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WHAT THE RESEARCH SAYS
Cities make some of their own weather.
Brick, tarmac and roofs soak up the sun all day and give the heat back after dark. Scientists call the result an urban heat island, and trees, water and lighter roofs are what push it back.
UP TO 5°C warmer in a London model
Compare surfaces
THE SHORT ANSWER
Three things do most of the work.
Each one also points to a way to cool a street.
AFTER DARK
Buildings and roads release the day's heat slowly, so heat islands often grow after sunset. In a University of Reading model of London, the urban effect was largest at night.
Why nights matterROOFS
The US EPA says conventional roofs can reach 66°F above the air temperature on a warm day. Reflective cool roofs absorb less of the sun.
Compare surfacesTREES
Trees cool the air by shading it and by evaporating water from their leaves. A review of 308 studies found urban forests were 1.6°C cooler on average.
See the numbersA QUICK SURFACE CHECK
Pick a surface you might pass on a sunny afternoon to see how it behaves. The bar shows how much heat it tends to add to a city.
Based on US EPA heat island guidance. Figures come from US research reviews; UK streets follow the same physics.
TWO NUMBERS WORTH KNOWING
One figure from a model of London,
one from a study of 93 European cities.
up to 5°C
Urban warming in a London weather model
Met Office model runs with London's surfaces, compared with the same area as grass, found an urban increment of up to 5 K (5°C). It was largest at night, when the air near the ground is shallow.
LONDON · UNIVERSITY OF READING
≈ 1.5°C
Average warming from the urban heat island
Across 93 European cities in summer 2015, the population-weighted average extra warmth from urban heat islands was 1.5°C, ranging from 0.5 to 3.0°C between cities.
93 CITIES · THE LANCET 2023
The same Lancet study estimated that raising tree cover to 30% would cool cities by an average of 0.4°C and could have prevented about 2,644 of the 6,700 premature summer deaths it linked to urban heat islands.
FOUR THINGS TO NOTICE
You can see a heat island at work on any sunny day once you know what to look for. These are the features the research keeps coming back to.
Tree-lined streets, parks and ponds form cooler pockets inside a city, while dense, paved centres run warmest.
Pale, reflective roofing absorbs less sun than dark materials. The EPA lists cool options for both flat and pitched roofs.
Trees cool by shade and evaporation, and the EPA says urban trees can also absorb 15 to 27% of annual rainfall.
Calm, clear days make heat islands stronger. Wind and cloud weaken them by carrying heat away and blocking the sun.
WHY NIGHTS MATTER
In the University of Reading's model of London, the lowest layer of air over the city kept warming after sunset as heat stored in the buildings and streets was released. The urban effect was largest at night, when that layer is shallow.
The US EPA reports the same pattern from a review of US studies: urban areas were about 1 to 7°F warmer than outlying areas by day and about 2 to 5°F warmer at night. Narrow streets between tall buildings also block the breezes that would carry heat away.
Questions
What is an urban heat island?
It is a built-up area that is warmer than the countryside around it. Buildings, roads and other structures absorb and re-emit more of the sun's heat than forests or water, and cities have less greenery to cool the air.
Why are cities warmer at night?
Bricks, concrete and tarmac store heat during the day and release it slowly after sunset. In a University of Reading model of London, the urban warming effect was largest at night.
How much cooler is it under trees?
A review of 308 studies, cited by the US EPA, found urban forests were on average 1.6°C (3.0°F) cooler than urban areas without greenery. Trees cool by shading and by evaporating water from their leaves.
What is a cool roof?
A roof with high solar reflectance, so it absorbs less of the sun, and good thermal emittance, so it sheds the heat it does absorb. The EPA says cool roofs can lower the hottest indoor temperatures in homes without air conditioning by 1.2 to 3.3°C.
Does the heat island affect the UK?
Yes. The Met Office notes that buildings, waste heat, narrow streets and fewer trees all add warmth in UK cities, and suggests planting trees in open spaces and streets to reduce heat risk.
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