Turbulence

Turbulence in the atmosphere describes the random motion of a parcel of air at a specific point in space. This random motion in the atmosphere is a primary factor in determining the dispersion of air pollutants. Atmospheric turbulence is governed by mechanical processes such as wind and topography, and by thermal forces such as solar heating.

Under high wind speeds, the frictional force of air movement over the ground will create mechanical turbulence. Mechanical turbulence is the dominant mechanism for atmospheric mixing on windy days and at night.

When the sun heats the earth's surface, pockets of warm air will form near the ground. This less dense, warm air will rise from ground level to be replaced by cooler air. The cooler air is, in turn, heated and thus ascends creating thermal or convective turbulence. This vertical motion in the atmosphere becomes more intense as solar heating increases. Convective turbulence causes vigorous atmospheric mixing which allows air pollutants to disperse rapidly. During daylight hours, when winds are light to moderate, atmospheric overturning, or convection, is the dominant mechanism for pollutant dispersion.