Pyrocumulus

Pyrocumulus, or cloud of smoke, is a dense cumuliform cloud associated with fire or volcanic activity. A Pyrocumulus cloud is produced by intense heating of the surface air. Intense heat induces convection, which causes the air mass to rise to a point of stability, usually in the presence of moisture.

Volcanic eruptions, forest fires, burning and occasionally industrial activities can induce cloud formation. The detonation of a nuclear weapon in the atmosphere can also produce a Pyrocumulus in the form of a mushroom cloud, which is formed by the same mechanism.

The presence of a low level of jet stream can improve its formation. Condensation of ambient humidity (moisture already present in the atmosphere) as well as evaporated moisture from burnt vegetation or volcanic degassing occurs easily in ash particles.

Pyrocumulus are often gray to brown in color, because of the ashes and smoke associated with fire. It also tends to expand because the smoke and ash in the cloud increase the amount of condensation nuclei, and this can cause a Cumulonimbus.

A Pyrocumulus cloud can help or disrupt a fire. Sometimes the moisture in the air condenses into clouds and then falls like rain, often extinguishing the fire. However, if the fire is large enough, the cloud continues to grow and becomes a Cumulonimbus.

A large Pyrocumulus, particularly associated with a volcanic eruption, can produce lightning. This is a process not fully understood, but it is probably somehow associated with charge separation induced by severe turbulence, and perhaps by the nature of the ash particles in the cloud. Large Pyrocumulus may contain temperatures well below freezing. Pyrocumulus that produces lightning is a type of induced Cumulonimbus, in which case it is called Pyrocumulonimbus (or PyroCb).