Place a small parcel at the trough and ridge lines andobserve the way the flow will spin the parcel, causingvorticity. The diameter of the parcel will be rotated fromthe solid line to the dotted position (due to the northerlyand southerly components of the flow on either side ofthe trough and ridge lines).Note that we have counterclockwise rotation at thetrough (positive vorticity), and at the ridge line we haveclockwise rotation (negative vorticity). At the pointwhere there is no curvature (inflection point), there is noturning of the parcel, hence no vorticity. This isdemonstrated at point Pin figure 1-5.Combined EffectsTo find the relative vorticity of a given parcel, wemust consider both the shear and curvature effects. It isquite possible to have two effects counteract each other;that is, where shear indicates positive vorticity butcurvature indicates negative vorticity, or vice versa (fig.1-6).To find the net result of the two effects we wouldmeasure the value of each and add them algebraically.The measurement of vorticity will be discussed in thenext section.It must be emphasized here that relative vorticity isobserved instantaneously. Relative vorticity in theatmosphere is defined as the instantaneous rotation ofvery small particles. The rotation results from windshear and curvature. We refer to this vorticity as beingrelative, because all the motion illustrated was relativeto the surface of the Earth.ABSOLUTE VORTICITYWhen the relative vorticity of a parcel of air isobserved by a person completely removed from theEarth, he or she observes an additional component ofvorticity created by the rotation of the Earth. Thus, thisFigure 1-6-Illustration of shear effect opposing the curvatureeffect in producing vorticity. (A) Negative shear and positivecurvature; (B) positive shear and negative curvature.Figure 1-7.-Contour-isotach pattern for shear analysis.person sees the total or absolute vorticity of the sameparcel of air.The total vorticity, that is, relative vorticity plus thatdue to the Earth’s rotation, is known as the absolutevorticity. As was stated before, for practical use inmeteorology, only the vorticity about an axisperpendicular to the surface of the Earth is considered.In this case, the vorticity due to the Earth’s rotationbecomes equal to the Coriolis parameter. This isexpressed as 2oI sin Ø, where w is the angular velocityof the Earth and Ø is the latitude. Therefore, theabsolute vorticity is equal to the Coriolis parameter plusthe relative vorticity. Writing this in equation formgives: (Za = absolute vorticity)Za=2cosin0+ZrEVALUATION OF VORTICITYIn addition to locating the areas of convergence anddivergence, we must also consider the effects ofhorizontal wind shear as it affects the relative vorticity,and hence the movement of the long waves anddeepening or falling associated with this movement.The two terms curvature and shear, whichdetermine the relative vorticity, may vary inversely toeach other. Therefore, it is necessary to evaluate bothof them. Figures 1-7 through 1-10 illustrate some of thepossible combinations of curvature and shear. SolidFigure 1-8.-Contour-isotach pattern for shear analysis.1-9

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