Find the coiaponent of the wind which on the preceding map 

 blew in the direction of the vdnd on the current map. Subtract 

 one-fourth of the difference between these two velocities from 

 the greater velocity. The result is considered the average 

 velocity during the time interval between the maps. 



If the wind is decreasing this rule should be applied only 

 if the velocity remains above 15 knots. If the velocity drops 

 below 15 knots the effect of a following wind should be ex- 

 amined . 



This procedure may have to be modified according to the 

 experience of the forecaster. 



In the tropics the wind velocities have to be obtained from 

 observations on board ships or at exposed stations on islands. 



3. Fetch ( 8) . The fetch is the length of the generat- 

 ing area in the direction of the wind, that is, the stretch between 

 the rear and the front boundaries of this area. In general, the 

 boundaries are detenained by coast lines or by one of the following: 

 (a) fanning out of isobars, (b) meteorological fronts, or (c) cur- 

 vature of isobars, as shown schematically in Figure 11. When the 

 boundaries have been decided upon the fetch is measured on the map. 

 When the isobars have a great curvature two fetches should be meas- 

 ured, as shown in Figure IIG. Computations of wave height and wave 

 period at the end of both fetches should be carried out, since 

 inspection alone will not indicate which fetch should be used. 

 In making the forecast consideration should be given the higher 

 values . 



43 



