coded symbols, letters, and numbers placed at definite points in relation 

 to the station dot. A sample model report, showing the amount of informa- 

 tion possible to report on a chart, is shown in Figure 3-10. Not all of 

 the data shown on this plot are included in each report, and not all of 

 the data in the report are plotted on each map. 



Figure 3-11 may be used to facilitate computation of the geostrophic 

 wind speed. A measure of the average pressure gradient over the area is 

 required. Most synoptic charts are drawn with either a 3- or 4-millibar 

 spacing. Sometimes when isobars are crowded, intermediate isobars are 

 omitted. Either of these standard spacings is adequate as a measure of 

 the geographical distance between isobars. Using Figure 3-11, the distance 

 between isobars on a chart is measured in degrees of latitude (an average 

 spacing over a fetch is ordinarily used), and the latitude position of the 

 fetch is determined. Using the spacing as ordinate and location as abscissa, 

 the plotted or interpolated slant line at the intersection of these two 

 values gives the geostrophic wind speed. For example, in Figure 3-9, a 

 chart with 3-millibar isobar spacing, the average isobar spacing (measured 

 normal to the isobars) over F2, located at 37°N. latitude, is 0.70° of 

 latitude. Using the scales on the bottom and right side of Figure 3-11, 

 a geostrophic wind of 67 knots is found. 



Geostrophic wind speeds are generally higher than surface wind speeds. 

 The following instructions, U.S. Fleet Weather Facility Manual (1966), are 

 recommended for obtaining estimates of the surface wind speeds over the 

 open sea from the geostrophic wind speeds: 



(a) For moderately curved to straight isobars - no correction is 

 applied. 



(b) For great anticyclonic (clockwise movement about a high pressure 

 center in Northern Hemisphere and counter-clockwise in Southern Hemisphere) 

 curvature - add 10 percent to the geostrophic wind speed. 



(c) For great cyclonic (counter-clockwise movement about a low 

 pressure center in Northern Hemisphere and clockwise in Southern Hemisphere) 

 curvature - subtract 10 percent from the geostrophic wind speed. 



Frequently the curvature correction can be neglected since isobars 

 over a fetch are often relatively straight. The gradient wind can always 

 be computed if more refined computations are desired. 



To correct for air mass stability, the sea-air temperature difference 

 must be computed. This can be done from ship reports in or near the fetch 

 area, aided by climatic charts of average monthly sea surface temperatures 

 when data are too scarce. The correction to be applied is given in Table 

 3-1. (U.S. Fleet Weather Facility Manual, 1966.) 



Over oceans, the surface winds generally cross the isobars toward low 

 pressure at an angle of 10° to 20°. 



3-24 



