The cumulonimbus flammagenitus cloud (CbFg), also known as the pyrocumulonimbus cloud, is a type of cumulonimbus cloud that forms above a source of heat, such as a wildfire or volcanic eruption, and may sometimes even extinguish the fire that formed it. It is the most extreme manifestation of a flammagenitus cloud. According to the American Meteorological Society’s Glossary of Meteorology, a flammagenitus 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 CbFg is a fire-aided or –caused convective cloud, like a flammagenitus, but with considerable vertical development. The CbFg 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 combined effects of these phenomena can cause greatly increased fire-spread and cause direct dangers on the ground in addition to 'normal' fires.
The CbFg was first recorded in relation to fire 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 CbFg clouds can persist for weeks, and with that, reduce ground level sunlight in the same manner as the “nuclear winter" effect.
In 2002, various sensing instruments detected 17 distinct CbFg in North America alone.
On August 8, 2019, an aircraft was flown through a pyrocumulonimbus cloud near Spokane, Washington, to better study and understand the composition of the smoke particles as well as get a better look at what causes these clouds to form, plus see what kinds of effects it has on the environment and air quality. It was one of the most detailed flights through CbFg to date.
In 2021 alone, an estimated 83 cumulonimbus flammagenitus had formed.
Alternative names and World Meteorological Organization terminology
Alternate spellings and abbreviations for cumulonimbus flammagenitus that may be found in the literature include Cb-Fg, pyrocumulonimbus, pyro-cumulonimbus, pyroCb, pyro-Cb, pyrocb, and volcanic cb, having developed amongst different specialist groups In the media and in public communications, fire-driven examples are often referred to as fires 'making their own weather'.
The World Meteorological Organization does not recognize the CbFg as a distinct cloud type, but instead classifies it simply as the cumulonimbus form of the flammagenitus cloud, and uses Latin as the root language for cloud names ('pyro' is of Greek origin). This was formalised in the 2017 update to the WMO International Cloud Atlas, which states that any Cumulonimbus that is clearly observed to have originated as a consequence of localised natural heat sources will be classified by any appropriate species, variety and supplementary feature, followed by flammagenitus.
Notable events
1945 Hiroshima firestorm, Japan
On 6 August 1945, an intense cumulonimbus-like cloud was photographed above Hiroshima, long after the cloud generated by the atomic bomb had dissipated. The cloud was a result of the firestorm that had by then engulfed the city. Some 70,000–80,000 people, around 30% of the population of Hiroshima at the time, were killed by the blast and resultant firestorm.
1991 Pinatubo 'volcanic thunderstorms', Philippines
Volcanic eruption plumes are not generally treated as CbFg, although they are convectively driven to a large extent and for weaker eruptions may be significantly enhanced in height in convectively unstable environments. However, for some months after the climactic eruption of Mt Pinatubo in the Philippines in 1991, meteorological observers from the US military observed what they termed 'volcanic thunderstorms' forming near the summit: cumulus cloud complexes formed near the top of the buoyant ash plume, and frequently developed into cumulonimbus clouds (thunderstorms).