Figure 3-15.-Infrared, local midnight, third night.vector, letting it intersect the “y” vector. This is line c infigure 3-16.5. The angle formed at the intersection of the “y”vector and the perpendicular originating from the “x”vector is labeled q(theta). Measure angle qto thenearest degree with a protractor, and determine thevalue of its sine by using trigonometric tables or a sliderule.6. Let side a of the right triangle formed in step 4represent the value of the geostrophic wind obtained instep 1, and call it “Cgs.” Solve the triangle for side b bymultiplying the sine of qby the value of Cgs. Theresulting value of b is the component of the windnormal to the front, giving it its forward motion. Theformula isb= Cgs x sin qFigure 3-16.-Geostrophic wind method.In the sample problem, if the Cgs was determined tobe 25 knots and angle qto be 40°, b is 19.1 knots, sincethe sine of angle qis 0.643.As you can see, the components normal to the frontshould be equal on both sides of the front, and that inreality, it would matter very little where the componentis computed in advance of or to the rear of the front. Incold fronts the reason that the component to the rear ischosen is that this flow, as well as this air mass, is theflow supplying the push for the forward motion. In thecase of a warm front, the receding cold air mass underthe warm front determines the forward motion, becausethe warm air mass is merely replacing the retreatingcold air, not displacing it.OTHER CONSIDERATIONS.— The foregoingdiscussion neglected to discuss the effects of cyclonicand anticyclonic curvature on the isobars, and the effectof vertical motion along the frontal surfaces. Theupslope motion along the frontal surfaces reduces theeffective component normal to the front. Furthermore,the cyclonic curvature in the isobars indicatesconvergence in the horizontal and divergence in thevertical, further reducing the effective componentnormal to the front. For these reasons, the componentnormal to the front is reduced at the surface only by thefollowing amounts for the different types of fronts andisobaric curvature:Slow moving cold front,anticyclonic curvature . . . . . . . . 0%Fast moving cold front,cyclonic curvature . . . . . . . . 10-20%Warm front . . . . . . . . . . . . .20-40%Warm occluded fronts . . . . . . . . 20-40%Cold occluded fronts . . . . . . . . . 10-30%3-19

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