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Abstract
A review of the literature on observations of dust devils suggests that these whirls form like the rolling-up of a vertical vortex sheet when the stratification is slightly unstable. The linearized equations governing the stability of such flows are formulated. These are solved when the basic flow is a vertical vortex sheet in a fluid with uniform Brunt-Väisälä frequency. Also, the effects of a shear layer of finite width are considered. It is found how unstable stratification strongly intensifies the shear instability. Increase of stable stratification may decrease stability of some special disturbances.