Dance of the Dust Devils

A hitchhiker I toted on the Dalton Highway recently told me he'd seen a "dust devil" dance down the road like a mini tornado. I felt deprived because I'd never seen one. As if by design, one appeared in front of me the next day.

While driving down a dry gravel road, I saw dust rotating on the ground like a dog chasing its tail. The funnel of twisting air grew until it rocked the tips of nearby spruce trees. Then it ran to the surface of a nearby river, where it touched down with the sound of a plastic tarp shredding in the wind.

Dust devils, also called whirlwinds and willy-nillys (in Australia), happen frequently in the desert and occasionally on Alaska roads, parking lots, and other hot surfaces.

Dust devils are born when sun-warmed air over a road or parking lot rises rapidly through layers of cooler air above, said Dave Hefner, a meteorologist with the National Weather Service in Fairbanks. As a blob of warm air quickly rises, it's turned in a corkscrew motion by winds whipping around nearby objects or by other air disturbances as slight as those caused by a passing car.

As the bubble of warm air quickly rises and begins spinning, the air pressure at ground level drops. Warm air from the ground that rushes to fill the vacancy within the cone of the dust devil begins spinning faster and faster. The effect is similar to that of a figure skater who increases the speed of a spin by pulling her arms closer to her body, Hefner said.

As air spins faster, the air pressure in the center of the whirlwind drops lower. As more hot air is drawn within the cone, the tiny tornado feeds on itself. The rotating column of air becomes a dust devil when airborne dirt and debris create a funnel we can see.

The dance of the dust devil may last from a few minutes to several hours. Dust devils eventually burn themselves out when the air pressure within the cone rises to the pressure of the surrounding environment. Along their paths, dust devils mostly carry dust, dry leaves, and other feathery things, but they also may pick up enough strength to pull shingles from a roof.

Jerry Dennis, author of It's Raining Frogs and Fishes, describes dust devils as the "smallest of the world's rotation, convection-fed storms." The largest convection-fed storm is a tornado. Born during a thunderstorm, a tornado forms from a mesocyclone--a column of warm air set spinning by inconsistent winds encountered at varying altitudes. A mesocyclone sometimes forms on the ground, acting like a spinning, horizontal tube of air. Warm updrafts force the funnel of moving air to stand up. It then gains strength and grows into the champagne-glass shape of a tornado.

Alaska isn't especially tornado prone because stormclouds big enough to create tornadoes rarely form here. Anvil-shaped cumulonimbus clouds over Alaska don't generate enough winds at their summits--from 10,000 feet elevation to about 30,000 feet--to sustain tornadoes, Hefner said. Cumulonimbus clouds grow much larger in the "tornado belt," which stretches from central Texas to Nebraska.

In a recent 25-year period, 1,326 tornadoes were reported in Oklahoma alone, according to a tally published in the book Meteorology Today. Only one tornado was reported in Alaska during that quarter century. After reading about black funnel clouds roaring down on the Kansas plains with the noise of 1,000 freight trains, I'm glad I live in Alaska. I'll take dinky dust devils any day.