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Scattered Clouds
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Wildfire weather explained

Published On: Aug 21 2013 11:12:00 PM CDT

Large wildfires make a lot of smoke, and if they get big enough can even create their own weather.

Residents can tell the fires are starting to have an effect on the weather when the puffy, white pyrocumulous clouds start to form on the smoke plumes.

It all comes down to basic physics and buoyancy.  And it basically works like a fire-powered thunderstorm. Hot air is less dense than cool air, and a wildfire creates a whole lot of very hot air which rises very quickly.

The rapidly rising air at the surface creates a vacuum, and since nature abhors a vacuum, the surrounding air rushes in to fill the void.

That rush of air is the wind we feel, and that wind can drive the fire’s intensity to extremes.

The winds created by this mechanism can be very strong, sometimes over 75 mph which can blow down trees, making fighting the fire that much more difficult.

During the day these winds blow towards the flames, feeding the fire.  But towards evening the whole cloud can collapse pushing air from higher up in the atmosphere back down to the ground.  This is called a downdraft.

The winds in the downdraft can be just as strong – or stronger – then the winds during the day.  But in the opposite direction. 

The downdraft winds can topple more trees, and spread the fire into new territory and/or back towards the fire lines.

These systems are very complex and – like thunderstorms – can even create their own lightning.

It is hard to believe, but the smoke plumes do contain a lot of water.

Each tree generally contains about 50 gallons of water, all of which when burned is evaporated and carried upwards by the fire’s updraft.

In very rare cases, the clouds created by these fires can cause rain, which then help to extinguish the fire below.