Pyrocumulus, or fire cumulus, is a dense cumuliform cloud usually found at an altitude of 1500 m. This meteorological formation is actually rare.
Formation
As its etymology might suggest, pyrocumulus is produced by the heating of the air and the air convection it causes, often also in the presence of heavy moisture. Phenomena such as a volcanic eruption, forest fire or industrial activities can be at the origine of this cloud. Very little wind is required, so that water vapor released by the heat can form updrafts sufficiently high to reach condensation level, without dispersing into the ambient air.
Appearance
Pyrocumulus is often greyish to brown in color, because of the ash and smoke associated with the fire. In size, the cloud is an enormous cumulus. It also tends to expand, because the smoke and the ash involved in the cloud's formation increase the amount of condensation nuclei. This actually poses a problem, as the cloud can trigger a thunderstorm and emit lightning, setting off new forest fires.
Pyrocumulus can develop into a cumulonimbus in particularly instable conditions.