Horseshoe-vortices are one of the more bizarre cloud forms. They can form if there is a lot of vorticity (a measure of rotation) in the air. Shallow cumulus is ideal for the formation of horseshoe-vortices, although the vortices are rare nevertheless. The vortices form when the small cumulus updraft interacts with the surrounding air and forms a dipole vortex; if the cumulus is small it will dissipate quicker than the spinning vortex so the leftover vortex is seen at the top of the cumulus. The vortex stays visible longer since the vortex interacts (mixes) less well with the ambient air.
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