always have to be computed, but when sorae experience has been 

 gained the computations Indicated In Table VI need not be carried 

 out but an estimate of the final values can be made directly. In 

 the following discussion the numbers In parentheses refer to cor- 

 responding terms in Tables IV and VI. 



Deterralnatlon of Wind, Fetch, and Duration 



1. ■ Wind direction . Outside of the tropics the wind direc- 

 tion over the ocean is obtained from the course of the isobars, 

 applying the rule that the wind deviates SO degrees to the right 

 of the pressure gradient in the Northern Hemisphere and 80 degrees 

 to the left in the Southern Hemisphere. Where the Isobars are 

 nearly straight (fig. 11, A and B) the winds to be considered in 

 forecasting swell are those with directions within 30° of a line 

 joining the generating area and the locality for vjhich forecasts 

 are to be made . Where the Isobars are curved (fig. 110) the winds 

 to be considered are those with directions within 45° of a. line 

 joining the generating area and the locality for which forecasts 

 are to be made . The generating areas to be considered are limited 

 by these restrictions. The reasons for these rules are that the 

 course of the Isobars is not exactly knovm and that the swell prob- 

 ably spreads out somewhat when entering areas of calm. The_ spread- 

 ing out will be greater from a region with curved Isobars. 



In the tropics the wind direction must be obtained from 

 observations on board ships or at exposed island stations. 



3S 



