The behaviour of a plume emitted from an elevated source such as a tall stack depends on the degree of instability of the atmosphere and the prevailing wind turbulence. There are following geometric forms of plume as per atmospheric stability are discussed below:
i. Looping ( Strong instability): This is a common type of plume form occurs under super adiabatic condition with height to moderate wind speed on a hot summer
afternoon when large scale thermal eddies are present. These eddied carry portions of the plume to the ground level for short time periods, causing momentary high surface concentration of pollutants near the stack.
ii. Coning (Near neutral stability): Coning plume can occur under cloudy skies both during day and night when the lapse rate is essentially neutral. It causes less concentration of the pollutant near the stack.
iii. Fuming (Surface inversion): It occurs when the plume is dispersed in the presence of very light winds as a result of strong atmospheric inversion. For high stack fuming is considered a favorable meteorological condition because plume does not contribute to the ground pollution
iv. Fumigation (A loft inversion): It occurs under the uniform lapse rate but when the lapse rate changes from stable to unstable. A situation usually arising when an inversion is breaking up in early morning when the Sun comes up. This is called a fumigation. It is favored by clear skies and light wind and is more common in the summer season.
v. Lofting (Inversion below stack): When pollutants are emitted above the inversion layer they are dispersed vigorously in the upward wind direction.
vi. Trapping (Inversion below and above stack): When almost all emissions are trapped below the inversion layer thus creating one of the worst pollution situations.
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