Urbanization dramatically alters the land surface and converts preurban local or regional climates into distinctively "urban climates." Probably the best known and most intensively studied urban-climatic feature is the "urban heat island," although considerable attention has also been directed toward the effects of urbanization on precipitation, humidity, wind, and the air quality regimes of cities. Using integrated programs of fieldwork and numerical modeling, geographers have been at the forefront of assessing urban influences (especially the effects of urban surface materials and morphology) on local and regional climates (Oke, 1987). Research by geographers also is beginning to suggest that extensive land surface changes associated with urban-and suburbanization occurring worldwide may be contributing to global climate change.

Sue Grimmond and Tim Oke have been particularly effective at integrating in-the-field measurement programs with the numerical modeling of urban climates (Grimmond and Oke, 1995; Grimmond et al., 1996). Making and evaluating heat and moisture flux observations, as well as compiling surface character databases, their research teams have examined a number of North American cities, including Los Angeles, Chicago, Miami, Vancouver, Sacramento, Tucson, and Mexico City. Not only have they documented the considerable variability that exists both within and between cities, but their analyses show that daily patterns of the fluxes and the timing of the peaks are remarkably similar among the cities. Their measurements further indicate that evapotranspiration is even higher than expected in many residential areas, owing to the irrigation of planted vegetation. Evapotranspiration in other parts of the city tends to be quite low, as available energy mostly warms the urban fabric.

Grimmond and colleagues have also been able to use geographic information systems (GISs) to help synthesize land surface information, field measurements, and model simulations (Grimmond and Souch, 1994). Their innovative approaches are revealing the often elusive source regions of the heat and moisture fluxes (e.g., evapotranspiration), as well as the character of the land cover in those source regions (see Plate 6). Although others have investigated source regions, Grimmond and colleagues are identifying and quantifying them more precisely than ever before and in turn are clarifying the spatial and temporal relationships between urbanization and attendant climate change. Their results have the potential to help isolate the influences of built environments on global climate change.