Vorticity

Relative Vorticity: The spin or rotation around anything, where positive relative vorticity is defined as being counterclockwise. Relative vorticity is usually smaller than planetary vorticity.

You probably know that air around highs goes clockwise and air around lows goes counterclockwise. Well, meteorologists need a way to distinguish between the clockwise and counterclockwise rotation. If you were to go back up to the north pole again and look down on the earth, you would see that the earth rotates counterclockwise. Scientists decided to use that as a basis for positive vorticity. Anything else that also rotates counterclockwise is considered to have positive relative vorticity in the Northern Hemisphere, and anything that rotates clockwise has negative vorticity in the Northern Hemisphere.

There are two ways to get relative vorticity; through shear and curvature. Shear is merely a difference in wind speed or direction. This can cause the air to rotate. Notice in the figure that the faster winds on the top of the diagram are forcing the pinwheel to rotate clockwise.


Figure 4


Curvature builds on this concept. When parcels of air travel along, moving parallel to the height lines, they move through troughs and ridges. Winds on the outside of the turns are faster than on the inside. Look at Figure 5 below:


Figure 5


Notice that the pinwheel turns counterclockwise (positive relative vorticity) in the trough and clockwise (negative relative vorticity) in the ridge.