less can be disregarded in computing the 

 fetch. This practice is allowable because 

 waves raised by a wind no greater than 20 

 knots will be reduced to negligible propor- 

 tions in traveling through a distance of decay 

 of 500 miles or more. 



c. The fetch is the distance between the 

 forward and rear boundaries of the generat- 

 ing area. However, if over the generating 

 area the surface wind speed, as determined 

 by individual isobar spacings, varies by an 

 amount of one force or more on the Beaufort 

 scale, two fetches should be selected, one for 

 the greater and one for the lesser wind 

 speed. Likewise, when the isobars in the 

 generating area have a large curvature 

 (fig. II. C) two fetches should be measured. 



Moving Fetch 



If successive weather maps show that the 

 fetch is moving, the wave forecast must be 

 modified to take account of this fact. The gen- 

 eral problem is complicated since the move- 

 ment can take place at any angle to the wind 

 direction within the fetch, as shown by figure 

 II.2. No fully satisfactory results can be given, 

 but qualitative adjustment of the forecast can 

 be made, at least in the cases where wind di- 

 rection is perpendicular or parallel to the di- 

 rection of fetch motion. 



If the fetch is moving perpendicularly to the 

 wind within it (F-2 and F-4 in fig. II.2), the 

 wave height will be limited by the time during 

 which the wind acts over the water which is 

 not moving. This maximum duration time may 



DIRECTION OF MOTION OF PRESSURE SYSTEM. 

 (WIND DIRECTION SHOWN BY ARROWS WITHIN PRESSURE 

 SYSTEM.) 



Figure II. 2. — Possible combinations of fetch movement 

 and wind direction. 



be determined by dividing the width of the 

 fetch by the speed with which it is moving. 

 This will give the maximum duration time. If 

 the fetch has not been in existence this long, 

 the actual duration time must be used in com- 

 puting the waves. 



The fetch movement parallel to the wind di- 

 rection has the effect of changing the fetch dis- 

 tance, making it longer if the motions are in 

 the same direction (F-3 of fig. II.2), and mak- 

 ing it shorter if the motions are opposite (F-1 

 of fig. II.2) . In the latter case the fetch dis- 

 tance cannot be divided by the speed to get a 

 duration time as the waves themselves are 

 traveling out of the fetch. It has been sug- 

 gested that the duration time computed by 

 dividing the length of the fetch by its speed be 

 reduced about 40 percent, but since the effec- 

 tive duration time is dependent on the speed 

 of the wind and waves as well as that of the 

 fetch, no hard and fast rules can be given. The 

 best approach is probably to determine the 

 group velocity of the waves as though the fetch 

 were not moving ; then, by using this figure and 

 the fetch speed, determine when the fetch and 

 waves will be separated so that the fetch will 

 no longer be effective in increasing wave height 

 and period. 



A similar approach can also be used with the 

 fetch moving in the same direction as the wind 

 within it (F-3), i. e., determine the group 

 velocity as though the fetch were stationary, 

 then determine where the waves will be in re- 

 lation to the fetch on subsequent maps, and 

 modify the forecast as required. 



Wind Duration 



The duration of the wind is determined from 

 a comparison of current and preceding weather 

 maps. The duration of the average wind speed 

 equals the time interval between the current 

 and preceding maps plus a correction deter- 

 mined from the height of the waves present at 

 the beginning of the preceding map. These 

 waves should be known from computations 

 based on the preceding map. Only the waves 

 which travel at an angle less than 45 degrees 

 from the average wind direction should be ex- 

 amined. The correction is obtained by the 

 following procedure : 



Enter plate IV or V with the average wind 

 velocity and follow a horizontal line to the 

 curve of the wave height on the preceding 



18 



