Vorticity, from the Greek word for whirlpool, is when a small air parcel spins horizontally about a vertical axis. Cyclonic circulation is referred to as positive vorticity and anticyclonic circulation is referred to as negative vorticity. If the air above a cyclonic vortex is diverging, the air in the vortex is drawn upwards, leading to an increase in the vorticity, or rate of spin. Conversely, if the winds aloft are converging, there will be low-level divergence and the region of vorticity will flatten and spread out and vorticity will decrease. The analogy is often drawn to the effect on spin when an ice skater stretches out their arms.
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