millibars (a bar) equals 750 mm (29.53 inches) of mercury and is 98.7 percent 

 of normal atmospheric pressure. 



A simplified surface chart for the Pacific Ocean drawn for 27 October 1950 

 at 0030Z (0030 Greenwich mean time) is shown in Figure 3-16. Note the area 

 labeled L in the right center of the chart and the area labeled H in the 

 lower left corner of the chart. These are low- and high-pressure areas; the 

 pressures increase moving outward from L (isobars 972, 975, etc.) and 

 decrease moving outward from H (isobars 1026, 1023, etc.) 



Scattered about the chart are small arrow shafts with a varying number of 

 feathers or barbs. The direction of a shaft shows the direction of the wind; 

 each one-half feather represents a unit of 5 knots (2.5 meters per second) in 

 windspeed. Thus, in Figure 3-16 near the point 35° N. latitude, 135° W. longi- 

 tude, there are three such arrows, two with 3-1/2 feathers which indicate a 

 wind force of 31 to 35 knots (15 to 17.5 meters per second) and one with 3 

 feathers indicating a force of 26 to 30 knots (13 to 15 meters per second). 



On an actual chart, much more meteorological data than wind speed and 

 direction are shown for each station. This is accomplished by using coded 

 symbols, letters, and numbers placed at definite points in relation to the 

 station dot. A sample plotted report, showing the amount of information 

 possible to report on a chart, is shown in Figure 3-17. Not all of 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 3-18 may be used to facilitate computation of the geostrophic wind- 

 speed. The figure is a graphic solution of equation (3-30). 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 3-18, 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 inter- 

 section of these two values gives the geostrophic windspeed. For example, in 

 Figure 3-16, a chart with 3-millibar isobar spacing, the average isobar 

 spacing (measured normal to the isobars) over Fo , located at 37° N. latitude, 

 is 0.70° of latitude. Using the scales on the bottom and left side of Figure 

 3-18, a geostrophic wind of 345 meters per second (67 knots) is found. 



b. Procedure for Estimating Surface Wind from Free Air Wind. After the 

 free air wind has been estimated by the method above, the windspeed at the 

 surface (10-meter level) must be estimated. First the geostrophic windspeed 

 is converted to the 10-meter level velocity by multiplying by R as given in 

 Figure 3-19; R is a function of the geostrophic windspeed (iree air wind- 

 speed) U . The resulting velocity is then adjusted for stability effects by 

 the factor R^ given in Figure 3-14 and discussed in Chapter 3, Section IV, 

 2. The duration-averaged windspeed is estimated in Chapter 3, Section 

 IV,l,b. The wind stress factor is computed from the windspeed in Chapter 3, 

 Section IV,1 ,e. 



3-35 



