In the late 1960s, Pasquill developed a method for classifying atmospheric conditions which was later modified by Gifford (1975), resulting in six stability classes, labeled A through F. The method was based on the amount of incoming solar radiation, cloud cover, and surface wind speed as shown in Table 5.8.1.
Stability greatly affects plume behavior as demonstrated by the dispersion curves discussed above. Classes E and F indicate stable air in which stratification strongly dampens mechanical turbulence, typically with strong winds in a constant wind direction. These conditions can produce a fanning plume that does not rise much and retains a narrow shape in the vertical dimension for a long distance downwind as shown in part (a) of Figure 5.8.8.
A situation where a plume in a stable layer is brought quickly to the surface by turbulence in a less stable layer is termed fumigation and is shown in part (b) of Figure 5.8.8. This can occur as the result of heat convection in the morning.
Class D stability is neutral, with moderate winds and even mixing properties. These conditions produce a coning plume as shown in part (c) of Figure 5.8.8. Classes A, B, and C represent unstable conditions which indicate various levels of extensive mixing. These conditions can produce a looping plume as shown in part (d). If the effective stack height exceeds the mixing height, the plume is assumed to remain above it, and no ground-level concentrations are calculated. This effect is known as lofting and is shown in part (e).
Rough terrain or heat islands from cities increase the amount of turbulence and change the classification of ambient conditions, usually upward one stability class.
In general, a plume under stability class A conditions affects areas immediately near the emission source with high concentrations. Class F stability causes the plume to reach ground level further away, with a lower concentration (unless terrain is a factor).
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