Air moves in two ways wind and convection. Wind is caused by the sun, which creates weather systems in the troposphere. Convection is the manner which air rises and falls, depending upon air temperature. Cold air, is heavier than warm air; falls and it pushes the warmer, lighter air up.
Movement of air is an important factor in keeping the air free of pollutants. It is in stagnant air that pollution becomes isolated and localized. In temperature
inversion, a body of warm air moves over cooler surface air. When this happens convection can not take place and the air easily becomes polluted.