GEOSTROPHIC AND GRADIENT WIND SCALES
Wind scales are used to compute wind speed or isobaric spacing in sparse data areas. There are many sizes, types, and shapes of scales, but theyall have one thing’ in common. They are in the form of tables and graphs which solve geostrophic or gradient wind equations. Those types com-monly used by personnel in our rating are
presented in the publication Meteorological
Wind-Scales, NAVPERS 50-1P-551.
Wind scales are designed based on
map projection (Polar Stereo-graphic, Mercator,
and Lambert Conformal) and
presented in the publication Meteorological Wind-Scales, NAVPERS 50-1P-551. Wind scales are designed based on map projection (Polar Stereo-graphic, Mercator, and Lambert Conformal) andmap scale. They can also be designed for surface or upper air use. Most maps have a wind scale printed on them, but many overlay types have also been developed.
GEOSTROPHIC and GRADIENT WINDS are approximations of the true wind. When these winds are computed, the effect of friction is dis-counted, and the pressure field is considered un-changing. Geostrophic wind parallels straight isobars, and gradient wind parallels curved isobars. The gradient wind is a better approxima-tion of the true wind, because the path of true wind is more curved than straight, as evidenced by pressure patterns. Even though gradient wind is more representative, gradient wind scales are not widely used. This is due to the curvature fac-tor, which is very complicated and difficult to determine.
Use of Geostrophic Wind Scale (Printed on base map)
See illustration of this type scale in figure 7-2-4.
At the point on your map where you want to DETERMINE THE WIND SPEED, measure the perpendicular distance between the isobars on either side of the point. You can use a pair of dividers or simply mark the distance on a piece of paper. Make note of the latitude at this point. You will need to know both to use the wind scale. Let’s suppose the distance between the two isobars is equal to line AB as shown in figure 7-2-4, and your point of latitude is 40°N. Starting at the left side of the wind scale, measure off the distance AB along the line for 40°. You can see point B falls on a curved line. Follow down this line to the base of the scale. Read the geostrophic wind (10 knots). This example used a latitude and isobar spacing that fell directly on lines of the wind scale. If your point of latitude falls between lined increments on the scale, simply measure mid-way between the increments. When the measured spacing fails between speed curve lines, you must interpolate the wind speed.
TO DETERMINE ISOBARIC SPACING, you must know the wind speed and latitude. Taking the known wind speed, move up the curve to the latitude increment corresponding to the point of latitude you’ve chosen. Measure the distance from this point to the scale’s left edge.
GEOSTROPHIC WIND SCALE-SEA LEVEL SURFACE
This distance is the correct spacing. As an
This distance is the correct spacing. As anexample, use a wind speed of 10 knots and a latitude of 40° as in the preceding example. Move up the speed curve marked 10 knots to the 40° line. Measure from this point (point B) to the left edge of the scale (point A). The line BA is the correct spacing for the isobars on either side of the 10-knot wind speed point on your map.