Pyrocumulonimbus clouds

Above: Bureau of Meteorology, Australia

(Originally published at 11:38 a.m. MST January 11, 2017)

The Australian Government’s Bureau of Meteorology has an excellent article about clouds that can form over rapidly burning vegetation fires. Pyrocumulus clouds can develop into pyrocumulonimbus that can generate lightning miles away from the fire.  Below are excerpts from the article.


What are pyrocumulonimbus clouds?

They’re a thunderstorm that forms in the smoke plume of a fire (or nuclear bomb blast, or volcanic ash cloud). In Australia they most commonly form in large and intense bushfire smoke plumes. (The official name for clouds that form this way is ‘flammagenitus‘, but they’re commonly known as pyrocumulonimbus.)

How do they form?

The intense heat from the fire causes air to rise rapidly in the smoke plume. The rising hot air is turbulent and draws in cooler air from outside the plume, which helps cool the plume as it rises. As the plume rises to higher and higher elevations the atmospheric pressure reduces, causing the plume air to expand and cool even further. If it cools enough, the moisture in the plume air will condense and forms cumulus cloud, which, because it comes from the fire plume, we call ‘pyrocumulus’. The condensation process causes latent heat to be released, which makes the cloud warmer and more buoyant and causes the cloud air to accelerate upwards. Further expansion and cooling causes more moisture to condense and the cloud air to accelerate upwards even more. In the right conditions the cloud can accelerate into the lower stratosphere before losing buoyancy. Collisions of ice particles in the very cold upper parts of these clouds cause a build-up of electrical charge, which is released by giant sparks—lightning. Having produced a thunderstorm, the cloud is now known as ‘pyrocumulonimbus’.