Read an Academic Passage

[1]On a summer afternoon, the center of a large city can be several degrees warmer than the countryside around it. Scientists call this pattern an urban heat island. The difference is often greatest at night, because buildings and roads slowly release the heat they absorbed during the day, while fields and forests lose their heat quickly after sunset.

[2]Several features of cities produce this effect. Dark surfaces such as asphalt absorb most of the sunlight that strikes them instead of reflecting it. Tall buildings trap warm air and block the wind that would otherwise carry heat away. Cities also have fewer trees and fields, so less water evaporates from plants, a process that normally cools the air. Finally, cars, air conditioners and factories add heat of their own.

[3]Because heat islands raise energy use and can endanger health during heat waves, many cities now try to reduce them. Some paint roofs white so that they reflect sunlight. Others plant trees along streets or cover roofs with gardens. Each of these measures is modest on its own, but researchers estimate that together they can lower afternoon temperatures in a neighborhood by one or two degrees.