vorticity

Vorticity is a measure of rotation in the atmosphere, or “spin,” in the air. If the spin, as viewed from above, is counterclockwise in the Northern Hemisphere, it is called a vorticity cyclonic and given a positive value. If the spin is clockwise, it is called a vorticity anticyclonic and given a negative value. The opposite conventions apply in the Southern Hemisphere. In forecasting large-scale weather events, one is concerned primarily with “relative vorticity” and focuses on the rotation of air around a vertical axis, much like the rotation of a hurricane over the earth’s surface. The vorticity caused by the Coriolis effect is another matter. Consider a large volume of air rotating around a common center just as a very large wheel would spin around its axis. One component of the relative vorticity is due to its pure rotation. The greater the wind speed, or V, in this circular motion and the smaller the radius of rotation, R, the greater the vorticity. The contribution toward relative vorticity from rotation alone is V/R. Because tornadoes have high wind speeds and nominal radii of only hundreds of meters, the vorticity associated with them is very high. Hurricanes have high winds, too, but the radius of rotation is greater, so their relative vorticity is less than for tornadoes. A second source of relative vorticity arises from wind shear, which can be defined as a change in wind speed between two locations. Wind shear in the horizontal imparts a spin to volumes of air caught in the flow. This shear gives rise to cyclonic vorticity because it imparts a counterclockwise spin to volumes of air caught in the flow. There is a good correlation between large values of vorticity and centers of low pressure in the atmosphere. The association of low pressure and high vorticity with foul weather is a generalization that is quite often true.