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Fire whirl

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A fire whirl is a spinning column of air created by a fire.

Fire-whirl
Fire-whirl

These phenomena are also called fire devils or fire tornadoes. They are often composed of flames and ash. While they look like tornadoes, they are actually different. Most fire whirls do not extend from the ground to the cloud base. Their spinning comes from surface winds and heat-induced lifting. This is different from a classic tornado, which comes from a mesocyclone high in the atmosphere.

To understand how a fire whirl works, we must look at the movement of air. It begins with a whirl of wind, often made visible by smoke. Intense heat rises quickly from a fire, creating an updraft. This rising heat combines with turbulent wind conditions to create whirling eddies of air. These eddies can contract into a tight, tornado-like vortex. As the vortex spins, it ingests combustible gases and debris.

Some of these whirls can be very large. They can be more than 100 meters tall and several meters wide. They can even reach wind speeds over 160 kilometers per hour. Some can persist for more than 20 minutes.

Scientists recognize three main types of fire whirls based on their position. Type 1 whirls are stable and stay centered over the burning area. Type 2 whirls are either stable or transient and occur downwind of the fire. Type 3 whirls are steady or transient and center over an open area next to an asymmetric fire. There is also a broader classification system proposed by Forman A. Williams. This system includes five categories: whirls generated by fuel distribution, whirls above fuel in pools or water, tilted whirls, moving whirls, and whirls modified by vortex breakdown.

Fire whirls play a significant role in how wildfires spread. They can uproot trees that are 30 meters tall or even larger. These whirls aid in a process called "spotting." This happens when the whirl lifts burning materials, like tree bark, into the air. Stronger winds aloft then blow these burning embers away from the main fire. This allows the wildfire to propagate and start new fires in different locations.

History shows us how powerful these events can be. In 1871, a fire whirl burned the community of Williamsonville, Wisconsin. In 1923, a massive firestorm during the Great Kantō earthquake in Tokyo produced a gigantic fire whirl. This event killed 38,000 people in the Hifukusho-ato region in just fifteen minutes. In 1926, lightning strikes at an oil facility in California caused fire whirls that damaged buildings far from the fire. Scientists also studied fire whirls produced by firebombings in World War II and the atomic bombings of Hiroshima and Nagasaki.

Modern observations have provided even more specific data. During the 2003 Canberra bushfires in Australia, a violent fire whirl was documented. It had horizontal winds of 240 kilometers per hour and vertical air speeds of 120 kilometers per hour. This was the first known fire whirl in Australia to reach EF3 wind speeds on the Enhanced Fujita scale. In 2020, the U.S. National Weather Service issued a tornado warning for a pyrocumulonimbus near Loyalton, California. This was the first time such a warning was issued for a fire-induced cloud.

Fire whirls are connected to larger atmospheric systems. When large fires create clouds, these are called pyrocumulus or pyrocumulonimbus clouds. These clouds can sometimes spawn landspouts, waterspouts, or even tornadoes. In laboratory settings, researchers have discovered a specific mode called a "blue whirl." This happens during controlled experiments where soot production is very low. Because there is almost no soot, the typical yellow color of the fire disappears, leaving a blue flame. These blue whirls have a smaller flame length and a slower burning rate than standard fire whirls.

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