distance, D. Inset I gives the length and velocity of a deep-water 

 wave for which the period is known. Inset II gives the factor by 

 which the oeriod at the end of the fetch, T-^, must be multiplied in 

 order to find the period at the end of the distance of decay, T^. 

 This factor depends only upon the reduction factor for the wave 

 height, H^/H-p. The use of the diagrams will be described when dis- 

 cussing the forecasting of swell. 



Effect of Following or Opposing Winds 



The effect of a following or an opposing wind on the decrease 

 of the height of the swell is also found from a special solution of 

 the fundamental equation of the "energy budget" of the wave. 

 It is assumed that the increase in wave velocity over the distance 

 of decay is not influenced by following or opposing winds. Although 

 this assumption has little basis in either theory or observation it 

 probably leads to approximately correct results. In the case of a 

 following wind the computed wave heights may be somewhat too high 

 and the wave periods somewhat too low, whereas in the case of an 

 opT50sing wind the heights laay be too low and the periods too high. 

 Consistent differences between values computed on this basis 

 and observed values may later be used to improve the theoretical 

 approach. 



The following or opposing wind may blow over only a part of 

 the distance of decay. The problem is to determine how much more 

 or how much less the wave height decreases in any given distance 

 as compared to its decrease in the absence of any wind. This 



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