. Compendium of meteorology. Meteorology. -^ii . J I I 150 mb ^4===/=-200 300 400 500 600 values, as far as the lower troposphere is concerned, will be found in the frontal zone where 2U^ — dvg/dr) is at a minimum. As shown in Fig. 7, the warm air over the lower and intermediate portion of the polar-front surface has anticyclonic isentropic shear, increasing to great values in the upper troposphere, whereas the air above the upper part of the frontal surface has cyclonic shear, likewise increasing to high values in the upper tropo- sphere. The dividing line between anticyclonic and cyclonic sh
. Compendium of meteorology. Meteorology. -^ii . J I I 150 mb ^4===/=-200 300 400 500 600 values, as far as the lower troposphere is concerned, will be found in the frontal zone where 2U^ — dvg/dr) is at a minimum. As shown in Fig. 7, the warm air over the lower and intermediate portion of the polar-front surface has anticyclonic isentropic shear, increasing to great values in the upper troposphere, whereas the air above the upper part of the frontal surface has cyclonic shear, likewise increasing to high values in the upper tropo- sphere. The dividing line between anticyclonic and cyclonic shear runs almost vertically through the maxi- mum of west-wind velocity, which in the average con- dition represented by Fig. 7 is located above the place where the frontal surface intersects the 500-mb level. Isentropic upgliding or downgliding as defined by equa- tion (11) will reach larger values south of the velocity maximum than north of it. It is likely that this differ- ence in V, values north and south of the velocity maxi- mum does give rise to important horizontal divergence effects because the Tj-component represents a nongeo- strophic part of the total wind. The y,-divergence effect in the jet-stream region should work out as shown sche- matically in Fig. 8. Where there is "confluence" of the winds into the western beginning of a "jet stream," equa- tion (11) indicates a superimposed isentropic upgliding Vn> Q and "isentropic convergence" dv^/dr^ < 0 north of the line of maximum | y, | . Where the wind velocity decreases along the streamlines in the "delta" of a jet stream, equation (11) indicates isentropic downgliding «;, 0, north of of maximum | y, | values the isentropic divergence dv„/dri changes sign, as shown by the hatching in Fig. 8. In figuring out the effect of the isentropic divergence in changing the pressure field we may think of the dis- tribution of dVn/dri as representing in the first approxi- mation
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