Discussion
A dust devil forms on a hot day with no synoptic scale rotation present in the region.
Where does its vorticity come from?
Answer: B
At this scale the Rossby number is enormous, so the planetary contribution is entirely negligible, and the rotation comes from ambient horizontal shear concentrated by the convergence beneath the thermal. This is why dust devils rotate in either sense with roughly equal frequency. The same argument explains why the popular claim about bath water is mistaken, since both are strongly ageostrophic small-scale flows. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.
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