Cities That Trap Heat

(1) On a summer evening, the center of a large city can be several degrees warmer than the surrounding countryside. Climate scientists call this phenomenon the "urban heat island." Although it can be measured during the day, the difference is often most pronounced after sunset, when rural areas cool rapidly while streets and buildings continue to release the heat they have stored.

(2) Several features of the urban landscape contribute to the effect. Dark surfaces such as asphalt roads and tar roofs absorb much of the sunlight that reaches them, rather than reflecting it, and materials such as concrete and brick store this energy and give it back slowly. In addition, cities have relatively few trees and little exposed soil. In the countryside, plants release water vapor through their leaves, a process that cools the air around them much as sweat cools the skin. Finally, cars, factories and air conditioners all discharge waste heat into the streets, so that, ironically, efforts to cool individual buildings can make the neighborhood outside them warmer.

(3) The consequences are not merely a matter of comfort. During heat waves, high nighttime temperatures deprive the body of the chance to recover from the heat of the day, and elderly people and those with chronic illnesses are particularly at risk. Studies in several countries have also found that the burden is not evenly distributed: within the same city, poorer neighborhoods often have less vegetation and more paved surfaces than wealthier ones, and are therefore hotter.

(4) Urban planners have begun to respond. Some cities encourage "cool roofs," coated with light-colored materials that reflect sunlight, while others plant trees along streets or install gardens on rooftops. Such measures are rarely dramatic on their own, but researchers argue that, when combined, they can noticeably reduce local temperatures, and that, as heat waves become more frequent, the question is no longer whether cities should adapt, but how quickly.

(1) On a summer evening, the center of a large city can be several degrees warmer than the surrounding countryside. Climate scientists call this phenomenon the "urban heat island." Although it can be measured during the day, the difference is often most pronounced after sunset, when rural areas cool rapidly while streets and buildings continue to release the heat they have stored.

(2) Several features of the urban landscape contribute to the effect. Dark surfaces such as asphalt roads and tar roofs absorb much of the sunlight that reaches them, rather than reflecting it, and materials such as concrete and brick store this energy and give it back slowly. In addition, cities have relatively few trees and little exposed soil. In the countryside, plants release water vapor through their leaves, a process that cools the air around them much as sweat cools the skin. Finally, cars, factories and air conditioners all discharge waste heat into the streets, so that, ironically, efforts to cool individual buildings can make the neighborhood outside them warmer.

(3) The consequences are not merely a matter of comfort. During heat waves, high nighttime temperatures deprive the body of the chance to recover from the heat of the day, and elderly people and those with chronic illnesses are particularly at risk. Studies in several countries have also found that the burden is not evenly distributed: within the same city, poorer neighborhoods often have less vegetation and more paved surfaces than wealthier ones, and are therefore hotter.

(4) Urban planners have begun to respond. Some cities encourage "cool roofs," coated with light-colored materials that reflect sunlight, while others plant trees along streets or install gardens on rooftops. Such measures are rarely dramatic on their own, but researchers argue that, when combined, they can noticeably reduce local temperatures, and that, as heat waves become more frequent, the question is no longer whether cities should adapt, but how quickly.

(1) On a summer evening, the center of a large city can be several degrees warmer than the surrounding countryside. Climate scientists call this phenomenon the "urban heat island." Although it can be measured during the day, the difference is often most pronounced after sunset, when rural areas cool rapidly while streets and buildings continue to release the heat they have stored.

(2) Several features of the urban landscape contribute to the effect. Dark surfaces such as asphalt roads and tar roofs absorb much of the sunlight that reaches them, rather than reflecting it, and materials such as concrete and brick store this energy and give it back slowly. In addition, cities have relatively few trees and little exposed soil. In the countryside, plants release water vapor through their leaves, a process that cools the air around them much as sweat cools the skin. Finally, cars, factories and air conditioners all discharge waste heat into the streets, so that, ironically, efforts to cool individual buildings can make the neighborhood outside them warmer.

(3) The consequences are not merely a matter of comfort. During heat waves, high nighttime temperatures deprive the body of the chance to recover from the heat of the day, and elderly people and those with chronic illnesses are particularly at risk. Studies in several countries have also found that the burden is not evenly distributed: within the same city, poorer neighborhoods often have less vegetation and more paved surfaces than wealthier ones, and are therefore hotter.

(4) Urban planners have begun to respond. Some cities encourage "cool roofs," coated with light-colored materials that reflect sunlight, while others plant trees along streets or install gardens on rooftops. Such measures are rarely dramatic on their own, but researchers argue that, when combined, they can noticeably reduce local temperatures, and that, as heat waves become more frequent, the question is no longer whether cities should adapt, but how quickly.