Definition

Convective cloud triggered by a heat source on the surface.

Description

A forest fire, volcano, or similar large heat source can cause the formation of cumulus type clouds. In extreme cases the cloud may develop into a Cumulonimbus (Cb) cloud.

The “mushroom cloud” which is a characteristic of a nuclear explosion is a form of pyrocumulus cloud.

A pyrocumulus cloud from the August 2009 Station fire in southern California

As the heat source (the fire or “pyro” which gives pyrocumulus its name) heats the air, the air rises and then expands and cools. If the air's liquid water content is such that the dewpoint temperature during the cooling is reached, condensation and cloud formation occurs. The air is very unstable due to the effect of the heat source and so the formation is cumuloform.