VORTICITYLEARNING OBJECTIVES: Recognize the twocomponents of relative vorticity. Define theterm absolute vorticity. Determine vorticityimpacts on weather processes.Vorticity measures the rotation of very small airparcels. A parcel has vorticity when it spins on its axisas it moves along its path. A parcel that does not spinon its axis is said to have zero vorticity. The axis ofspinning or rotation can extend in any direction, but forour purposes, we are mainly concerned with therotational motion about an axis that is perpendicular tothe surface of the Earth. For example, we could drop achip of wood into a creek and watch its progress. Thechip will move downstream with the flow of water, butit may or may not spin as it moves downstream. If itdoes spin, the chip has vorticity. When we try to isolatethe cause of the spin, we find that two properties of theflow of water cause the chip to spin: (1) If the flow ofwater is moving faster on one side of the chip than theother, this is shear of the current; (2) if the creek bedcurves, the path has curvature. Vorticity always appliesto extremely small air parcels; thus, a point on one ofour upper air charts may represent such a parcel. Wecan examine this point and say that the parcel dots ordoes not have vorticity. However, for this discussion,larger parcels will have to be used to more easilyvisualize the effects.Actually, a parcel in theatmosphere has three rotational motions at the sametime: (1) rotation of the parcel about its own axis(shear), (2) rotation of the parcel about the axis of apressure system (curvature), and (3) rotation of theparcel due to the atmospheric rotation. The sum of thefirst two components is known as relative vorticity, andthe sum total of all three is known as absolute vorticity.RELATIVE VORTICITYRelative vorticity is the sum of the rotation of theparcel about the axis of the pressure system (curvature)and the rotation of the parcel about its own axis (shear).Figure 1-4.-Illustration of vorticity due to the shear effect.The vorticity of a horizontal current can be broken downinto two components, one due to curvature of thestreamlines and the other due to shear in the current.ShearFirst, let us examine the shear effect by looking atsmall air parcels in an upper air pattern of straightcontours. Here the wind shear results in each of thethree parcels having different rotations (fig. 1-4).Refer to figure 1-4. Parcel No. 1 has stronger windspeeds to its right. As the parcel moves along, it will berotated in a counterclockwise direction. Parcel No. 2has the stronger wind speeds to its left; therefore, it willrotate in a clockwise direction as it moves along. ParcelNo. 3 has equal wind speeds to the right and left. It willmove, but it will not rotate. It is said to have zerovorticity.Therefore, to briefly review the effect of shear-aparcel of the atmosphere has vorticity (rotation) whenthe wind speed is stronger on one side of the parcel thanon the other.Now let’s define positive and negative vorticity interms of clockwise and counterclockwise rotation of aparcel. The vorticity is positive when the parcel has acounterclockwise rotation (cyclonic, NorthernHemisphere) and the vorticity is negative when theparcel has clockwise rotation (anticyclonic, NorthernHemisphere).Thus, in figure 1-4, parcel No. 1 has positivevorticity, and parcel No. 2 has negative vorticity.CurvatureVorticity can also result due to curvature of theairflow or path. In the case of the wood chip flowingwith the stream, the chip will spin or rotate as it movesalong if the creek curves.To demonstrate the effect of curvature, let usconsider a pattern of contours having curvature but noshear (fig. 1-5).Figure 1-5.-Illustration of vorticity due to curvature effect.1-8