Have you noticed how evenings cool off more in rural areas than they do in cities? Urban areas also tend to get hotter during the day than any nearby areas with lots of greenery.
It's mainly caused by the difference in materials that cover the ground in urban areas and the countryside. In the country, evaporation(蒸发) of water from soil and the leaves of plants helps to cool the air. Having fewer plants, cities have less evaporation and are unable to cool down the temperature.
Dark colours are another problem. Dark objects absorb all wavelengths of light, making the temperature increase more noticeably. In contrast, white objects reflect all wavelengths of light energy. Sadly, most parts of cities are covered by asphalt(沥青), steel, roofs and bricks which are often dark in colour.
As people drive cars, heat buildings, and run air conditioners, cities are generating waste heat and pouring it into the atmosphere directly. The waste heat adds to the solar energy trapped by the tall buildings.
But cities don't have to be so hot. Some cities have lightened their streets. This is done by covering black asphalt streets, parking lots, and dark roofs with a more reflective gray coating.
Having more green spaces also helps. Plants serve as a natural air conditioner. They catch heat, release vapour(蒸汽) into the air, and take away heat, contributing to cooler, fresher cities.
A. Cities also produce more heat than suburban areas.
B. The higher the temperature, the shorter the wavelength.
C. This phenomenon is known as the urban heat-island effect.
D. So it will not be transformed into heat which makes the air hot.
E. Changes in building materials have a minimal effect on city temperatures.
F. Much of the soil in cities, by contrast, has been covered with roads and buildings.
G. These changes can decrease air temperatures dramatically, especially in summer.
