See illustration shown in figure 7-2-5. TO DETERMINE THE WIND SPEED, select the appropriate wind scale for your map and map scale. Determine the latitude at the point you want to compute the wind speed. Place the corresponding latitude number from your overlay on one side of the isobars closest to your point. Count the number of millibars between the latitude mark on the overlay and the overlay index (interpolation may be required). Multiply this number by 10 to obtain the geostrophic wind speed.

In our illustration, the point of latitude is 45. Align the 45-degree mark on the over-lay with the 1012 isobar at this latitude. The index mark rests on what would be the 1010 mb. Therefore, the number of millibars between the 45-degree mark and the index is 2 mb. Mul-tiplying this number (2) times 10 gives us a geostrophic wind speed of 20 knots.

Figure 7-2-5.Use of geostrophic wind scale (overlay type).

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