A pyrocumulus cloud is produced by the intense heating of the air from the earths surface. The intense heat induces convection, which causes the air mass above the heat source to rise. As the air rises it expands and cools. If the water content in the rising air mass cools to the dew-point temperature in the atmosphere around it (the temperature at which condensation forms) then cloud formation occurs. This is the same mechanism that causes thunderstorms to form on a hot day.
Image credit: ©Amery Drage. Large pyrocumulus cloud from a fire in the Kalbarri National Park, 13th March 2014. The base of this fire is between 50-60km away from the photographer. Notice how the winds at a higher altitude are pushing the top of the cloud over to the right.
Image credit: ©Jordan Cantelo. Arial view of the Boddington fire, Feb 2015.
Phenomena such as volcanic eruptions, forest fires, and occasionally industrial activities can form of this type of cloud. The detonation of a nuclear weapon in the atmosphere will also produce a pyrocumulus, in the form of a mushroom cloud. Condensation of the moisture already present in the atmosphere, as well as moisture evaporated from burnt vegetation or volcanic steam, occurs readily on the particles of ash in the cloud. In reality, a pyrocumulus cloud is made up of smoke, ash and condensed water vapour. This is why they always have a greyish to brownish colour, with the tops looking more cloud like than the smoke plume closer to the ground.
Image credit: ©Brayden Marshall. View of the Boddington fire from the Perth metro area. The base of this fire is approximately 120km away from the photographers location.
Image credit: ©Craig Eccles. Small pyrocumulus cloud starting to form. Notice how the majority of the smoke is blown to the right but the hot air rising straight up is forming a cloud as water vapour condenses.
Pyrocumulus clouds often contain strong updrafts, which can cause strong wind gusts at the surface. This effect can make an already dangerous fire even more so. As the hot air mass rises, fresh air is pulled into the base. Even a relatively small fire can create it’s own in-draft, which further fuels the fires capacity to burn. A large pyrocumulus, particularly one associated with a volcanic eruption, may also produce lightning. A pyrocumulus which produces lightning is usually called a pyrocumulonimbus cloud, but not all pyrocumulonimbus clouds produce lightning.
There have also been many recorded examples of pyrocumulonimbus clouds producing rain. There are even a few examples where pyrocumulonimbus clouds, caused by forest fires, actually quenched the fire that spawned them.
Image credit: ©Sam Kaye – Sam posted this photo into PWL on 3rd Feb 2015. It shows the pyrocumulus cloud from the fire near Boddington. Notice how the smoke turns to cloud at about the same altitude and the conventional clouds.
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Article source: http://en.wikipedia.org/wiki/Pyrocumulus_cloud