Do Thermals Spin?

by Peter Gray


    Of course, dust devils spin, so thermals probably do also, at least when they're close to the ground. For many years, I was convinced that dust devils rotated in random directions. However, based on a few flights in northern Washington State, where I kept more careful mental notes, I would guess that about 2/3 to 3/4 of the dust devils there spin counterclockwise, in agreement with the Coriolis Effect. Contrary to popular belief, the smaller-scale equivalent, water going down a toilet or drain, is essentially unaffected, and the rotation, if it is biased toward one spin direction, is the result of the geometry of each such basin (check it out for yourself!).

    The dust devils formed by thermals seem to be just large enough to be affected somewhat by the Coriolis Effect. If I have the choice, I usually opt for circling against the dust devil's rotation, most often to the right, in case this will produce a better climb rate by reducing my circling ground speed, thus bank angle and sinkrate.

    However, when I have reversed direction several times in one climb, I have rarely detected a significant advantage in one direction. What little angular momentum thermals start with, they seem to lose through drag in the surrounding air, and they probably start with little spin anyway. As with water going down a drain, very little spin momentum at a large radius can translate to a rapid spin when the radius shrinks to that of a dust devil.