How Cities Keep Cool
A Cities are often several degrees warmer than the countryside around them, a pattern known as the urban heat island effect. The main reason is the materials used to build them. Dark roads and roofs absorb sunlight during the day and release the heat slowly at night, while buildings block the wind that would otherwise carry it away. Heat from air conditioners, vehicles and factories adds to the problem. For residents the result is more than discomfort. In long heatwaves, hot nights stop the body from recovering, and the risk of illness rises, especially for older people.
B One obvious response is to plant trees. Trees cool in two ways: their leaves cast shade, and they release water vapour, which draws heat from the air as it evaporates. A street lined with mature trees can be noticeably cooler than a bare one. However, trees take years to grow, need water, and cannot be planted on every street, particularly in older districts where the space beneath the pavement is crowded with pipes and cables.
C Another option is to change the colour of surfaces. White or light-coloured cool roofs reflect much of the sunlight that would otherwise be absorbed, and cool pavements can be made with light aggregate or special coatings. These measures are cheap compared with large building projects, and some cities have offered grants to property owners who adopt them. A drawback is that reflective surfaces can add glare, and in cold winters they may slightly raise heating costs.
D Green roofs, covered with soil and plants, combine several benefits. They insulate the building below, absorb rainwater and provide a small habitat for insects and birds. Critics point out that they are heavy, so older buildings often need to be strengthened, and that they cost more to install than a conventional roof. Supporters reply that the extra cost is repaid over time through lower energy bills.
E Planners increasingly agree that no single measure is enough. The most successful programmes combine trees, lighter surfaces and open spaces with careful design of streets so that breezes can move between buildings. Involving residents is just as important. Programmes tend to work best where people understand why changes are being made, because householders control many of the roofs and gardens in a city. The writer suggests that cities that begin this work now will be better prepared for hotter summers in the future.