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02 November 2011

Real Time Cloud Observation: Horse-shoe Vortex


Sheldon Strydom writes:@SAWDIS @zerkerx my first ever horse-shoe vortex! Image shows it going behind a cumulus cloud. Phone image=bad quality.

SAWDIS:  Well done Sheldon and congratulations!!!  You captured one of the rarest of all clouds the ‘horseshoe vortex cloud’. 
This cloud forms in a region of rotating air, or vortex. Such vortices usually form vertically, sometimes leading to waterspouts or even tornados, but occasionally they can develop with a horizontal axis and give rise to a gently rotating crescent of cloud. Such a movement of air seems to happen when an updraught is sent into a spin upon reaching shearing horizontal winds. Rarely are conditions right for a cloud to appear within the spin.
Some say that a horseshoe brings luck only when it is pointing upwards and holding the luck in. Others claim that, when pointing down, it allows luck to fall onto those below. Horseshoe vortex clouds tend to point downwards and so any cloudspotters already fortunate enough to see one will want to stand below it to top up on their luck.  Jokes aside.

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.