Satellite Remote Sensing of Urban Heat Islands: Current Practice and Prospects

Abstract

Climatologists have long been interested in the differences in observed ambient air temperature between cities and their surrounding rural regions (Landsberg, 1981). Urban climate studies have traditionally focused on the magnitude of such differences, which collectively describe the Urban Heat Island (UHI) effect. Urban development usually results in a dramatic alteration of the Earth’s surface, as natural vegetation is removed and replaced by non-evaporating, non-transpiring surfaces (e.g., stone, metal, concrete, etc.). Under such alteration, the partitioning of incoming solar radiation into fluxes of sensible and latent heat is skewed in favor of increased sensible heat flux as evapotranspirative surfaces are reduced. Urban areas generally have higher solar radiation absorption, and a greater thermal capacity and conductivity. The urban-rural temperature differences are usually modest, averaging less than one degree centigrade, but occasionally rising to several degrees when urban, topographical, and meteorological conditions are favorable for the UHI to develop (Mather, 1986). The UHI effect is not restricted to large metropolitan areas; in fact, it has been detected in cities with populations of less than 10,000 people (Karl et al. 1988). Two types of UHI can be distinguished pertinent to the methods of temperature measurement: the urban canopy layer (UCL) heat island, and the urban boundary layer (UBL) heat island (Oke, 1979). The former consists of air between the roughness elements, e.g., buildings and tree canopies, with an upper boundary just below roof level. The latter is situated above the former, with a lower boundary subject to the influence of urban surface.