What is an urban heat island?
An urban heat island is the measurable tendency for cities to be warmer than surrounding rural areas — commonly several degrees, and occasionally by considerably more at night.
What causes it:
Surface materials. Asphalt, concrete and brick have high thermal mass, absorbing heat through the day and releasing it slowly overnight. Dark surfaces also have low albedo, reflecting less sunlight and absorbing more. This is the single largest contributor.
Loss of evapotranspiration. Vegetation and soil cool their surroundings by evaporating water, which consumes energy as latent heat. Replacing them with impermeable surfaces removes that cooling entirely — and drains rainfall away before it can evaporate.
Urban geometry. Streets lined with tall buildings form canyons that trap radiation through multiple reflections and restrict the sky view through which heat escapes at night. Buildings also reduce wind, limiting ventilation.
Waste heat. Vehicles, industry, buildings and — importantly — air conditioning, which pumps heat from indoors to outdoors, creating a feedback where cooling buildings warms the street.
The nighttime effect is the strongest and the most consequential. Rural areas cool rapidly after sunset; cities release stored heat through the night. That matters because the human body relies on overnight cooling to recover from heat stress, and heatwave mortality correlates strongly with nighttime minimum temperatures rather than daytime peaks.
The impacts: excess deaths during heatwaves, concentrated among older and isolated people; higher energy demand for cooling; worse air quality, since ozone forms faster in heat; and an unequal distribution — lower-income neighbourhoods typically have less tree cover and more hard surface, so heat exposure tracks deprivation closely.
What reduces it: urban trees, which provide shade and evapotranspiration and are the most effective single measure; green and cool (high-albedo) roofs; permeable surfaces and water features; and street layouts preserving ventilation.