On a hot summer evening, people driving out of a large city often notice that…

On a hot summer evening, people driving out of a large city often notice that the air in the countryside feels much cooler. This is not their imagination. Scientists have known for more than a century that built-up areas tend to be warmer than the land around them, an effect known as the "urban heat island". The difference is often a few degrees Celsius, and it is usually most noticeable after sunset, when fields and forests cool down quickly but streets and buildings go on releasing the heat they have stored during the day.

The main cause lies in the materials that cities are made of. Asphalt roads, concrete pavements and dark roofs absorb a large share of the sunlight that falls on them, rather than reflecting it back into the sky. They warm up during the day and then give off that heat slowly, long into the night. In the countryside, by contrast, much of the ground is covered by plants. Trees provide shade, and they also cool the air around them as water evaporates from their leaves, in much the same way that sweat cools the human body.

The shape of a city matters too. Tall buildings standing close together trap heat between them and block the winds that might otherwise carry it away. On top of this, cities produce heat of their own. Car engines, factories and, ironically, air-conditioning units all release warm air into the streets, so that the more people try to cool their homes, the hotter the outdoor air becomes.

These extra degrees are more than a minor discomfort. During heatwaves, hospitals in large cities admit more patients suffering from heat-related illness, and older people living alone are particularly at risk. Higher temperatures also speed up the chemical reactions that produce ground-level ozone, a gas that irritates the lungs. And because people use more electricity for cooling, power stations burn more fuel, adding to the greenhouse gases that are warming the planet as a whole.

The good news is that many solutions are surprisingly simple. Painting roofs white or covering them with light-coloured materials can reflect much of the sun's energy. "Green roofs", planted with grass and small plants, keep buildings cooler and also absorb rainwater. Above all, experts recommend planting more trees along streets and creating small parks wherever space allows. In Paris, for example, the city has begun replacing the asphalt in many school playgrounds with trees, plants and surfaces that let water through, so that the playgrounds can also serve as cool places for local people during heatwaves.

None of these measures will stop global warming on its own. But as more and more of the world's population moves into cities, keeping urban areas cool is likely to become one of the most important tasks facing city planners in the coming decades.