2. Wind velocity ( 5) . Outside the tropics the mnd velocity 

 over the generating area is obtained frora the pressure distribution. 

 Instead of computing the gradient wind It is sufficient to compute 

 the geostrophic wind (4) and to multiply the value so obtained by a 

 reduction factor which takes into account the curvature of the 

 isobars . 



The following factors appear to be sufficiently accurate to 

 dispose of the somewhat uncertain computation of the gradient wind: 



Great cyclonic curvature of isobars 0.60 



Small cyclonic curvature of isobars 0.63 



Straight lsobai:s 0.65 



Small antlcyclonlc curvature of isobars 0.6? 



Great antlcyclonlc curvature of isobars 0.70 



The computations may have to be carried out for different 

 parts of the fetch in order to obtain the average wind velocity 

 in the generating area. Ships' observations should be used as a 

 check on the computed value. A difference of not more than one 

 on the Beaufort Scale betv/een computed and observed velocity is 

 a satisfactory check. 



The wind velocity obtained In this manner applies to the 

 current v;eather map and may differ from the wind velocity over 

 the same area according to the preceding map. A constant wind 

 velocity v/as assumed in the preparation of Plates II to V which 

 are used to determine the wave height, and it is therefore 

 necessary to introduce an average wind velocity (6) which can 

 be considered applicable to the entire time Interval between 

 the two maps. Although the manner in which the velocity has changed 

 is not knov/n, the fact that strong winds raise waves more rapidly 

 permits the application of the following crude procedure: 



42 



