Cities are less sunny

One reason people find the countryside attractive may be that they believe it to be sunnier. They are quite right. On average, city dwellers experience 5 percent to 15 percent fewer hours of sunshine over the year, 15 percent to 20 percent less solar radiation in total, and 5 percent less ultraviolet

Cities are less sunny

radiation in summer and 30 percent in winter. The drawing shows why this is so. Buildings shade the ground and the taller the buildings are, the deeper the shade that they cast. The extent of the shade depends partly on the height of the Sun in the sky. The streets are more shaded in winter when the Sun is low than they are in summer. This is also why there is such a large reduction in the amount of ultraviolet radiation reaching the ground in winter. A street that is shaded by the tall buildings on either side is known as an urban canyon. Depending on its orientation, the walls of the buildings lining the canyon may receive different amounts of solar radiation, or receive similar amounts but at different times of day.

Very little of the sunshine reaches street level, but as it is reflected back and forth most of the radiation is absorbed by the fabric of the buildings. As the buildings grow warmer they begin to radiate their heat. Where the buildings are tall, most of the absorption and radiation take place some distance above street level. It is shadier at street level, but the air is somewhat warmer than it would be otherwise, because the walls lining the street are

Solar radiation in the city radiating heat. This effect is greater in summer than in winter, but in winter large amounts of warm air leaks from heated buildings, so the air on the street is still relatively warm.

The temperature difference is especially noticeable at night. Out in the countryside, as the night falls, the ground, plants, and other objects begin radiating the heat they absorbed during the day and the temperature drops sharply. This also happens in the city, but the warm air escaping from buildings offsets one-quarter or even more of the heat that is being lost by radiation.

Shading is the main reason for the reduced sunshine at ground level, but there is another. City air is much dustier than country air—it is more polluted. The extent of the radiation loss varies according to the height of the Sun because of the varying distance that solar radiation must travel through the polluted air. When the Sun is low in the sky the city may lose almost one-third of its incoming solar radiation. The most dramatic losses used to occur when coal was widely used for domestic heating, so city air contained large amounts of smoke. This used to reduce the duration of sunshine by about 44 minutes a day in the center of London, England. The smoke also absorbed infrared radiation from the surface, so it helped to make the nights warmer. Cities have been much sunnier since coal burning was banned.

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