A temperature inversion in the atmosphere in which the temperature, instead of falling, increases with height above the ground. With the colder and heavier air below, there is no tendency to form upward currents and turbulence is suppressed. Inversions are often formed in the late afternoon when the radiation emitted from the ground exceeds that received from the sinking sun. Inversions are also caused by katabatic winds, that is cold winds flowing down the hillside into a valley, and by anticyclones. In inversion layers, both vertical and horizontal diffusion is inhibited and pollutants become trapped, sometimes for long periods. Low-level discharges of pollutants are more readily trapped by inversions than high level dischargers, hence the case for high stacks. Furthermore, high level discharges into an inversion tend to remain at a high level because of the absence of vertical mixing.
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