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Wildfire in Yellowstone National Park produces a pyrocumulus cloud

A pyrocumulus, or, literally, fire cloud, is a dense cumuliform cloud associated with fire or volcanic activity.[1]

A pyrocumulus is similar dynamically in some ways to a firestorm, and the two phenomena may occur in conjunction with each other. However, one may occur without the other.

Contents

Formation

Wildfire in Yellowstone National Park produces pyrocumulus clouds
A pyrocumulus cloud from the August 2009 Station fire in southern California

A pyrocumulus cloud is produced by the intense heating of the air from the surface. The intense heat induces convection which causes the air mass to rise to a point of stability, usually in the presence of moisture. Phenomena such as volcanic eruptions, forest fires, and occasionally industrial activities can induce formation of this cloud. The detonation of a nuclear weapon in the atmosphere will also produce a pyrocumulus in the form of a mushroom cloud which is made by the same mechanism. The presence of a low level jet stream 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. 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 cloud viewed from above

Pyrocumulus is often grayish to brown in color, because of the ash and smoke 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 nuclei. This poses a problem, as the cloud can trigger another thunderstorm 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 cloud. This generates lightning strikes that can start up other fires as stated above.[2]

References

  1. ^ Pyrocumulus entry in the AMS Glossary
  2. ^ "Pyrocumulus by The Airline Pilots". http://www.theairlinepilots.com/met/pyrocumulus.htm. 
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