Fire Storms

A major fire is news when it consumes homes and claims lives--but when it makes weather it's science.

By Mark Wheeler|Sunday, May 01, 1994
Fire fighters hate winds. They can make a fire fall to a puff or explode to a roar; they can make it jink to the left or right, or halt and spin in a circle. Nature's winds are fickle and unpredictable, and they can be a fire fighter's worst enemy. What's more, when conditions are right, fires spawn their own winds as the flames gobble up oxygen, sucking it in from all sides. These winds in turn can create a bizarre natural phenomenon: microweather, born of fire. Odd little weather systems, occurring within the larger system, can give rise to tornadoes filled with fire and noxious gases. They can cause huge thunderheads to form in a cloudless sky; they can make rain fall and lightning flash.

All these wonders start with the plume that is formed as the heated air rises from the fire in a column. Usually a strong prevailing wind quashes such a plume before it can grow. But when the fire is especially hot and the wind is weak, the plume can prevail. "Wind is the most critical weather component for fires," says Margaret Gross, a meteorologist for the National Weather Service in Riverside, California. "It determines more than anything else how fast a fire will spread. But when the power of the fire is greater than the wind, these large plumes can rise high into the atmosphere. Those are the fires that usually generate weather."

For a plume to develop, says Gross, the fuel load--leaves, twigs, and brush--must be high and its moisture content low. And there must be a little instability in the atmosphere at somewhere between 10,000 and 18,000 feet. In stable conditions, slowly sinking cold air stops a plume from developing, but the warm, rising currents of an unstable atmosphere are prime breeding grounds. Given these conditions, a plume--full of hot air, smoke, ash, and noxious gases--can grow and generate tremendous heat.

As the hot, smoky air rises, moisture in the atmosphere begins to condense on the ash and smoke particles, creating the right conditions for cloud development. Usually the cloud is a cumulonimbus--a thunderhead. Such clouds can be astoundingly huge. The one shown at right, from a 1993 fire in Santa Barbara, California, topped out at 38,000 feet.

As the cloud grows, the droplets start to collide and coalesce until they grow heavy enough to start falling as rain; the Santa Barbara cloud produced .4 inch of rain and several flashes of lightning in an otherwise clear sky. Unfortunately the rainfall from such a cloud rarely helps put out the flames, since the thundercloud usually drifts downwind, away from the fire.

Plumes can also breed tornadolike fire whirls. Like dust devils that form in desert sands, fire whirls grow from a heat source such as freshly burned, still-smoldering undergrowth or a burning log. Again a bit of instability is needed--warm, rising currents to support the whirl's lift. The circular motion can start when a light prevailing wind curves, perhaps from hitting a cliff or some other obstacle. A slight slope to the ground can also enhance the circulation.

Such whirls can grow to be 300 to 400 feet tall and 20 to 50 feet wide. They move at speeds of five to seven miles an hour and more, often igniting new fires in unburned areas as they steer their way over an erratic course. The good news is that fire whirls are much more short-lived than regular tornadoes, usually lasting no more than a minute until they twist away from the heat source and dissipate.

The cool of evening can allow a plume fire to play yet one more trick. Because the ground cools more quickly than the air, the air closest to Earth's surface normally cools faster than air higher up. In canyons or valleys, where horizontal circulation is hindered, the cooling can create a nighttime inversion layer, in which cold air near the surface is trapped by warmer air on top. A fire burning in a canyon can thus be partially smothered as the shield created by the cool night air descends.

Deprived of a continuous oxygen supply, the fire slows to a crawl, and it spends the night depleting the oxygen trapped under the inversion. But the fire is not truly dying; instead it's gathering its forces for a morning surge.

"As it uses up the available oxygen, the fire is also preheating the unburned fuel load that's surrounding it," says Gross. The heat radiating from the fire can decompose plants, causing a release of combustible gases that are also trapped by the inversion.

In the morning the sun begins heating the air; the inversion breaks up as the warming air starts to rise, and the fire receives a rapid infusion of fresh oxygen. The combination of preheated fuel, oxygen, and combustible gases can make a fire explode.

Such vagaries are the reason fire fighters hate fire weather, marvelous though it may be to science buffs. Indeed, in its training sessions California's Department of Forestry and Fire Protection uses a videotape of a particularly nasty fire whirl that occurred in 1989 during a major wildfire in northern California's Lassen County. A Reno, Nevada, television reporter was preparing his story when a huge fire whirl formed about 100 yards behind him. The journalist's expletive could be heard as the tall, roaring vortex, flinging embers and ash, shot across a highway into three parked fire trucks. The next segment of film shows a group of fire fighters scrambling to help a colleague. He never had a chance to run and was badly burned by the aberrant whirl.
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