Answer to Question 1
Answer: Urban heat islands occur because of modification to the energy balance that results when natural surfaces are replaced by artificial materials and when human activities release heat into the local environment.
Answer to Question 2
Answer: Urban heat islands occur because of modifications to the energy balance that result when natural surfaces are paved and built on and when human activities release heat into the local environment. When the sun is low in the skynear sunrise and sunset, and during much of the day at high latitudes in the winterdirect sunlight that would otherwise reach the horizontal surface hits the vertical walls of buildings. This causes the angle of incidence to become closer to perpendicular and increases surface heating, which leads to a higher temperature. The presence of buildings also affects the rate of heating by changing the surface albedo. Darker buildings, of course, absorb more sunlight than lighter ones, and, in general, urban surfaces (asphalt streets, roofing materials) have lower albedos than the natural surfaces they replace. The presence of buildings also affects the amount of absorption by causing multiple reflections to occur, as shown in Figure 1419. As sunlight penetrates the urban landscape and hits the side of a building, some of the energy is absorbed and some is scattered back as diffuse radiation. Some of the scattered radiation strikes an adjacent building, where once again a portion is absorbed. This process goes on repeatedly, with each successive reflection at least partially absorbed upon contact with another wall. This increases the total absorption so the albedo of the urban area is actually lower than the albedo of the individual surfaces.
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