A pyrocumulus cloud, which is often grayish to brown in color, is formed during volcanic eruptions and large forest fires. This type of cloud is a dense cumuliform cloud, this means it has strong vertical development and clearly defined edges. Sometimes a pyrocumulus cloud can be produced by heavy industrial activity as well. The mushroom cloud produced by the detonation of a nuclear weapon is also a form of pyrocumulus.
In some ways, a pyrocumulus cloud is similar to a firestorm, a conflagration intense enough to produce and sustain its own wind system. Although, the two can occur at the same time, one is capable of forming without the other.
The intensely heated air from a fire or a volcanic eruption, induces convection, which is the difference of temperature difference layer in the atmosphere. Usually, in the presence of moisture, this causes the air mass to ascend to a point of stability, thus creating a pyrocumulus cloud. A low level jet stream can further encourage the cloud's formation.
In particles of ash, condensation easily occurs from moisture that already exists, and evaporated moisture from the conflagration's destruction, and volcanic out-gassing. The ash involved in the pyrocumulus cloud's formation increases the amount of condensation nuclei (cloud seeds). This may trigger a thunderstorm, from which lightning can spark another fire.
A pyrocumulus cloud that is created by a volcanic eruption is also capable of producing lightning. Although this occurrence is not fully understood, it is likely linked with charge separation induced by severe turbulence, and the nature of ash particles in the cloud. A pyrocumulus that is capable of producing lightning is actually form of thundercloud or cumulonimbus, and it's named pyrocumulonnimbus or pyroCb. This is the most extreme form of pyrocumulus.
A pyrocumulus can either aid or foil a fire. Because this type cloud contains sever turbulence, it can produce strong wind gusts, which can give a conflagration a boost of speed and strength. On the other hand, there have been numerous occasions where large firestorms were extinguished by the pyrocumulus clouds they created. This can happen, if the moisture from the air condenses in the cloud, and then rains down on the fire.
Although, these two types of "fire clouds", pyrocumulus and pyrocumulonimbus, are complex and unusual, the World Meteorological Organization doesn't recognize them as distinct cloud types. They are instead classified as cumulus and cumulonimbus.