고등학교 공통영어2 (YBM 박준언) 연계 글

On a summer night, a city often feels warmer than the fields and forests around it. Scientists call such a city an urban heat island, a patch of heat surrounded by cooler land. In American cities, for example, daytime temperatures are about 0.5 to 4 degrees Celsius higher than in nearby rural areas. At night, the difference is about 1 to 2.5 degrees. The gap is usually largest in big cities with dense populations.

What a city is built from matters a great deal. Its streets and parking lots are paved with black asphalt, and many of its roofs are dark. Dark surfaces absorb more of the sun's energy than light ones do and turn it into heat. Asphalt and concrete also hold heat well. Much of what they absorb during the day is released slowly after sunset, so a city often stays warm long into the night.

What the countryside has and most cities lack is a thick cover of plants. Trees shade the ground, and their leaves help cool the air around them. In a city, however, much of the ground is paved or built on, so there is little room for plants. Cars, air conditioners, and factories also emit heat into the crowded streets. This extra heat is more than a matter of comfort. On the hottest days, people use more electricity for fans and air conditioners, and some become seriously ill.

Fortunately, a heat island can be cooled. One way is to cover roofs with white or other light-colored materials. A cool roof sends back much of what a dark roof would absorb, so the rooms below it and the nearby air stay cooler. Such a roof keeps working well as long as it stays clean, because dirt makes a bright surface less reflective.

Street trees help, too. Pavement in a tree's shade stays far cooler than pavement in full sun, provided that the tree has enough leaves to block the light. Parks and other green spaces can also become cool spots within a city.

Each of these changes is small on its own. Together, however, they can make even a crowded city far more comfortable, provided that the city plans for them as carefully as it plans its roads.