekern, P.C, (1964) D i r e c t i n t e r c e p t i o n of cloud water on Lanaihale, Hawaii. Proc. S o i l S c i . Soc. Am., 28, pp. 419 - 421. 161 Fogg, G.E., (1.944) Diurnal f l u c t u a t i o n s i n the physical property of l e a f c u t i c l e . Nature, 154, pp. 515. Fogg, G.E., (1947) Quantitative studies on the wetting of leaves by water. Proc. Roy. Soc., Ser. b, 134, pp. 503 - 522. Fournier D'Albe, E.M., (1957) Some observations; of the geographical d i s t r i b u t i o n of giant hygroscopic n u c l e i . In: A r t i f i c k l Stimulation  of Rain. Proc. 1 s t Conference on physics of cloud and p r e c i p i t a t i o n , Woods Hole Oceanograph Inst., 1955, pp. 73 - 80. Fuchs, N., and Petrjanoff, J . , (1937) Microscopic examination of fog, cloud, and r a i n droplets. Nature, 139, pp. I l l - 112. Gates, F., (1914) Winter as a f a c t o r i n the xerophily of c e r t a i n evergreen ericads. Bot. Gaz., 57, PP. 445 - 489. Geiger, R. , (1950) The Climate near the Ground. Second Ed. Cambridge, Harvard Univ. Press, pp. 482.. Geiger, R., (1965) The Climate near the Ground. T h i r d Ed. Cambridge, Harvard Univ. Press, pp. 6 l l . George, J . J . , ( l 9 5 l ) Fog. Compendium of Meteorology", pp. 1179 - 1189.Am.Met.Soc, Boston, pp. 1334. Gessner, F., (1956) Die wasseraufnahme durch b l a t t e r und samen. Encycl. Plant P h y s i o l . . 3, pp. 215 -246. Gindel, I., (1966) A t t r a c t i o n of atmospheric moisture by woody xerophytes i n a r i d climates. Commonwealth For. Rev., 45, pp. 297 - 321. Ginsburg, J.M., (1930) Studies on penetration of o i l s i n t o plant t i s s u e . N.J. Agric. Exper. Sta.. Annual Report, 51» PP. 163 - 167. Grossenbaeher, K.A., (1939) Autonomic cycle rate of exudation from plants. Am. Journ. Bot., 26, pp. 107 - 109. Grubb, P.J., and Whitmore, T.C., (1966) A comparison of montane and lowland r a i n f o r e s t i n Ecuador: I I . The climate and I t s effects; on the d i s t r i b u t i o n and physiognomy of the f o r e s t s . J . E c o l , 54, pp. 303 - 333-162 Grunow., J. ,, (1952) Nebelniederscblag: bedeutung und erfassung einer zusatkomponente de niederschlags. Berichte Deutsch Wetterd., U.S. Zone, 7, pp. 30 - 3k. Grunow, J. , (1959) The productiveness of fog p r e c i p i t a t i o n i n r e l a t i o n to the cloud droplet spectrum. A.G.U. Geophys. Monograph No. 5 s Physics of  P r e c i p i t a t i o n . Guest, P.L., and Chapman, H.D., (19^9) Investigations on the use of i r o n sprays, dusts, and s o i l a p p l i c a t i o n s to control i r o n chlorosis: of citrus;. Proc. Am. Soc. Hort. S c i . . $k, pp. 1 1 - 2 1 . Hagan, R.M., (19^9) Autonomic cycles i n the water r e l a t i o n s of non - exuding de-topped root systems. Plant Physiol, 2k, pp. kkl - k5k. Haines, F.M., (1952) The absorption of water by leaves i n an atmosphere of high humidity. J . Exp. Bot., % PP. 95 - 98. j Haines, P.M., (1953) The absorption of water by leaves i n fogged a i r . J . Exp. Bot., k, pp. 106 - 107. H i l t n e r , E., (1930) Der tau und seine bedeutung f u r den pflanzenbau. Wi ss;. Arch. Landwi r t s ch. , Ges. , Abt-. A, Archiv Pflanzenbau, 3, pp. 1 -70. H i l t n e r , E., (1932) Der tau, ein vernachlassigter lebenfaktor der pflanzen. M i t t . Deutsch  Landwirtsch. Ges., kl, pp. 825 - 827. H i r s t , J.M., (1954) A method f o r recording the formation and persistence of water droplets on plant shoots. Q.J.R.M.S., 80, pp. 227 - 231. Hohn, K., (1951) Untersuchungen uber das wasserdampfaufnahme und wasser dampfabgabe vermogen hoherer landpflanzen. Beitrage. B i o l . Pflanzen., 30, pp. 159 - 178. H o r i , T., (1953) Ed. Studies on fog i n r e l a t i o n to fog -preventing f o r e s t . Tanne Trading Ca, Sapporo, Hokkaido, Japan, pp. 399-163 Hrudieka, B. , (1938) Zu den optisclien uber alcustischen eigenschaften des klimas einer gross stadt. Gerland Beitrage zur Geophysik, 53, pp. 337 -344. Also quoted i n Geiger, R". , 1965, o p . c i t Huzioka, T., and Tabata, T., and Matsumura, N., (1953) D i s t r i b u t i o n of fog water contents i n a f o r e s t . In Hori, T., 1953, op. c i t . . pp. 205 - 220. Isaac, L.A., (1946) Fog d r i p and r a i n i n t e r c e p t i o n i n coastal f o r e s t s . .Forestry Research Notes. No. 34, P a c i f i c N. W. For. Exp. Sta., August 1 5 t h , 1946, pp. 15 - 16. Janes, B.E., (1954) Absorption and l o s s of water by tomato leaves i n a saturated atmosphere. S o i l  Science,78, pp. I 8 9 - 197. Jensen, R.D., (1961) E f f e c t of temperature on water transport through plants;. Plant P h y s i o l . , 36, pp. 639 - 642. Jensen, R.D., Taylor, S.A., ahd Wiebe, H.H., (1961) Negative transport and resistance to water • flow through plants. Plant Physiol., 36, pp. 633 " 638. Jones;, R.L. , (1957) The e f f e c t of surface wetting on the t r a n s p i r a t i o n of leaves. Physiol. Plant., 10, pp. 281 - 288. K e l l e y , V.W.. , (1930) E f f e c t of c e r t a i n hydrocarbon oils; on r e s p i r a t i o n of f o l i a g e and dormant twigs of the apple. I l l i n o i s A g r i c . Exp. Sta., B u l l . , 348, pp. 371 - 406. Kendall, M.G., (1955) Rank C o r r e l a t i o n Methods. London, pp. 1 K e r f 0 0 1 , 0!. , (1968) Mist p r e c i p i t a t i o n on vegetation. Forestry Abstracts, 29, pp. 8 - 20. K i r i g i n , B., (1959) Beitrag zum problem der nebelnieder-schlagsmessungen. Berichte Deutsch ¥etterd., 8, pp. 96 - 105. Kittredge, J . , (1948) Forest influences. Ch. 12: Fog d r i p , pp. 115 - 119. N.Y., McGraw H i l l , p p . 364. 164 Knight, H.J., and Chamberlain, J.C., and Samuels, CD., (1Q2Q) Some l i m i t i n g f a c t o r s i n the use of saturated petroleum o i l s : as i n s e c t i c i d e s . Plant Physiol., 15, pp. 299 - 321. Kramer, P.J., (1940) Root resistance as a cause of decreased water absorption by plants at low temperatures. Plant Physiol, 15, pp. 63 -79. Kramer, P.J., (1942) Species d i f f e r e n c e with respect to water absorption at low s o i l temperatures. Ahi-. Journ. Bo tan. . 29, pp. 828 - 832. Kraslkov, P.N., (1948) Protection against the early and l a t e f r o s t by means of smoke and fog. Trans. Main. Geophys. Obs., No. 12, pp. 74. Krause, H., (1935) Beitrage zur kenntis der wasseraufnahme durch oberirdische pflanzenorgane. Ost. Botan. Z e i t s c h r . . 84, pp. 2 4 l - 270. Kummerow, J . , (1962a) Quantitative messungen des nebel -nlederschlages im walde von Fray Jorge an der Nordchilenschen kuste. Naturwi s s ens ch., 49, pp. 203 - 204. Kummerow, J . , (1962b) Quantitative measurements of fog i n the Fray Jorge National Park. Bol.  de l a Univ. de C h i l e . Santiago. No. 28, pp. 36 - 37. Kuroiwa, D., (1953) E l e c t r o n microscope study of atmospheric condensation n u c l e i . In Hori, T., Ed., 1953» op. c i t . . pp. 349 - 145. Kuroiwa, D., and K i n o s i t a , S., (1953) A balloon fog meter and the v e r t i c a l d i s t r i b u t i o n of l i q u i d water contents i n the lower atmosphere. In Hori, T., Ed., op. c i t . , pp. I87 - 204. Kyriazopoulos, B.D., Llvada, G.C., and Angouridakis, V.F., (1968) Olympus Cumulus Project. I. A r t i f i c i a l d r a i n i n g of summer ground! clouds. Preliminary Report, N.A.T.O. Landsberg, H., (1938) Atmospheric condensation n u c l e i . Ergebnisse der Kosmlschen Phvsik. 3, PP* 207 - 201. Langmuir, I., (1948) Production of r a i n by a chain r e a c t i o n i n cumulus clouds at temperatures above fr e e z i n g . J. Meteorol.. 5, pp. 175 - 209. 165 L a n g m u i r , I . , (1961) M a t h e m a t i c a l i n v e s t i g a t i o n of* w a t e r d r o p l e t t r a j e c t o r i e s ; . R e p o r t RL - 224, 1945. I n S u i t s , G . , E d . , C o l l e c t e d Works  o f I r v i n g L a n g m u i r . N . Y . , Pergammon, 12 v o l s . L a i d l a w , C . G . P . , and K n i g h t , R , C . , (1916) A d e s c r i p t i o n and a n o t e o n s t o m a t a l b e h a v i o u r d u r i n g w i l t i n g . A n n . B o t . , 30, p p . 47, - 56. L a u s b e r g , T . , (1935) Q u a n t i t a t i v e u n t e r s u c h u n g e n u b e r d i e k u t i k u l a r e e x k r e t i o n des; l a u b b l a t t e s . J a h r b u c h W i s s . B o t a n . , 81, p p . 769 - 806. L i n k e , F . , (1921) N i e d e r s c h l a g s m e s s u n g e n u n t e r baumen M e t e o r o l . Z e i t s c h r . , 38, p p . 277 - 278. L l o y d , F . E . , (1905) The a r t i f i c i a l i n d u c t i o n o f l e a f f o r m a t i o n i n t h e o c t i l l o . T o r r e y a , 5, p p . 175 - 179. M a l r o u s , M . A . , (1954) D r o p s i z e s i n s e a m i s t s . Q . J . R . M . S . , pp. 99 - 101. M a n n , H . B . , (1945) Non - p a r a m e t r i c t e s t s a g a i n s t t r e n d . E c o m o m e t r i c a , 13, p p . 245 - 259. M a r l o t h , R . , (1904) R e s u l t s * o f e x p e r i m e n t s o n T a b l e M o u n t a i n f o r a s c e r t a i n i n g t h e amount o f w a t e r d e p o s i t e d f r o m t h e S . E . c l o u d s . P r o c . S . A f r i c a n P h i l . S o c . 14, p p . 403 - 408. M a r l o t h , R . , (1907) N o t e s on t h e a b s o r p t i o n o f w a t e r b y a e r i a l o r g a n s o f p l a n t s . T r a n s . R o y . S o c S . A f r i c a . 1, p p . 429 - 433. M a r l o t h , R . , (1908) Das K a p l a n d . W i s s e r g e b n i s s e der. D e u t s c h e n T i e f s e e E x p e d . , B d . I I , T e i l 3, J e n a . M a r l o t h , R . , (1926) W e i t e r e b e o b a c h t u n g e n u b e r d i e w a s s e r d e r p f l a n z e n d u r c h o b e r i r d i s c h e o r g a n e . B e r i c h t e D e u t s c h B o t . G e s . . 44, p p . 448 - 455. M a r t e n s , P . , (1934) R e c h e r c h e s s u r l a c u t i c u l e . P r o t o p l a s m a , 20, p p . 483 - 515. M a s o n , B . J . , (1957) The P h y s i c s o f C l o u d s . O x f o r d , C l a r e n d o n P r e s s , p p . 481. M a t s u m u r a , N . , (1953) On t h e c a l i b r a t i o n o f a f o g m e t e r . I n H o r i , T . , E d . , 1953, o p . c i t . , p p . 175 - 186. 1 6 6 Meidner, H., ( 1 9 5 4 ) Measurements of water intake from the atmosphere by leaves. New/ Phytologist, 5 3 , pp. 4 2 3 - 4 2 6 . Meyer, B.S., ( 1 9 4 5 ) A c r i t i c a l evaluation of the terminology of d i f f u s i o n phenomena. P1ant Physio 1 , 2 0 , pp. 142: - 1 6 4 . Mosteller, F., and Rourke, R.E.K., and Thomas, G.B. , ( 1 9 6 1 ) P r o b a b i l i t y and S t a t i s t i c s . Reading, pp. 3 9 5 . Myers, JN., ( 1 9 6 7 ) The use of vegetation i n the control of shallow r a d i a t i o n fog. Weather. 2 2 , pp. 2 8 9 - 2 9 1 . Myers, J.N., ( 1 9 6 8 ) Fog. S c i e n t i f i c American. 2 1 9 , pp. 7 5 - 82. Nagel, J.F. , (1956-) Fog p r e c i p i t a t i o n on Table Mountain. QJ J.R.M.S. , 82, pp. 4 5 2 - 4 6 0 . Nagel, J.F., ( 1 9 6 2 ) Fog p r e c i p i t a t i o n on A f r i c a ' s south west coast. Notes, 1-1, pp. 5 1 - 6 0 . Nakaya, U., ( 1 9 5 7 ) E l e c t r o n microscope study of the n u c l e i of sea fog. In: A r t i f i c i a l Stimulation of  Rain. Proc. 1 s t Conference on the physics of cloud and p r e c i p i t a t i o n p a r t i c l e s , Woods Hole Oceanographic Ins t . , 1 9 5 5 -Oberlander, G.T., ( 1 9 5 6 ) Summer fog p r e c i p i t a t i o n on the San Francisco Peninsula. Ecology. 37» pp. 8 5 1 - 8 5 2 . Ogiwara, S., and Okita, T., ( 1 9 5 2 ) E l e c t r o n microscope study of cloud and fog n u c l e i . T e l l u s , 4 , pp. 2 3 3 - 2 4 0 . Okita, T., (I962) Observations of the v e r t i c a l structure of a stratus cloud and r a d i a t i o n fogs; i n r e l a t i o n to the mechanism of d r i z z l e formation. T e l l u s , 1 4 , pp. 3 1 0 - 3 2 2 . Oura, H., ( 1 9 5 3 ) On the capture of fog p a r t i c l e s by a f o r e s t . In: Hori, T., Ed., 1 9 5 3 , op. c i t . , pp. 2 3 9 - 2 6 0 . Palmiter, D.H., Roberts, E.A., and Southwick, M.D., ( 1 9 4 6 ) Apple l e a f structure i n r e l a t i o n to penetration by spray s o l u t i o n s . Phytopath, 3 6 , pp. 681 - 6 8 9 . 167 Pane-fsky, H.A. , and B r i e r , G.W. , (1968) Some app l i c a t i o n s off s t a t i s t i c s to meteorology. Penn.State Univ. pp.224. Parsons, J . J . , ( i 9 6 0 ) Fog d r i p from coastal stratus with s p e c i a l reference to C a l i f o r n i a . Weather, 15, pp. 58 - 62. Penman, H.L., (1963) Vegetation and Hydrology. Technical Communication 53, Commonwealth Bureaux of Agr i c u l t u r e , Farnham Royal, England, pp. 124. Pettersen, S., (1939) Some aspects of the formation and d i s s i p a t i o n off fog. Geo phys . Publikas.ioner, 12, pp. 22 - 33. P h i l i p , J.R., (1958) Osmosis and d i f f u s i o n i n t i s s u e : h a l f times and i n t e r n a l gradients. Plant  P h y s i o l . . 33, pp. 275 - 278. P h i l i p , W., (1932) Transpirationversuche mit betaruben i n laboratium und f r e i l a n d . Wiss. Arch. Landwirtsch., Abt. A, Archiv Pflanzenbau, 8, pp. 70 - 120. Pincock, G.L., and Turner, J.A., (1956) Advection fog along the B r i t i s h Columbia coast and over the north P a c i f i c Ocean during l a t e summer and early autumn. Proc. Eighth Pacific.  Science Congress. 1953:, V o l . IIA, Geology, Geophysicsffli-and Meteorology, pp. 955 -962. Richards, P.W., (1952) The t r o p i c a l r a i n f o r e s t : an ec o l o g i c a l study.Cambridge, Univ.Press, pp. 450 Roberts, E.A., Southwick, M.D., and Palmiter, D.H., (1948) A microchemical examination of Mcintosh apple leaves showing r e l a t i o n s h i p s of c e l l wall constituents to penetration of spray solutions. Plant Physiol., 23, pp. 557 -559. Rohrbaugh, P.W., (1934) Penetration and accumulation of petroleum; spray o i l s i n the leaves, twigs, and f r u i t of c i t r u s trees. Plant Physiol., 9, PP. 699 - 730. Rubner, K., (1932) Fog p r e c i p i t a t i o n and i t s measurement. Tharandter F o r s t l i c h e s Jabrbuch, 86, pp. 330 - 342. 168 S h i f r i n , K.S., ( l 9 5 l ) The e f f e c t of fog on net r a d i a t i o n . Trans. Main. Geophys. Obs., No. 27, pp. 89. S h i f r i n , K.S., and Bogdanova, N.P., (1955) The e f f e c t of mist on net r a d i a t i o n . Trans. Main. Geophvs. Obs.. No. 46, pp. 108. Siccama, T., (1968) A l t i t u d i n a l d i s t r i b u t i o n of for e s t  and vegetation i n r e l a t i o n to s o i l and  climate on the slopes of the Green Mountains, Unpubl. Ph.D. Thesis, Univ. of Vermont;, Burlington, Vt. Simpson, G.C., ( l 9 4 l ) Sea s a l t and condensation n u c l e i . Q.J.R.M.S., 67, pp. 99 - 110. Skoog, F., Broyer, T.G., and Grossenbacher, K.A., (1938) E f f e c t s of auxin on rates, p e r i o d i c i t y , and osmotic r e l a t i o n s i n exudation. Am. Journ.  Bot. , 25, pp. 749 -• 759. Slatyer, R.O., Slatyer, R.O,, Slatyer', R.0. , Steubing, L., Stockers, 0., Stone, E.C. , Stone, E.C., Stone, E.C., (1956) Absorption of water from atmospheres of d i f f e r e n t humidities and i t s transport through plants. Austr. J. B i o l . S c i . , 9, PP. 552 - 558. ( i 9 6 0 ) Absorption of water by plants. Botan. Rev., 26, pp. 331 - 392. (1967) Plant -Water Relationships. N.Y., Academic Press, 366 pp. (1949) Beitrage zur tauwasseraufnahme hoherer pflanzen. B i o l . Zentr., 68, pp. 252 - 259. (1933) T r a n s p i r a t i o n und wasserhaushalt i n verschiedenen klimazonen. Jahrbuch Wi s s.  Botan.. 78, pp. 751 - 856. (1957) Dew. as an e c o l o g i c a l f a c t o r : (l)Review of the l i t e r a t u r e ; (2)The e f f e c t of a r t i f i c i a l dew on the s u r v i v a l of Pinus ponderosa. Ecology, 38, pp. 407 - 420. and Fowells, H.A., (1955) S u r v i v a l value of dew under laboratory conditions with Pinus: ponderosa. For. S c i . , 1, pp. 183 - 188. Shachori, A.Y., and Stanley, R.G., (1956) Water absorption by needles of Ponderosa pine seedlings, and i t s i n t e r n a l r e d i s t r i b u t i o n . Plant P hysiol., 31, PP. 120 - 126. 169 Stone, E.C., Went, F.W. , and Young, C.L., (1950) Water absorption from the atmosphere by plants growing i n dry s o i l . Science, 111, pp. 546 - 548. Stone, R.G., (1936) Fog i n the United States and adjacent regions. Geogr. Rev., 26, pp. I l l - 134. Suring, R., (1915) Quoted i n Linke, 1921, op. c i t . S u t c l i f f e , J . , (1968) Plants and Water. I n s t i t u t e of Biology, Studies i n Biology, No. l 4 . Sverdrup, H.U., (1942) Oceanography for. Meteorologists. Prentice H a l l , New Jersey. Taylor, G.I., (1917). The formation of fog and mist. Q.J.R.M.S., 43, pp. 241 - 268. Thut, H.F., and Loomis, W.E., (1944) Relation of l i g h t to the growth off plants. Plant Physiol., 19, pp. 117 - 130. Tintner, G., (1952) Econometrics. Trabert, W., ( l 9 0 l ) Die extinktion des l i c h e s i n einem truben medium. Meteorol. Z e i t s c h r . , 36, pp. 518 - 524. T u r e l l , F.M., (1947) C i t r u s l e a f structure: stomata, composition, and pore size i n r e l a t i o n to the penetration of l i q u i d s . Botan. Gaz.. 108, pp. 476 - 483. Twomey, S., (1956) The c o l l e c t i o n of cloud water by a v e r t i c a l wire mesh. B u l l . Observ. du Puy de Dome, No. 3. Twomey, S., (1957) P r e c i p i t a t i o n by d i r e c t i n t e r c e p t i o n of cloud water. Weather, 12, pp. 120 - 122. U.S.D.A., (1941) Climate and Man. Yearbook of A g r i c u l t u r e . Vaadia, Y., and Waisel, Y., (1963) Water absorption of the a e r i a l parts of plants. P h y s i o l . Plant., 16, pp. 44 - 51. Van den Honert, T.H., (1948) Water transport as a catenary process. Disc. Faradav Soc.. No. 3, pp. 146 - 153. 170 Vogelmann, H.W., Sic c a m a , T . , Leedy, D., and O v i t t , D.C., (1968) P r e c i p i t a t i o n f r o m f o g m o i s t u r e I n t h e Green M o u n t a i n s o f Vermont. E c o l o g y . 49, pp. 1205 - 1207. W a d l e i g h , CH., and A y e r s , A.D., (1945) Growth and b i o c h e m i c a l c o m p o s i t i o n o f bean p l a n t s as c o n d i t i o n e d by s o i l m o i s t u r e t e n s i o n and s a l t c o n c e n t r a t i o n . P l a n t P h y s i o l . , 20, pp. 106 - 132. W a d l e i g h , C H . , and Gauch, H.G. , (1948) R a t e o f l e a f e l o n g a t i o n as a f f e c t e d by the i n t e n s i t y o f the t o t a l s o i l m o i s t u r e s t r e s s . P l a n t  P h y s i o l . . 23, pp. 485 - 495. W a d l e i g h , C H . , Gauch, A.D. , and M a g i s t a d , O.C, (1946) Growth and r u b b e r a c c u m u l a t i o n i n g u a y u l e as c o n d i t i o n e d by s o i l s a l i n i t y and i r r i g a t i o n r e g i m e . U.S.D.A., Tech. B u l l . No. 925. Went, F.W. , (1955) Fog, mist., dew, and o t h e r s o u r c e s o f w a t e r . U.S.D.A., Yearbook o f A g r i c u l t u r e , Water, 1955. Wheland, G.W., (1944) The t h e o r y o f r e s o n a n c e and i t s a p p l i c a t i o n t o o r g a n i c c h e m i s t r y . N..Y. W i l e y . PP. 316. : W h i t e , G . , ( l 7 8 9 ) The n a t u r a l h i s t o r y o f S e l b o u r n e , L e t t e r XXIX, p. 294. Whiteman, P . C , and K o l l e r , D. , (1964) S a t u r a t i o n d e f i c i t s o f t he m e s o p h y l l e v a p o r a t i n g s u r f a c e i n a d e s e r t h a l o p h y t e . S c i e n c e , 146, pp. 1320 - 1321. W i l l e t t , H.C., (1944) D e s c r i p t i v e m e t e o r o l o g y . W i l s o n , C. T.R. , ((.1897) C o n d e n s a t i o n o f w a t e r v a p o u r i n t h e p r e s e n c e o f d u s t f r e e a i r and o t h e r g a s e s . P h i l . T r a n s . Roy. S o c , A, I 8 9 , pp. 265 -285. Wood, J.G., (1925) The s e l e c t i v e a b s o r p t i o n o f c h l o r i n e i o n s and the a b s o r p t i o n o f w a t e r by t h e l e a v e s i n t h e genus A t r i p l e x . A u s t r . J . E x p e r . B i o l . Med. S c i . , 2, pp. 45 - 56. Woodcock, A.H., ((1950) Sea s a l t i n a t r o p i c a l s t o r m . J . M e t e o r o l . , 7, pp. 397 - 399. 171 Woodward, W.H. , ( l 9 k l i ) Fog and s t r a t u s a t S e a t t l e . B u l l . Am. Met. Soc., 22, pp. 242 - 24°. Yevfimov, N.G., ( l 9 5 l ) T o t a l s o f net r a d i a t i o n f o r some p l a c e s i n the U.S.S.R. and o t h e r q u a n t i t i e s c h a r a c t e r i z i n g n e t r a d i a t i o n under a clo u d y sky. Trudy G.G.O., No. 26, 88. " / I n : Kondratyev, K. Ya., R a d i a t i v e Meat Exchange  i n the Atmosphere, p. 223. I 9 6 5 . Y o s i d a , Z., (1953) A t h e o r e t i c a l study off the d i s t r i b u t i o n o f f o g d e n s i t y i n a f o r e s t . I n H o r i , T., Ed., 1953 1, op. c i t . , pp. 105 - 113. Y o s i d a , Z., and Kuroiwa, D. , (1953) Wind f o r c e on a c o n i f e r t r e e and the q u a n t i t y off water th e r e b y c a p t u r e d . I n H o r i , T., 1953, op. c i t . , pp. 261 - 278. Z a i t s e v , D., (1950) Vodnost' i r a s p r e d e l e n i e k a p e l v kuchvykh ob l a k a k h . ( L i q u i d water content and d i s t r i b u t i o n o f drops i n cumulus c l o u d s . ) T r a n s . Trudy G l a v n o i G e o f l z . Observ., 19, pp. 122 - 130. Z a m f i r e s c u , N., ( l 9 3 l ) C e r c e t a r i asupra a b s o r t i u n i i p r i n o r g a n e l e a l e p l a n t e l o r . B u l l . Min. Agr., Domenilor, 3» pp. 3 - 6 . Z a t t l e r , F., (1932) A g r a r m e t e o r o l o g i s c h e b e i t r a g e zum t a u problem auf grund von messungen im Hopfgarten. Wiss. A r c h . L a n d w i r t s c h . , Abt. A, A r c h i v Pflanzenbau, 8, pp. 371 - 4o4. P u b l i s h e d d a t a s o u r c e s . Department o f T r a n s p o r t . G e n e r a l Summaries o f H o u r l y Weather O b s e r v a t i o n s i n Canada. (Ceased p u b l i c a t i o n i n 1 9 6 1 ) . Department o f T r a n s p o r t . H o u r l y Data Summaries, ( i n d i v i d u a l S t a t i o n s . ) Vancouver I n t e r n a t i o n a l A i r p o r t , and V i c t o r i a I n t e r n a t i o n a l A i r p o r t . 

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            data-media="{[{embed.selectedMedia}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
https://iiif.library.ubc.ca/presentation/dsp.831.1-0103983/manifest

Comment

Related Items