Frontal movement is forecasted by the geostrophic wind at the surface, and at the 700-hPa level at several points along the front. The basic idea is to determine the component of the wind at the surface and aloft, which is normal (perpendicular) to the front and, therefore, drives the front. Determination of the component normal to the front is made by triangulation.

1. Select two to four points along the front where a regular and reliable pressure gradient exists, and  determine the geostrophic wind by use of the geostrophic wind scale. (NOTE: Do not use the observed wind.) The wind speed should be determined a short distance behind a cold front and a short distance ahead of the warm front where a representative gradient can be found. The points on the isobars in figure 3-16 serve as a guide to the proper selection of the geostrophic wind measurements.

2. Draw a vector toward the front, parallel to the isobars from where the geostrophic wind was determined. The vectors labeled "y" in figure 3-16 illustrate this step.

originating at the point where the "y" vector intersects the front, and label this vector "x," as illustrated in figure 3-16.

4. At a convenient distance from the intersection, along the "x" vector, construct a perpendicular to the "x"

vector, letting it intersect the "y" vector. This is line c in figure 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 q to the nearest degree with a protractor, and determine the value of its sine by using trigonometric tables or a slide rule.

6. Let side a of the right triangle formed in step 4 represent the value of the geostrophic wind obtained in step 1, and call it "Cgs." Solve the triangle for side b by multiplying the sine of q by the value of Cgs. The resulting value of b is the component of the wind normal to the front, giving it its forward motion. The formula is

In the sample problem, if the Cgs was determined to be 25 knots and angle q to be 40, b is 19.1 knots, since the sine of angle q is 0.643.

As you can see, the components normal to the front should be equal on both sides of the front, and that in reality, it would matter very little where the component is computed in advance of or to the rear of the front. In cold fronts the reason that the component to the rear is chosen is that this flow, as well as this air mass, is the flow supplying the push for the forward motion. In the case of a warm front, the receding cold air mass under the warm front determines the forward motion, because the warm air mass is merely replacing the retreating cold air, not displacing it.

OTHER CONSIDERATIONS. The foregoing discussion neglected to discuss the effects of cyclonic and anticyclonic curvature on the isobars, and the effect of vertical motion along the frontal surfaces. The upslope motion along the frontal surfaces reduces the effective component normal to the front. Furthermore, the cyclonic curvature in the isobars indicates convergence in the horizontal and divergence in the vertical, further reducing the effective component normal to the front. For these reasons, the component normal to the front is reduced at the surface only by the following amounts for the different types of fronts and isobaric curvature:

Slow moving cold front, anticyclonic curvature . . . . . . . . 0%

. If the pressure gradient is forecast to increase, decrease the component by the least percentage.

. If the pressure gradient is forecast to decrease, decrease the component normal to the front by the highest percentage value.

. If the pressure gradient is forecast to remain static, decrease the component normal to the front by the middle percentage value as listed above.

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