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Why cities stay warm after dark

On a still summer evening, someone cycling from the centre of a large city out into the surrounding countryside may notice that the air becomes cooler within a short distance. This contrast, known as the urban heat island effect, has been observed in cities on every inhabited continent. Interestingly, the difference between town and country is often greatest not in the afternoon, when the sun is strongest, but several hours after sunset.

The main cause lies in the materials from which cities are built. Asphalt roads and dark roofs absorb much of the sunlight that falls on them, while brick, stone and concrete store heat during the day and release it slowly at night. In the countryside, by contrast, much of the ground is covered by soil and vegetation. Plants draw water from the soil and release it through their leaves, and this evaporation uses energy that would otherwise warm the air. When fields and woods are replaced by paved surfaces, this natural form of cooling largely disappears.

The shape of a city matters as well. Where tall buildings line narrow streets, much of the heat given off by walls and roads at night is absorbed by neighbouring buildings instead of escaping to the open sky. The same buildings can slow the wind that would otherwise carry warm air away. Cities also produce heat of their own: engines, factories and air-conditioning units all release warmth into the streets, so on hot days, when air conditioners are working hardest, they add to the very problem they are designed to solve.

Warm nights are more than an inconvenience. Many people find it difficult to sleep well when their bedrooms stay warm, and several poor nights in a row can leave them less able to cope with the heat during the day. Older people, young children and those who are already unwell are generally considered the most vulnerable during long spells of hot weather. Higher temperatures also increase the demand for electricity for cooling, which can place extra pressure on power networks.

Fortunately, the effect can be reduced. Planting trees is one of the most widely recommended measures, because trees both shade the ground and cool the air around them. Roofs can be covered with plants, or painted in light colours that reflect sunlight rather than absorb it, and some cities have experimented with lighter-coloured pavements. Parks, rivers and fountains can also create pockets of cooler air.

None of these solutions is perfect, however. Trees need space, regular care and, in dry regions, a reliable supply of water, and their roots can damage pipes and pavements if species are poorly chosen. Highly reflective surfaces may bounce sunlight onto nearby walls or into the eyes of people walking past. For this reason, urban planners increasingly argue that there is no single answer, and that the best combination of measures depends on the climate, the layout and the needs of each particular city.