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On a hot summer afternoon, the centre of a large city can be several degrees warmer than the countryside around it. This effect, known as the urban heat island, has several causes. Dark surfaces such as asphalt roads and roofs absorb sunlight during the day and release the stored heat slowly at night, so cities often cool down far less after sunset than rural areas do. Buildings packed closely together trap warm air and block breezes, while vehicles, air conditioners and factories add heat of their own. At the same time, cities have fewer trees, parks and areas of open soil, which in the countryside cool the air as water evaporates from leaves and the ground. The consequences are more than a matter of comfort. During heatwaves, high night-time temperatures prevent the body from recovering, and older people and those with heart or lung conditions face a greater risk of serious illness. Higher temperatures also increase demand for air conditioning, which raises electricity use and, where power comes from fossil fuels, adds to the emissions that drive climate change. Because the problem is largely created by design, it can also be reduced by design. Planting street trees and creating parks provides shade and cooling. Roofs painted in light colours or covered with plants absorb less heat, and paving materials that reflect more sunlight can lower surface temperatures. None of these measures will make a city as cool as the countryside, but together they can make summers noticeably safer for the people who live there.