Role of soil moisture in the determination of urban heat island intensity in different climate regimes

A precipitation pattern scheme was combined with Koppen climate classes to form a five-class Global Climate Scheme for Urban Climate Studies (GCS/UCS) that explains observed diurnal variations of urban heat island (UHI) magnitude in different climatic zones. The validity of this scheme was demonstrated using urban/rural micrometeorological field measurements obtained during summer and winter in four cities. Nighttime UHI magnitude was found to increase monotomicly with increasing population, independent of season, climate type, or causal mechanism. Daytime UHI magnitude versus population values, however, showed an interesting dichotomy between results for dry and wet rural soils. Wet-soil rural sites produced daytime UHIs larger than their corresponding nighttime values due to high rural latent heat flux values and high rural thermal inertia values (relative to those of urban materials). With dry rural soils, daytime UHIs were smaller than corresponding nighttime values due to the low thermal inertia of the rural soil (relative to that of urban materials). This produces a rapid daytime rural warming/nighttime cooling and hence weak daytime/strong nighttime UHIs.