To this point we have discussed two of the factors that control the spin of air parcels. The third factor is Earths rotation. The effect of this rotation is seen only from space. There, we can see the effect of Coriolis force. An air parcel traveling from the equator toward the poles has its trajectory altered because Earth is rotating beneath the parcel. A parcel moving north in the Northern Hemisphere appears to curve to the right. Since it curves, a spin is imparted to it. This is known as the Coriolis factor or parameter. All three factors, the wind shear, curvature created by an air parcels position in a pressure pattern, and the curvature created by Earths rotation (the Coriolis parameter), impose a spinning motion.

Figure 8-5-3.Illustration of shear effect opposing the curvature effect in producing vorticity. (A) Negative shear and positive curvature; (B) Positive shear and negative curvature.

Therefore, absolute vorticity is equal to the Coriolis parameter plus relative vorticity.

Learning Objective: Define constant absolute vorticity trajectories and describe their use with upper-air charts.

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