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Tests of an operational algorithmic urban heat island scheme Barton, Mark


This study presents the tests of an algorithmic scheme to calculate the magnitude of the heat island of the urban canopy layer, on an hourly basis throughout the year. The algorithmic scheme has been developed so that operational climatologists can apply it in all cities. Input requirements for the scheme are readily available meteorological data and simple numerical descriptors of the urban canyon geometry and rural surface conditions. The scheme comprises four components. The first, (ΔT[sub u –r (max)]), is an empirical relation that gives the absolute maximum urban heat island, experienced in a city as a function of its canyon height-to-width ratio in the urban core (Oke, 1981). Three 'phi' terms, (Φ[sub m] Φ[sub w] Φ[sub t]), adjust this maximum downward as a result of the combined effects of rural surface thermal admittance, weather and time of day, respectively. The rural surface thermal admittance effect term, (Φ[sub m]), is derived from the results of an energy balance model (Oke et al., 1991). Two datasets from Uppsala, Sweden and one from Lodz, Poland are used to test the scheme. The nocturnal performance of the scheme is promising. Average predicted nocturnal heat island magnitudes are typically within plus or minus half a degree of the measured heat island, (MAE 0.5 °C), in Lodz, Poland (1998). However, the daytime utility of the scheme is poor. The regular occurrence of small, (~1 °C), daytime heat and cool islands is largely missed. Degradation of performance appears to be due, in part, to the inability of the scheme to produce negative heat islands (cool islands). Given that, applied (pollution) climatologists are primarily concerned with heat island effects when they attain magnitudes of several degrees; this is not considered a significant limitation.

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