Why Fire Whirls Form

Goodness, gracious, great spinning walls of fire!

Several fire whirls were reportedly spotted by firefighters out in the Mojave Desert this weekend while working on the York Fire, according to the National Park Service.

At least one was captured on camera.

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The smaller cousins of the more extreme fire tornadoes, these dust devil-like phenomena can last from a few seconds to a few minutes, scattering embers all over, making a fire less predictable and more dangerous to fight.

They aren't all that uncommon when hot air and winds are present, but there's not much to do about them when they pop up besides wait.

Why do they form?

Two ingredients are needed for a fire whirl to form:

• Hot air rising.
• Rotation or spin in the atmosphere, usually near the ground. 

On a fire like York, vegetation is burning quite hot, meaning there’s an awful lot of air rising into the atmosphere. The hotter the fire the more intense the updrafts.

Add even a tiny bit of circular movement, say from winds swiftly changing direction out in the desert, and a fire whirl can take off.

What about fire tornadoes?

A National Weather Service document from 1978 gives some pretty clear definitions for various wind driven fire phenomena.

From smallest to largest they are:

• Fire devils: Can be anywhere from 3 to 33 feet in size, with winds weaker than 22 mph. They do not tend to impact fire spread all that much.
• Fire whirls: Can reach 100 feet in size with winds up to 67 mph, and can have a substantial impact on fire spread. The hazard only increases from here.
• Fire tornadoes: These are an "extreme hazard," as they can grow to 1,000 feet in size with winds up to 90 mph. 
• Fire storm: The scariest of the bunch as it drives the most violent, least predictable fire behavior with wind speeds rising above 110 mph. They can reach 10,000 feet in size, meaning they can also cover a huge area. The great fire storm of 1910 comes to mind. It was so bad that it completely changed how we view and fight fires in the United States.