Urban Heat Islands

In summer, large cities are often several degrees hotter than the countryside around them. Scientists call this effect an "urban heat island." One major cause is the materials used to build cities. Concrete, brick, and asphalt take in heat from the sun during the day and then slowly release it during the night. （ 1 ）, the air in a city stays warm even after the sun has gone down. Tall buildings make the problem worse, because they block the wind that would normally help to cool the streets.

This extra heat can be dangerous. On very hot days, more people become ill, and the city uses an enormous amount of electricity for air conditioning. The heat also makes the air dirtier and less pleasant to breathe. To deal with these problems, many cities are （ 2 ）. For example, they are planting more trees and creating small parks among the buildings. The leaves of trees provide cool shade and release water into the air, which helps to lower the temperature of the whole area.

Other cities are trying a different idea: they are painting the roofs of buildings white or other light colors. （ 3 ）, which absorb the sun's heat, light-colored surfaces send much of that heat back into the sky. Some cities are even building "green roofs" that are covered with plants. These simple and fairly cheap ideas are already helping to make hot cities a little more comfortable, and they are likely to spread to more places in the future. As the world grows warmer, keeping our cities cool will only become more important. Every tree and every white roof, however small, can be part of the answer.