Social Sustainability
Urban Heat Island (UHI)
The phenomenon of cities running several degrees hotter than surrounding countryside — a major amplifier of heatwave risk.
Definition
The urban heat island is the temperature difference between urban areas and their rural surroundings, caused by dark, heat-absorbing surfaces (asphalt, roofing), loss of vegetation and evaporative cooling, waste heat from vehicles and air conditioning, and canyon geometry trapping heat. Differences reach several degrees — strongest at night, when stored heat releases — and UHI amplifies heatwave mortality, raises cooling energy demand and worsens smog; climate warming and UHI combine additively in city heat risk.
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