The next noon the wind velocity decreases and It Is forecast 

 that from IS^^f^ on It will be nearly calm. What waves can be ex- 

 pected tc reach the eastern shore on the following morning at 06?f0? 



Assuming that the wind suddenly .died at 1500 the following 

 wave heights and periodr; are determinea from Plate III: 



Distance from Distance to Wave height Wave period 

 lee shore windward shore (feet) (seconds) 

 (naut. 111.) (naut. m. ) 



The tizae interval under consideration is 15 hours. From Plate 

 VI it is evident that the waves listed above v.ould travel 50 nauti- 

 cal miles or moro in 15 hours. Therefore, the waves have died out 

 before 0600 on the following morning. 



The procedure indicated in this exaiiple can be modified accord- 

 ing to the nature of the problem. The forecaster should attempt to 

 gain local experience and modify his use of the graphs accordingly. 



FORECASTING OF SWELL 



Forecasts of swell can be made v/ith considerable accuracy if 

 adequate consecutive weather maps are available from which (1) wind 

 direction, (2) wind velocity, (3) fetch, and (Zj) duration can be 

 determined for the wave-generating areas. The details of the pro- 

 cedure will depend upon the character of the weather maps but some 

 general principles can be outlined. In Tables IV, V, and VI are 

 listed the fundaiTLental. and the auxiliary quantities which are used 

 when preparing a forecast. Tlie quantities summarized in Table IV 



37 



