AUSTIN, Texas - Last week showed us just how quickly wildfires can spread.
The Big Sky Fire in Gillespie County even got big enough to produce clouds.
Wildfires burn everything in their path. But sometimes the most dangerous part of the fire is what's up above, in the smoke plume.
The extreme heat from a wildfire carries the smoke rapidly upwards and if there's any instability in the atmosphere a cloud can form. That cloud is called a pyrocumulus cloud and is an indication of extreme fire behavior.
The clouds are formed even if temperatures are very hot because the fire acts as an extremely hot point that can overpower the surrounding atmosphere even in Texas-like heat.
Wildfires can burn at temperatures hotter than 15,000 degrees which means the fire will always create some buoyancy.
If a fire is large enough the smoke plume can sometimes reach into the stratosphere. But that begs the question, clouds are made of water, so where does the water come from?
It comes from the trees and brush. Even in a severe drought, plants have lots of water inside them. Burning an entire forest evaporates literally tons of water contained in the trees, grass, and brush.
All that water is rushed upwards where it forms the pyrocumulus cloud when it cools, just like in a thunderstorm. And just like a normal thunderstorm, it can produce everything from gusty winds, lightning, and even rain.
When pyrocumulus clouds get that big, it's trouble since any lightning strikes can spread the fire farther away. Any rain that falls is a warning that the cloud is about to produce some big winds, sometimes in excess of 60 miles per hour. Those extreme winds blow embers which further spreads the wildfire and can bring down trees weakened from the fire's progress.
Texas can also get, and does get, pyrocumulus and pyrocumulonimbus clouds, though due to several factors, they don't happen as often as in the western states.
More often Texas pyrocumulus clouds are small and short-lived.