Page 310 - Fundamentals of Air Pollution

266 17. The Physics of the Atmosphere

Fig. 17-20. Local valley-ridge flow patterns. (Numbers refer to Table 17-4.)

and steel of urban areas heat quickly and have a high heat-storing capability
compared to the soil and vegetation of rural areas. Also, some surfaces of
buildings are normal to the sun's rays just after sunrise and also before
sunset, allowing warming throughout the day. The result is that the urban
area becomes warmer than its surroundings during the day and stores
sufficient heat that reradiation of the stored heat during the night keeps
the urban atmosphere considerably warmer than its rural surroundings
throughout most nights with light winds.
Under the lightest winds, the air rises over the warmest part of the urban
core, drawing cooler air from all directions from the surroundings (Fig.
17-21). Subsidence replaces this air in rural areas, and a closed torus
(doughnut)-shaped circulation occurs with an outflow above the urban