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SYFY WIRE Bad Astronomy

A Vortex of Fire. Bonus: Tumbleweedocalypse.

By Phil Plait
A fire tornado (really a fire dust devil) forms during a proscribed burn in Denver, Colorado in 2014. Credit: Thomas Rogers, from the video

On March 14, 2014, just a few weeks ago, firefighters began a “controlled burn” at Rocky Mountain Arsenal, a wildlife preserve between the city of Denver and the Denver airport. This is done (generally in cooler times of the year) to prevent the buildup of weeds and other plants that can fuel much larger fires once the weather gets hotter and drier.

On that day there were shifting winds, and a sudden change in the direction of the wind created—get this—a fire tornado! I’ve written about these spectacular events before (see below), but this time it has the added bonus of hundreds of tumbleweeds caught in the vortex, producing one of the more awe-inspiring displays of natural forces at work:

Yegads! I can only imagine what it must have been like to stand there and watch this ridiculously over-the-top phenomenon unfold. I hope they all had a change of underwear.

What causes a fire tornado? Well, it’s not actually a tornado by definition, since tornadoes descend from the bottoms of clouds. A fire tornado is pretty much a dust devil with fire added. As I wrote in an earlier post about a fire tornado:

Imagine: a fire starts. As the air is heated above the fire, it rises, and the upward motion can be very strong. This leaves a lower pressure spot at the fire, and the air from outside the fire rushes in to fill the gap. The air is very turbulent, and as the inward-moving air from one side hits air coming in from the other, swirls can form. These get amplified by the constant gale of air, and rotation on a larger scale can get started and sustained. The whirlwind gets pumped by the hot air rising, and the next thing you know you've got a full-blown tornado of fire. 

In general dust devils don’t have a lot of power; the rotation is rather weak and the wind isn’t blowing terribly hard. But their strength depends in part on how quickly the warm air in the middle rises, and when there’s fire in there, well, that adds a bit of a kick to it. In this case, the wind was enough to draw in all those tumbleweeds. Mind you, tumbleweeds have evolved to be picked up by wind; that’s kinda their whole point. Still, it does make for a jaw-dropping scene!

Tip o’ the firefighter helmet to Ed Yong.

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