On a summer evening, a thermometer in the heart of a city can read several degrees higher than one in the fields just beyond the suburbs, even though both are experiencing the same regional weather. This difference is not measurement error or coincidence. It is a real and well-documented phenomenon called the urban heat island, in which built-up areas run consistently warmer than their rural surroundings. As the world's population becomes ever more urban and the global climate warms, the urban heat island is shifting from a scientific curiosity to a serious concern for public health and city planning.
The cause of the urban heat island lies in the fundamental difference between a natural landscape and a built one, in how each handles the sun's energy. A field, forest or meadow is full of vegetation and moisture. Plants shade the ground and, crucially, cool their surroundings through transpiration, releasing water vapour in a process that carries heat away much as sweating cools a body. Moist soil and open water do the same, using incoming solar energy to evaporate water rather than to heat the surface. A natural landscape thus spends much of the sun's energy on evaporation and stays relatively cool.
A city works completely differently. Replace the vegetation and soil with concrete, asphalt, brick, stone and glass, and you have a landscape of dense, dark, dry materials that excel at absorbing and storing heat. These surfaces soak up solar energy through the day, heating up substantially, and because there is little moisture and little vegetation, almost none of that energy goes into cooling evaporation. Dark asphalt in particular can reach scorching temperatures under summer sun. The city becomes a vast thermal battery, absorbing heat all day.
The effect is most pronounced not at midday but after dark, and this nocturnal warmth is the urban heat island's most striking and consequential feature. All that heat stored in the mass of buildings and pavement through the day is released slowly through the evening and night. While the countryside cools rapidly after sunset, radiating its heat away to the sky, the city keeps releasing its stored warmth, so that overnight temperatures remain elevated. The gap between urban and rural temperatures is therefore often widest in the hours before dawn, when the countryside has cooled off but the city has not. This matters enormously for human health, because it is the failure of temperatures to drop at night that makes heat waves dangerous; the body needs cool nights to recover from hot days, and a city that stays warm around the clock denies its residents that relief.
Several other features of cities reinforce the effect. The tall buildings and narrow streets of urban cores form what are known as urban canyons, which trap heat by reflecting it between surfaces and reducing the sky view that would otherwise let heat radiate away. The geometry also blocks wind, reducing the ventilation that might carry heat off. On top of the stored solar heat, cities generate their own waste heat directly: from vehicles, air conditioning, industry and the sheer metabolic activity of millions of people and machines. Air conditioning is a particular irony, pumping heat out of buildings and into the streets, warming the city further and driving yet more demand for cooling. The lack of vegetation removes not just the cooling of transpiration but the shade that would keep surfaces from heating in the first place.
The consequences reach across many aspects of urban life. The most serious is health: elevated urban temperatures, especially at night during heat waves, increase heat stress, illness and mortality, and the burden falls hardest on the vulnerable — the elderly, the sick, the poor, and those without access to cooling or green space. The heat island also drives up energy demand for cooling, straining power grids exactly when they are most stressed, and it can worsen air quality, since heat accelerates the chemical reactions that form some pollutants. And it compounds the broader warming of the climate, so that city dwellers experience a double dose of heat: the global rise plus the local urban boost layered on top.
Encouragingly, the same understanding that explains the urban heat island points to ways of reducing it, and cities around the world are increasingly acting on them. The most powerful remedy is to bring nature back into the built environment: planting trees and creating parks and green spaces restores shade and the cooling of transpiration, and a well-treed street can be dramatically cooler than a bare one. Green roofs and walls do the same on the buildings themselves. Changing the surfaces helps too: lighter-coloured, more reflective roofs and pavements absorb far less solar heat than dark ones, and so-called cool roofs and cool pavements are being adopted to lower surface temperatures. Preserving water features and designing for ventilation to let cooling breezes flow through the city both help. None of these individually solves the problem, but together they can meaningfully lower urban temperatures and ease the burden on residents.
As cities grow and the climate warms, the urban heat island is becoming one of the more tangible ways that people experience a changing climate in daily life, felt directly on a hot city night when the air simply refuses to cool. It is also, unusually among climate challenges, a problem that individual cities can do a great deal about locally, through the way they build, plant and pave. Understanding why the concrete and asphalt around us hold the day's heat long into the night is the first step toward building cities that stay liveable in a warming world.