Dust Devils

Dust devils form most frequently in desert areas where the sun beats down on the poorly conducting sand surface. As Meg Hayes of Fairbanks has noted, they also occur in non-vegetated areas such as parking lots, especially if there are nearby large buildings to help cause irregular air currents. The hot desert or parking lot surface heats a thin layer of air just above--setting the stage for the dance of the sun devils.

Even though the hot near-surface air wants to rise up through the cooler, and therefore heavier, air above, quiet air tends to retain a degree of stability--a resistance to new air motion. One can visualize that resistance as something like the surface tension on water--it is strong but it does have a breaking point.

As the stability limit is approached, almost any sort of irregularity can cause the limit to be exceeded so that a bubble of hot surface air bursts upward. A place on the ground that is hotter than elsewhere will cause the limit to be surpassed there first. Or an irregularity caused by a minor gust of wind, the nearby motion of a car, and even the passage of a rabbit can initiate a dust devil.

The sudden uprush of hot air causes air to speed horizontally inward to the bottom of the newly-forming funnel. One of the rules of the physics of moving air is that its vorticity is preserved. The requirement that vorticity be preserved leads to a large speed-up in the circular motion of air spiraling inward to the bottom of the new funnel.

And so a dust devil is formed; it is an almost self-sustaining whirlwind that maintains a funnel-like chimney through which hot air moves both upward and circularly.

A dust devil may last less than a minute up to several hours. Air speeds up to 70 mph have been measured in vigorous dust devils. This enables them to pick up dust, leaves or, sometimes, rather large objects. Dust devils tend to form in groups; a large dust devil is sometimes seen to have little, shorter-lived devils traveling along behind it.