The 'Urban Heat Island Effect' describes the difference in temperature between a city and its surrounding countryside. Cities typically have less vegetation and more impervious surfaces, such as roofs and cement roads, which absorb solar radiation and re-radiate it as heat. Urban and suburban areas can by anywhere from 2 to 10°F (1 to 6°C) hotter than nearby rural areas. The presence of tall buildings and narrow streets can trap air and decrease air circulation, which also contributes to the formation of heat islands.

The EPA has a Heat Island Reduction Initiative focusing on ways to reduce this urban problem with the help of community groups, public officials, industry representatives and researchers. Cities do have higher rates of heat-related illnesses and death, however researchers are still studying the extent to which heat islands affect human health. Meanwhile it is important for cities to begin implementing heat island reduction strategies such as installing cool roofs (i.e. green roofs), using cool paving, and planting shade trees and vegetation.

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