How Fire Tornadoes Work

To a layperson, any funnel-shaped column of air is a tornado, but meteorologists tend to be more discriminating. They classify twisters into two broad categories -- supercell and nonsupercell -- based on how they form and how much damage they inflict. For a storm to be called a supercell tornado, it must have three characteristics [source: Williams]:

Contrary to popular belief, a tornado doesn't have to take the shape of a funnel sidewinding its way across the sky. Those without a funnel will appear as a broad cylinder or cone, sometimes a mile wide, swirling near the ground. Either way, these types of large tornadoes can cause significant damage, dismantling homes and buildings, flinging vehicles and uprooting trees.

Any other tornado that fails to meet these standards would fall into the nonsupercell category. Waterspouts, for example, are nonsupercell tornadoes that form over a body of water. Although they look, at first glance, like their larger, land-based buddies, they form in different ways, arising from cumulus clouds that haven't become thunderstorms and may never mature into thunderstorms. As a result, they have shorter life cycles and often wind speeds in the range of 33 to 45 mph (53 to 72 kph); that is, they rage a lot less than their stronger tornado cousins.

The same holds true for landspouts and gustnadoes. Landspouts occur over solid land, but they look and act like waterspouts. As a result, they cause far less damage than supercell tornadoes. So do gustnadoes, which are weak, short-lived (but awesomely named) tornadoes that can arise along the boundary between descending cold air and warm air at the surface. Meteorologists refer to such a boundary as a gust front, and although the strong winds spawned by such a front signal an approaching thunderstorm and possible supercell activity, they're not themselves full-blown tornadoes.

Finally, weather scientists recognize a third class of tornado-like storms known as whirlwinds. These common types of atmospheric systems occur when the sun heats dry terrain and causes a column of warm air to rise rapidly. As it does, the column of air will whirl, or rotate around a vertical axis, much like water draining from a basin. They become visible when they pick up debris from the ground and are often named to reflect the nature of that debris: dust whirls (or dust devils), sand whirls, snow whirls, even hay whirls.

Fire tornadoes develop when a blaze, not the sun, heats air above the surface of the Earth. They're not really tornadoes at all, but a special type of whirlwind (makes sense why they're known as fire whirls or fire devils in many parts of the world). Although they occur far less regularly than dust devils, fire tornadoes can develop readily over large fires. And they obey many of the same principles that govern the formation of true tornadoes.

Tornadoes, spouts and whirlwinds have something in common: They all serve as examples of atmospheric vortices -- air masses that spin about either a horizontal or vertical axis. Vortices can range in size from small eddies that swirl around the lee side of buildings to huge mesocyclones that churn in the gut of a thunderstorm. Large or small, most atmospheric vortices begin when air near the planet's surface is heated, either by the sun or by a fire on the ground.

Let's consider a vortex created on a hot day over dry terrain. In this situation, air near the ground absorbs more of the sun's energy and heats up faster than air higher in the atmosphere. As the temperature of the ground-level air rises, it becomes less dense and more buoyant. This superheated air then rises in columns or chimneys, creating strong updrafts that can extend thousands of feet into the air. In most cases, an upward-spiraling motion -- analogous to the whirlpool effect you observe when water drains from your bathtub -- develops within the column of air. Some of these vortices are weak and remain invisible. Others, spawned from intense updrafts, generate significant rotation as even more air is sucked into the spinning column. When they pick up dust or sand, they become clearly visible as whirlwinds.

Now imagine a different scenario: a wildfire burning out of control over several hundred acres of brush or timber. In this situation, it's the fire, not the sun, that increases the heat of the air near the Earth's surface. The results, however, are the same. Superheated air above the blaze rises rapidly in columns or, to use firefighter speak, in plumes. As the air rises, it begins to rotate, drawing in more air and slowly drawing the flames upward in a tight spiral. Most fire whirls stay small -- a foot or two in diameter. But some can grow to be 400 feet (122 meters) tall and 50 feet (15 meters) wide. In other words, they can assume proportions of a small tornado.

Notice that the axis of rotation in the examples above lies perpendicular to the ground. This is often the case, especially if extreme heating causes intense updrafts. But not every vortex begins its life standing straight up. Some begin on their backs (or bellies, if you prefer), with the axis of rotation oriented parallel to the ground. Then, an uplifting force tilts the horizontal vortex up until it stands on one end. Scientists now believe that this is how many fire whirls form. They also think the same processes explain how the vortex at the heart of a supercell tornado -- the mesocyclone -- develops and evolves. Up next, we'll look more closely at this peculiar aspect of vorticity and what conditions can create the perfect firestorm.

We're used to seeing tornadoes and fire whirls spinning in an upright position, so it's strange to think of them oriented any other way. Recently, however, scientists have proposed a new model to explain how vertical vortices develop. Here's how it works:

The fire whirl that results from this process may seem like a non-meteorological event, but many scientists would describe it as an example of microweather: a small-scale weather system created by a localized event. In the case of fire tornadoes, that event is a fire, usually a wildfire, but sometimes also a house fire or an urban conflagration caused by firebombing, nuclear detonations or natural disasters. The only requirement, then, is a large amount of dry fuel and an intense blaze concentrated in a relatively small area. When these conditions are met, fire tornadoes are likely to form.

As you can imagine, firefighters encounter these meteorological rarities far more often than average citizens. And they must be especially cautious. Fire tornadoes have been known to ignite new fires by moving into unburned territory. They also perform really nasty tricks, such as picking up flaming logs and hurling them great distances. Not surprisingly, many state forestry services include fire whirl basics in their training. The California Department of Forestry and Fire Protection, for example, shows its firefighters a 1989 video of a fire whirl that developed during a major wildfire and then raced forward to consume three parked fire trucks and a news crew filming the event.

Fortunately, fire tornadoes rarely cause mass casualties. Indeed, most of us will never see one in our lifetimes. Every now and then, though, thousands of people must face a roaring monster of flames, ash and noxious gases. In the next section, we'll look back at a few famous fire tornadoes that still smolder in the annals of history.

Given that a wildfire generally occurs in the countryside or wilderness area, most people never come in direct contact with the blaze or its secondary features, such as fire tornadoes. There have been occasions, however, where fire tornadoes raged in heavily populated areas, damaging property and claiming lives. Three of the most infamous cases involve urban conflagrations of epic proportions. One case led to the first serious study of fire tornadoes. Here they are, chronologically:

Oct. 8, 1871 -- The Great Peshtigo Fire. You probably know about Mrs. O'Leary's cow and the fire that tried to remove Chicago from the map, but did you know the same fate befell Peshtigo, Wis. (and sections of Michigan)? The blaze that consumed the booming mill town started on the same day as the Chicago fire and as a result of the same conditions: a dry summer, slash-and-burn farming practices and a powerful cold front that swooped in from the western U.S., bringing strong winds that turned a few small prairie fires into a raging inferno. Peshtigo, which contained hundreds of wooden structures and lumberyards overflowing with cut timber and wood scraps, sat in the middle of thick stands of pine and hardwood. When the fire reached the town, it found a ready supply of fuel. In a matter of minutes, a massive fire tornado swirled around Peshtigo, generating 100-mph (161-kph) winds and ambient temperatures topping out at 700 degrees Fahrenheit (371 degrees Celsius) [source: Hemphill]. People who didn't make it to the river, died in the flames. It still ranks as the worst U.S. fire disaster, with a final death toll of 2,000 [source: Hemphill].

Sept. 1, 1923 -- The Great Kanto Earthquake. When tectonic plates shifted far beneath Sagami Bay, 30 miles (48 kilometers) south of Tokyo, on Sept. 1, 1920, they unleashed the double whammy typical of such events: a strong earthquake followed by a tsunami. Those two disasters alone claimed thousands of lives in Tokyo and Yokohama. But then came the fire, which roared through neighborhoods of wooden houses and forced residents to flee ahead of the flames. Thousands raced to the Sumida River, hoping to take refuge near the water, only to be cut off by a roiling, 300-foot-tall (91-meter-tall) funnel of fire the locals referred to as a "dragon twist" [source: Hammer]. Nearly 45,000 people perished in the fire tornado, leaving the final death toll at 140,000 [source: Hammer]. When the fire finally stopped, 45 percent of Tokyo had been burned to the ground [source: Hammer].

April 7, 1926 -- San Luis Obispo Fire. On the morning of April 7, 1926, during a moderate thunderstorm over San Luis Obispo, Calif., lightning struck oil reservoirs at a Union Oil Company tank farm located 2.5 miles (4 kilometers) south of the city's center. The discharge ignited the oil itself or vapors above the oil, leading to a five-day fire that consumed approximately 6 million barrels of petroleum. Over the course of the disaster, eyewitnesses observed hundreds of tornadoes erupting as a result of the blaze.

J.E. Hissong wrote about the strange meteorological phenomenon in the April issue of Monthly Weather Review, marking the first scholarly treatment of fire whirls and their formation. In that paper, Hissong reported that the fire whirls were directly responsible for property damage and loss of life:

One of these whirlwinds left the vicinity of the reservoirs and traveling east-northeast about 1,000 yards picked up the Seeber cottage, just outside the tank farm, lifting it several feet in the air and carrying it about 150 feet north, where it was dropped in a field, a total wreck. Mr. A.H. Seeber and his son, who were in the house, were killed.

Hissong's paper also featured several photos of the fire tornadoes -- some of the first ever recorded. Today, thanks to the ubiquity of video cameras and YouTube, fire tornadoes are appearing more frequently on film. 2012 footage of a fire whirl in the Australian outback, shot by filmmaker Chris Tangey, has racked up more than 2.5 million views on YouTube. Tangey's video shows a red-orange braid of fire twisting nearly 100 feet (31 meters) into the air. It's an awesome sight and a good reminder that fire tornadoes -- part beauty and part beast -- are always best observed from a distance.