Pyrocumulonimbus cloud

Picture of a pyrocumulonimbus cloud, taken from a commercial airliner cruising at about 10 km altitude.

The pyrocumulonimbus cloud (pyroCb) is a type of cumulonimbus cloud that forms above a source of heat, such as a wildfire, and may sometimes even extinguish the fire that formed it. It is the most extreme manifestation of a pyrocumulus cloud. According to the American Meteorological Society’s Glossary of Meteorology, a pyrocumulus is "a cumulus cloud formed by a rising thermal from a fire, or enhanced by buoyant plume emissions from an industrial combustion process." Analogous to the meteorological distinction between cumulus and cumulonimbus, the pyrocumulonimbus is a fire-aided or –caused convective cloud, like a pyrocumulus, but with considerable vertical development. The pyroCb reaches the upper troposphere or even lower stratosphere and may involve precipitation (although usually light), hail, lightning, extreme low-level winds, and in some cases even tornadoes.

The pyroCb was named following the discovery in 1998, that extreme manifestations of this pyroconvection caused direct injection of large abundances of smoke from a firestorm into the lower stratosphere. The aerosol of smoke comprising PyroCb clouds can persist for weeks, and with that, reduce ground level sunlight in the same manner as the “nuclear winter" effect. A pyrocumulonimbus may often form from the eruption column of a volcano.

In 2002, various sensing instruments detected 17 distinct pyrocumulonimbi in North America alone.

Alternate spellings and abbreviations for pyrocumulonimbus that may be found in the literature include pyro-cumulonimbus, pyro-cb, pyro-Cb and pyrocb. The World Meteorological Organization does not recognize the pyrocumulonimbus as a distinct cloud type, but instead classifies it simply as cumulonimbus.

2003 Canberra Firestorm

On the 18 of January 2003, a supercell thunderstorm formed from a pyrocumulonimbus cloud associated with a severe wildfire, during the 2003 Canberra bushfires in Canberra, Australia. The supercell resulted in a huge fire tornado, rated at EF2 on the fujita scale, the first confirmed violent fire tornado. The tornado and associated fire killed 4 people and injured 492.

See also

• Atmospheric convection
• Pyrocumulus