Pyrocumulus cloud
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Pyrocumulus, or fire cumulus, is a dense cumuliform cloud associated with fire or volcanic activity.
Formation
Pyrocumulus is produced by the intense heating of the air from the surface and the convection it causes, usually in the presence of heavy moisture. Phenomena such as a volcanic eruption, forest fire or occasionally industrial activities can induce formation of this cloud. The presence of a low level jet stream can enhance its formation. Condensation of ambient moisture (moisture already present in the atmosphere) as well as moisture evaporated from burnt vegetation or volcanic outgassing occurs readily on particles of ash.Pyrocumulus clouds contain severe turbulence which also results in strong gusts at the surface which can exacerbate a large conflagration. A large pyrocumulus cloud, particularly one associated with a volcanic eruption, may also produce lightning, a process not fully understood as yet, but probably in some way associated with charge separation induced by severe turbulence, and perhaps, by the nature of the particles of ash in the cloud. Large pyrocumulus can extend well above the freezing level, and the electrostatic properties of any ice which forms may also play a role. A pyrocumulus cloud which produces lightning is properly a type of cumulonimbus, a thunderstorm.
Pyrocumulus is similar dynamically in some ways to a firestorm, and the two phenomena may occur in conjunction with each other. However, one may occur without the other.
Appearance
Pyrocumulus is often greyish to brown in color, because of the ash and smoke associated with the fire. It also tends to expand, because the smoke and the ash involved in the cloud's formation increase the amount of condensation nuclei. This poses a problem, as the cloud can trigger another thunderstorm whose lightning can set off new fires.
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