Air does not flow smoothly near the earth's surface; rather, it follows patterns of three-dimensional movement which are called turbulence. Turbulent eddies are produced by two specific processes: (1) thermal turbulence, resulting from atmospheric heating, and (2) mechanical turbulence caused by the movement of air past an obstruction in a windstream. Usually both types of turbulence occur in any atmospheric situation, although sometimes one prevails. Thermal turbulence is dominant on clear, sunny days with light winds. Although mechanical turbulence occurs under a variety of atmospheric conditions, it is dominant on windy nights with neutral atmospheric stability. Turbulence enhances the dispersion process although in mechanical turbulence, downwash from the pollution source can result in high pollution levels immediately downstream (Godish 1991).
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