vorticity

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vorticity The measure of the amount of rotary or circular motion in a water or air mass about a vertical axis. Two types can be distinguished: ‘relative vorticity’ is rotation relative to the Earth's surface (it is positive if cyclonic, with the rotation in the same direction as the Earth's direction of rotation, and negative if anticyclonic); ‘absolute vorticity’ is relative vorticity plus the component of the Earth's rotation about its axis. Absolute vorticity is at a maximum at the poles and zero at the equator.