In conjunction with the release of the 'fire tornado' video out of Brazil today (you may have seen the video on Yahoo's home page), I have decided to discuss the nature behind this rare phenomenon. The first video below, courtesy of the Associated Press, is the video made famous today on the web. The second video is from the 2006 Shekell Fire in Moorpark, California. After viewing both videos, you should have a pretty good idea of what a fire whirl looks like.
While the more technical name for this is a fire whirl, it is also commonly referred to as a fire tornado or fire devil. I first became aware of fire whirls when I began my internship at The Weather Museum in Houston, Texas. I never knew something so monstrous could happen within a fire, and I finally understood why battling a wildfire can prove to be a tough task.
I'll start off by discussing some basics first. A whirlwind is just a vortex of wind, or more simply put, a vertically rotating column of air. There are many different types of whirlwinds, including tornadoes, waterspouts, land spouts, fire whirls, dust devils, and snow devils. We can break whirlwinds down into two categories: Major Whirlwinds and Minor Whirlwinds. Major whirlwinds are produced from powerful storms that interact with other high altitude winds to create a funnel. Tornadoes, waterspouts, and land spouts fall into this category. Fire whirls, along with dust and snow devils, belong to what we refer to as minor whirlwinds. This category produces a funnel from local winds spinning on the ground, as opposed to higher in the sky. I will focus on fire whirls today and discuss some of the other whirlwinds in future posts.
Fire whirls are usually caused by forest fires, wildfires, and post harvest stubble burning; however, they can also form as a result of bonfires, oil fires, volcanic eruptions, or nuclear explosions. Most large fire whirls are typically associated with a wildfire. They can occur under many weather conditions, but are most common in calm or light winds because it allows the heat to build up more rapidly. The two main requirements for a fire whirl to occur are warm updraft and convergence. Some organized source of angular momentum must exist, whether it be from wind shear (an abrupt change in wind speed or direction) or the fire's convective column. This creates large swirl velocities as air is carried along into the fire plume. Fire whirls carry smoke, debris, and flames aloft, which can very easily increase the rate of fire spread.
Most fire whirls are approximately 30-200 feet tall, 1-2 feet in diameter, and last only a few minutes. There are some that can climb more than 3/5 of a mile high and carry winds over 100 miles per hour; these typically can last for twenty minutes or so. Even though a fire whirl is classified as a minor whirlwind, it is capable of causing quite a bit of damage as it can uproot trees that are nearly 50 feet tall. So where are fire whirls most likely to occur? The most common occurrence is on the lee side of a ridge where the heated air from the fire is sheltered from the general winds. Mechanical eddies (a current of air moving in a circular motion different from that of the main current) are produced when wind blows across the ridge, which in turn can trigger a fire whirl. If you're located on a flat terrain, the lee side of the fire near the outside edges of the front are where fire whirls will be most likely.
An extreme example of a fire whirl in history happened back in 1923 with the Great Kanto Earthquake that struck a region of Tokyo, Japan. Approximately 38,000 people were killed after a fire whirl roared over Rikugun Honjo Hifukusho, a Former Army Clothing Depot that they all packed inside for shelter. The fire whirl was a part of a firestorm that spawned as a result of the earthquake. It is believed that high winds from a nearby typhoon (another name for a hurricane) attributed to the fire whirl's creation. The cause of the fire whirl shown in the first video out of Brazil was due to strong dry winds impacting a brush fire.
Sources:
"1923 Great Kanto Earthquake: Facts, Discussion Forum, and Encyclopedia Article." AbsoluteAstronomy.com. Web. 26 Aug. 2010. http://www.absoluteastronomy.com/topics/1923_Great_Kanto_earthquake.
"Fire Whirl." Wikipedia, the Free Encyclopedia. Web. 26 Aug. 2010. http://en.wikipedia.org/wiki/Fire_whirl.
"Fire Whirls." Forest Encyclopedia Network. Web. 26 Aug. 2010. http://www.forestencyclopedia.net/p/p4/p140/p354/p450/p471.
"TORRO - Tornado FAQ's." TORRO. Web. 26 Aug. 2010. http://www.torro.org.uk/site/tfaq.php#whirlwind.
"Whirlwind (atmospheric Phenomenon)." Wikipedia, the Free Encyclopedia. Web. 26 Aug. 2010. http://en.wikipedia.org/wiki/Whirlwind_(atmospheric_phenomenon).
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