When wildfires or forest fires grow large enough and hot enough, they begin to control the weather around them.
Pyrocumulus clouds, where "pyro" refers to fire and "cumulus" refers to billowing, form above a fire.
Figure 1: A pyrocumulus cloud as seen from aloft.
Sometimes the process, which we will discuss now, is so intense that the pyrocumulus clouds create lightning and even rain. Unfortunately, the upper-level wind, which pushes storm clouds along, usually moves the raining part of the pyrocumulus away from the fire before it has a chance to rain on the fire itself.
There are three basic scientific and meteorological principles to keep in mind when trying to understand the pyrocumulus cloud.
1. Hot air rises.
2. Rising air cools.
3. Invisible moisture in air that cools condenses into visible cloud droplets.
The fire creates extreme heat, resulting in rising air. That rising air cools, condensing any moisture into visible cloud droplets.
Figure 2: A fire heats the air above it, causing it to rise.
Figure 3: Hot air above the fire rises. Rising air cools, causing the invisible moisture in the air to condense into visible cloud droplets.
Seems simple, right? Here's the fun(ky) part: on a super dry day in Arizona, during May or June when the fire season is peaking, there is very little moisture in the air. But for the cloud to have formed, there must have been moisture to work with.
So, where does the moisture to make the pyrocumulus cloud originate? It largely comes from the trees, brush, grasses and other vegetation that the fire is burning.
It's strange to think of the air over a fire having moisture from the plants being burned. But, it does!
Alas, a pyrocumulus cloud forms!
Figure 4: Putting it all together...
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