The Devil is in the details

The other day I was in the car with Mrs. Bad Astronomer, and while we were stopped at a light she suddenly pointed out the window and shouted, "Dust devil!" I took a look, and sure enough, there was a little swirl of wind circling around in front of the car. It was about a foot across, and had picked up some leaves and detritus, whirling around like a miniature and delicate tornado. Dust devils are in fact similar to tornados, but are not nearly so violent. I've seen lots of them, from small ones that can barely pick up leaves, to a big one once when I was in college. That one had formed -- I think -- due to the unusual shape of a building. Wind would come around the building, create a vortex, and was fed by warm air in the surrounding field. It was easily two ...