A pyrocumulus, or literally fire cloud, is a dense cumuliform cloud

Cumulus cloud

Cumulus clouds are a type of cloud with noticeable vertical development and clearly defined edges. Cumulus means "heap" or "pile" in Latin. They are often described as "puffy" or "cotton-like" in appearance. Cumulus clouds may appear alone, in lines, or in clusters...

 associated with fire or volcanic activity.

A pyrocumulus is similar dynamically in some ways to a firestorm

Firestorm

A firestorm is a conflagration which attains such intensity that it creates and sustains its own wind system. It is most commonly a natural phenomenon, created during some of the largest bushfires, forest fires, and wildfires...

, and the two phenomena may occur in conjunction with each other. However, one may occur without the other.

Formation

A pyrocumulus cloud is produced by the intense heat

Heat

In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...

ing of the air from the surface. The intense heat induces convection

Convection

Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

 which causes the air mass to rise to a point of stability, usually in the presence of moisture

Moisture

Humidity is the amount of moisture the air can hold before it rains. Moisture refers to the presence of a liquid, especially water, often in trace amounts...

. Phenomena such as volcanic eruptions, forest fires

Wildfire

A wildfire is any uncontrolled fire in combustible vegetation that occurs in the countryside or a wilderness area. Other names such as brush fire, bushfire, forest fire, desert fire, grass fire, hill fire, squirrel fire, vegetation fire, veldfire, and wilkjjofire may be used to describe the same...

, and occasionally industrial activities

Industry

Industry refers to the production of an economic good or service within an economy.-Industrial sectors:There are four key industrial economic sectors: the primary sector, largely raw material extraction industries such as mining and farming; the secondary sector, involving refining, construction,...

 can induce formation of this cloud. The detonation of a nuclear weapon

Nuclear weapon

A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission bomb test released the same amount...

 in the atmosphere will also produce a pyrocumulus in the form of a mushroom cloud

Mushroom cloud

A mushroom cloud is a distinctive pyrocumulus mushroom-shaped cloud of condensed water vapor or debris resulting from a very large explosion. They are most commonly associated with nuclear explosions, but any sufficiently large blast will produce the same sort of effect. They can be caused by...

 which is made by the same mechanism. The presence of a low level jet stream

Jet stream

Jet streams are fast flowing, narrow air currents found in the atmospheres of some planets, including Earth. The main jet streams are located near the tropopause, the transition between the troposphere and the stratosphere . The major jet streams on Earth are westerly winds...

 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

Lightning

Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...

. This is a process not fully understood as of 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. The World Meteorological Organization doesn't recognize pyrocumulus or pyrocumulonimbus as distinct cloud types, but rather classifies them respectively as cumulus

Cumulus

Cumulus is a type of cloud with the appearance of a lump of cotton wool.Cumulus may also refer to:*Cumulus Media, a radio broadcasting company*Cumulus , digital asset management software developed by Canto Software*Reinhard Cumulus, glider...

 (mediocris or congestus) and cumulonimbus.

Appearance

Pyrocumulus is often grayish to brown in color, because of the ash and smoke

Smoke

Smoke is a collection of airborne solid and liquid particulates and gases emitted when a material undergoes combustion or pyrolysis, together with the quantity of air that is entrained or otherwise mixed into the mass. It is commonly an unwanted by-product of fires , but may also be used for pest...

 associated with the fire. It also tends to expand because of the ash involved in the cloud's formation increase the amount of condensation nuclei

Cloud condensation nuclei

Cloud condensation nuclei or CCNs are small particles typically 0.2 µm, or 1/100 th the size of a cloud droplet ) about which cloud droplets coalesce. Water requires a non-gaseous surface to make the transition from a vapour to a liquid. In the atmosphere, this surface presents itself as tiny...

. This poses a problem, as the cloud can trigger a thunderstorm

Thunderstorm

A thunderstorm, also known as an electrical storm, a lightning storm, thundershower or simply a storm is a form of weather characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere known as thunder. The meteorologically assigned cloud type associated with the...

, from which the lightning can start another fire.

Effects on wildfires

A pyrocumulus cloud can help or hinder a fire. Sometimes, the moisture from the air condenses in the cloud and then falls as rain, often extinguishing the fire. There have been notable examples where a large firestorm has been extinguished by the pyrocumulus that it created. However, if the fire is large enough, then the cloud may continue to grow, and become a type of cumulonimbus cloud

Cumulonimbus cloud

Cumulonimbus is a towering vertical cloud that is very tall, dense, and involved in thunderstorms and other inclement weather. Cumulonimbus originates from Latin: Cumulus "Heap" and nimbus "rain". It is a result of atmospheric instability. These clouds can form alone, in clusters, or along a cold...

 known as a pyrocumulonimbus cloud, which may produce lightning and start another fire; however, this cloud type is not differentiated from cumulonimbus by the World Meteorological Organization

World Meteorological Organization

The World Meteorological Organization is an intergovernmental organization with a membership of 189 Member States and Territories. It originated from the International Meteorological Organization , which was founded in 1873...

.