Urban Heat Islands: Why Cities Are Hotter Than Nearby Land

An urban heat island is a city area that runs hotter than the countryside around it, mainly because dark surfaces absorb sunlight and buildings and roads store heat.

Category: Geography · Created: 2026-10-15 · Updated: 2026-10-15 · 3 min read

Illustration: Diagram of the urban heat island temperature profile across a city · Image: TheNewPhobia, Public domain, via Wikimedia Commons.

An urban heat island is a built-up area that is measurably warmer than the rural land around it. The gap is usually largest at night, when the city releases the heat it stored during the day while the countryside cools faster. Dark pavement, dense buildings, and the loss of trees and damp soil cause most of the difference. It is a local effect, unlike the greenhouse effect, which warms the whole planet.

What do cities do with sunlight?

Asphalt roads, dark roofs and concrete absorb a large share of the sunlight that reaches them, and they warm quickly. Sunlight is mostly visible and infrared radiation, as laid out in the electromagnetic spectrum. Many rural surfaces behave differently. Fields, forests and lakes reflect more sunlight, and moist soil and leaves use part of the incoming energy to evaporate water instead of heating the air.

Rain in a city runs off through drains rather than soaking into the ground, so there is less water left to evaporate. Urban materials also store heat. Concrete and brick absorb warmth during the day and release it slowly after dark. The city behaves like a storage heater that keeps giving heat back long after the sun has set.

Why does the heat last into the night?

Tall buildings form street canyons. Their walls block part of the open sky, so heat radiated from warm surfaces is trapped between them rather than escaping to space. Narrow streets also slow the wind, which would otherwise mix warm air away.

Machines add heat directly. Air conditioners, vehicles and factories release waste heat into the streets, and this is counted separately from heat that comes from sunlight. In dense districts with heavy traffic and many air conditioners, this direct heat can be significant.

Surface	What happens to incoming sunlight
Dark asphalt or roof	Absorbed in large amounts, then released as heat
Bare dry soil	Warms strongly, with little water to evaporate
Lawn or crop	Some warming, plus evaporation from moist soil and leaves
Water	Warms slowly and holds its heat for a long time
Tree canopy	Shades the ground and cools the air through evaporation

The surface matters more than the building itself. Replacing dark materials, adding shade and keeping water in the landscape can change the temperature at street level.

How large is the difference?

Differences of several degrees Celsius between city centres and the surrounding countryside are common. On calm, clear nights in large cities, some studies have reported gaps above 10 °C. The result depends on the method. Air temperature is measured near the ground with thermometers, while satellites measure surface temperature, which can differ from the air above it by many degrees.

Surface heat islands are often strongest in daytime, when sun-heated roofs and roads are hottest. Air heat islands tend to peak at night, when the air above the city stays warm after the countryside has cooled. Both are real, and they should not be compared as if they were the same number.

Who feels the heat most?

Heat islands fall hardest on people with the least shade and cooling. Studies in many cities find that neighbourhoods with fewer trees and more pavement are often hotter, and these are frequently areas with lower incomes. Heat waves in dense cities are also linked to higher numbers of deaths among older people and those with existing illnesses.

How can cities reduce the heat?

Planners use several measures. Light-coloured or reflective roofs and pavements send more sunlight back to space. Street trees and green roofs shade surfaces, and they cool the air as water evaporates from their leaves and soil, which returns moisture to the air as part of the water cycle.

Permeable paving lets rain soak into the ground, so more water remains available for evaporation. Parks and water features cool the air nearby, and that cooler air often reaches a short distance into the streets around them. Each measure helps alone, and the largest effect comes from combining them across a whole district.