A pyrocumulus or fire cloud is a dense cumuliform cloud associated with fire or volcanic activity.
A pyrocumulus cloud is produced by the intense heating
of the air
from the surface. The intense heat induces convection
which causes the air mass to rise to a point of stability, usually in the presence of moisture
. Phenomena such as volcanic eruptions
, forest fires
, and occasionally industrial activities
can induce formation of this cloud. The detonation of a nuclear weapon
in the atmosphere will also produce a pyrocumulus in the form of a mushroom cloud
which is made by the same mechanism. 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.
Pyrocumuli contain severe turbulence which also results in strong gusts at the surface which can exacerbate a large conflagration. A large pyrocumulus, particularly one associated with a volcanic eruption, may also produce lightning. This is a process not fully understood as yet, but is 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 pyrocumuli can contain temperatures well below freezing, and the electrostatic properties of any ice that forms may also play a role. A pyrocumulus which produces lightning is actually a type of cumulonimbus, a thundercloud and is called pyrocumulonimbus.
A 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.
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.