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Pyrocumulus Clouds, boiling up on the Weather Notebook.
Fri Oct 22, 2004
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Pyrocumulus Clouds, boiling up on the Weather Notebook.
Cumulus and cumulonimbus clouds often sprout from a heated surface and rising moist air. Usually the sun provides the heat, but where wildfires rage, a special brand of cumulus arises: the pyrocumulus cloud. Hi, I’m Bryan Yeaton, and this is The Weather Notebook.
“Pyro” comes from the Latin word for fire, and these clouds arise like the phoenix from burning vegetation. In large fires, the required moisture comes from the soil, living plant tissue and standing water evaporated by the heat.
These clouds often first develop amidst the rising smoke plume. When conditions are right, the hot, moist air forms thunderclouds. Pyro-generated thunderstorms often have a mixed blessing. Rain falling from them can quench a small fire or cool a larger one. However, lightning arcing out of the clouds can also start new fires, often springing up some distance from the storm’s location.
When a swiftly moving stream of very dry air flows over the fire zone, it can rapidly generate severe thunderstorms. Fire meteorologists call this a “blow up.” The greatest concerns to firefighters from pyro-thunderstorms are the storm-generated winds. They not only fan the flames but strong downdrafts and variable gusts can push the fire suddenly in different directions, potentially trapping firefighters.
In the extreme, the swirling fiery winds surrounding the fire create a firestorm with gale force winds and fire devils or small tornadoes. An extreme fire storm struck the Wisconsin logging community around Peshtigo in October 1871 — the same day as the great Chicago fire. The firestorm killed nearly twelve hundred people and destroyed over 1.25 million acres of timberland. To this day, the Great Peshtigo Fire ranks as the deadliest fire in US history, natural or otherwise.
Thanks to our contributing writer, meteorologist Keith Heidorn. The Weather Notebook is a program of the Mount Washington Observatory, funded by Subaru of America.
More Pyrocumulus Photos
http://www.astro.ucla.edu/~obs/t19990830.html
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