Urban heat islands (UHIs) are regions of characteristic warmth associated with cities. They are perhaps the clearest expressions of unintentional climate modification by humans. Weather patterns, geographic setting, and urban configuration cause UHIs to vary in space and time; the heat island effect is typically most pronounced in city centers where building and population densities are highest, and during calm, cloudless evenings that promote maximum thermal differences between the city and surrounding countryside. UHIs have several economic, ecological, and social effects, notably on energy consumption, dispersal of air pollutants, and human health and comfort. Climate-sensitive urban design demonstrates some potential for mitigation of these effects, where appropriate.

The UHI effect traditionally denotes a near-surface air temperature difference between the urban canopy-layer (the layer of atmosphere below ...