Anti-twister
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An anti-twister is a tornado which rotates in a clockwise direction (or in a counterclockwise direction in the Southern Hemisphere). In all other respects, it is indistinguishable from a typical tornado.
Tornadoes are formed in the updraft zone of severe thunderstorms which have a rotational component to their motion. This rotation is generally the result, either directly in the case of a supercell thunderstorm, or indirectly, in a normal frontal thunderstorm, of the Coriolis effect. Only a small percentage of frontal storms develop rotation, and of those, only a small percentage go on to produce tornadoes. Within that group, only a tiny percentage is of the clockwise variety. Hence, the odds of encountering an anti-cyclone are vanishingly small.
See also: