A
pyrocumulus, or, literally,
fire cloud, is a dense
cumuliform cloudCumulus 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
firestormA 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. The Great Peshtigo Fire and the Ash Wednesday fires are two examples of...
, 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
heatIn physics and thermodynamics, heat is the process of energy transfer from one body or system due to thermal contact, which in turn is defined as an energy transfer to a body in any other way than due to work performed on the body....
ing of the air from the surface. The intense heat induces
convectionConvection is the movement of molecules within fluids . Convection is one of the major modes of heat transfer and mass transfer...
which causes the air mass to rise to a point of stability, usually in the presence of
moistureMoisture generally refers to the presence of water, often in trace amounts.The moisture content is often an important aspect of foodstuffs including cheese and many dried goods such as tea where excess moisture can promote bacterial growth, decay, molding, or rotting over time.Excessive moisture...
. Phenomena such as volcanic eruptions,
forest firesA wildfire is any uncontrolled fire that occurs in the countryside or a wilderness area. Reflecting the type of vegetation or fuel, other names such as brush fire, bushfire, forest fire, grass fire, hill fire, peat fire, vegetation fire, and wildland fire may be used to describe the same phenomenon...
, and occasionally
industrial activitiesAn industry is the manufacturing of a good or service within a category. Although industry is a broad term for any kind of economic production, in economics and urban planning industry is a synonym for the secondary sector, which is a type of economic activity involved in the manufacturing of raw...
can induce formation of this cloud. The detonation of a
nuclear weaponA nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion...
in the atmosphere will also produce a pyrocumulus in the form of a
mushroom cloudA 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 streamJet streams are fast flowing, narrow air currents found in the atmosphere of planets at 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
lightningLightning is an atmospheric discharge of electricity 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.
Appearance
Pyrocumulus is often greyish to brown in color, because of the ash and
smokeThis article is about the substance. For other uses, see Smoke .Smoke is the 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...
associated with the fire. It also tends to expand, because the smoke and the ash involved in the cloud's formation increase the amount of
condensation nucleiCloud condensation nuclei or CCNs are small particles 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 solid or liquid particles called CCNs...
. This poses a problem, as the cloud can trigger another
thunderstormA thunderstorm, also known as an electrical storm, a lightning storm, a hailstorm, 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...
whose lightning can set off new fires.
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 putting the fire out. There have been some 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 continues to grow and becomes a type of
cumulonimbus cloudCumulonimbus is a type of cloud that is tall, dense, and involved in thunderstorms and other intense weather. It is a result of atmospheric instability. These clouds can form alone, in clusters, or along a cold front in a squall line. They create lightning through the heart of the cloud...
. This generates lightning strikes that can start up other fires as stated above.