Rockin' Roll: Fire Tornadoes and Horizontal Vorticity
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:
- Everything begins with a horizontal vortex, or roll. In the case of supercell tornadoes, the rolling motion occurs when fast winds high above the ground run into slower winds near the ground. This is known as a kind of wind shear called speed shear, and it imparts a horizontal rolling motion to the air as the opposing winds collide. Wind shear can also play a role in wildfires, but wildfires can generate horizontal vortices without the help of large-scale winds. This occurs on the forward side of the fire line, where horizontal temperature differences develop as hot air behind the fire meets cold air in front of the fire.
- Next, a horizontal roll encounters an updraft -- a column of warm air rising high into the atmosphere. As we discussed in the previous section, updrafts normally develop when the sun heats up the Earth's surface, but they can also occur over fires. If you think of the horizontal vortex as a Slinky and the updraft as a telescoping arm, you can form some concrete imagery around an otherwise invisible phenomenon. As the telescoping arm (updraft) lifts higher, the Slinky (horizontal roll) begins to bulge upward.
- If the updraft is strong enough, it will continue to raise the horizontal roll until a large section splits off and, eventually, stands upright. The now-vertical vortex obeys all of the same principles as a vortex that begins its life in the vertical position. It intensifies by sucking air into the "pipe" and then begins to elongate, pulling flames, smoke and embers high into the air.
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.