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This wildfire season is whipping up a flurry of "firenadoes" — pillars of flame that spring up from the inferno. Is there something in the weather this year that's sparking more of these bizarre fire whirls?
You can blame drought conditions in the western United States, but we may be seeing more firenadoes just because there are more smartphones out there to document them.
"I think when people saw them in the past, they just said, 'Wow,'" Bret Butler, a research mechanical engineer at the U.S. Forest Service's Missoula Fire Sciences Laboratory in Montana, told NBC News. "As long as nobody is hurt, they're impressive."
Butler and his colleagues study fire whirls as part of the Fire, Fuel and Smoke Research Program, headquartered at the Missoula Fire Lab. The flaming twisters can range in size from less than 3 feet (1 meter) in diameter to 2 miles (3 kilometers) in diameter and 3 miles (5 kilometers) in height. The winds can whip around at tornado speeds of more than 110 mph (50 meters per second).
Tens of thousands killed
The most devastating fire whirl on record killed more 38,000 people in less than 15 minutes during the Great Tokyo Fire of 1923, according to the lab's website. Other massive whirls were generated during the World War II bombings of Hamburg, Dresden and Hiroshima.
"Largely they formed because of the arrangement of buildings in the city, and the terrain," Butler said.
The conditions most likely to generate a firenado involve an intense wildfire that generates superheated air, rapidly rising amid low to moderate winds. The winds converge into the fire and generate a tornado-like vortex. The Weather Channel's Jon Erdman compares it to the way ice skaters speed up their spins by pulling their arms inward.
Extreme drought conditions, like the conditions we're seeing in California this season, can promote more powerful firenadoes. ""The drier the fuels, the more rapid the combustion and energy release," retired California firefighter Royal Burnett explained in a 2008 factsheet about the phenomenon.
"If there's anything that causes more intense fire behavior, the odds of having a fire whirl go up," Jason Forthofer, a mechanical engineer at the Missoula Fire Lab, told NBC News.
Forthofer said fire whirls are also more likely to arise on the lee side of a mountain — that is, the side facing away from the wind, where the air can become turbulent.
Fire whirls vs. fire tornadoes
The way some researchers tell it, fire whirls stay anchored to the ground — while "fire tornadoes" are true tornadoes that arise through a process of pyro-tornadogenesis. They anchor themselves to a storm cloud and lifting off the ground.
Either way, these twisters are super-dangerous. The Missoula Fire Lab notes that they can do damage equivalent to an EF-2 tornado — uprooting trees, tipping automobiles and tearing the roofs off houses.
The Missoula Fire Lab studies firenadoes primarily to provide the best guidance for firefighters battling wildfires. But the lab's advice may be of use to smartphone-toting witnesses as well.
"If they see a large fire, on the order of 100 feet, don't just stand there and watch it," Butler said. "You need to notify other people in the area, and back off."