What is Atmospheric Stability?
Atmospheric stability is a measure of atmospheric status which determines whether air will rise, sink, or be neutral. In general stability refers to air tendency to rise or to resist vertical motion.
Just as the stability of the air influences lifting and cloud formation, it also affects the vertical movement of pollutants. Recall that when the air temperature decreases slowly with height (or if it increases with height), the air is said to be stable. Stable air resists vertical displacement and leads to higher pollutant concentrations near the ground. Unstable air, on the other hand, enhances vertical mixing, and any material introduced near the surface is easily displaced upward. This reduces pollution concentrations near the surface.
Atmospheric Stability Classification
Atmospheric stability is defined in terms of the tendency of a parcel of air to move upward or downward after it has been vertically by a small amount. Unstable atmospheres of stability classes A tend to develop vertical updrafts which increase boundary-layer turbulence intensity. Stable atmospheres close to class F tend to suppress vertical updrafts and reduce turbulence intensity.
Since it is difficult to measure turbulence intensity, correlations are estimated through wind speed, cloud height, cloud cover, solar radiation, etc.
And there are several schemes to expect the atmospheric stability classes. The earliest stability classification is attributed to Pasquill and this scheme only requires solar radiation and wind speed. And there are classes respectively correlated with Richardson Number, vertical temperature gradient, horizontal wind direction, etc.
