the earth is receiving from the sun. As population and energy demand increase, the number and sizes of these areas will likely increase substantially. The question of concern is whether or not these perturbations of the heat balance of the lower atmosphere will produce significant climatic perturbations . Considerable work has been done to evaluate and describe the impact of large urban areas on weather and climate. Man-made heat islands have been studied by Landsberg and Maisel,18 Ludwig,1$ Clarke,20 and others. The urban heat island exists during both the day and the night but is much stronger at night. In large urban areas, temperatures can be as much as 5°C to 10°C warmer than nearby rural areas. Studies have indicated that the mean annual minimum temperature of a city may be as much as 2°C higher than surrounding rural areas.21>22 However, this temperature increase is not due solely to the waste heat released from the cities. Urbanization results in the replacement of natural vegetated surfaces with brick, asphalt, and concrete, which store heat much more effectively than vegetation and release more stored heat to the environment at night. In addition to the effects of waste heat on temperature, it has been observed that urban areas can affect precipitation patterns. There are several causal factors that increase precipitation around an urban center; these include combustion vapor added to the atmosphere, greater surface roughness to enhance mechanical turbulence, the presence of greater concentrations of condensation and ice nuclei in the urban atmosphere, and higher temperatures to increase thermal convection. Table 2.3 summarizes the urban effects on summer rainfall in nine metropolitan areas. In addition to increasing precipitation, cities quite possibly influence the occurrence of severe weather in their vicinities. Table 2.4 displays the increase in hail-days at several cities, where increases in thunderstorm occurrences of up to 50% at a given point have also been observed.23 Although these precipitation increases appear to be real, the exact causes have not been quantified. It is likely that waste heat has a role, although not a major one, in climatic change. The contribution of energy production to creation of urban heat islands and their precipitation impacts is somewhat less certain.
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