9th September 2005
Pyrocumulus
Source: Excerpt from The Book " Weather "
Distribution: Any area where wildfires occur. Height: 2000 to 30,000 feet. Cause:
Convection created by fire.Associated Weather: Can generate showers and storms. Hazard Warning:
Clouds may develop into cumulonimbus and trigger further fires through lightning strikes.This variation of cumulus derives its name from the fact that fire (pyro in Latin) creates both the lifting mechanism and the water vapor that combine to form this cloud. An extensive wildfire produces vigorous rising air currents and a large quantity of water vapor that is released by the air and vegetation during combustion.
The rising air lifts the water vapor to a level where it condenses and forms cumulus clouds that ride above the fire. The bases of these clouds are often difficult to discern, as they are usually hidden by the smoke from the wildfire, but the cloud tops are normally situated well above the smokescreen.
Pyrocumulus clouds vary widely in vertical extent, from humilis to congestus size. In some cases, the cloud can produce rain showers that limit or even extinguish the blaze below. However, particularly in subtropical regions where condensation results from an abundance of moisture in the surrounding air mass, the clouds may continue to grow until they reach the cumulonimbus stage. In this case, lightning strikes from the cumulonimbus may trigger further fire outbreaks.
Pyrocumulus clouds may be seen wherever wildfires occur. They are inevitably more common in highly fire-prone areas such as California, the French Riviera, and southeastern Australia.
Acknowledgement due: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.
Check out Skyscapes for cloud photos taken from the aircraft.
