GROWTH OF WIND WAVES 



A knowledg^e of the height, velocity, and direction of progress 

 of wind waves Is necessary If their arrival as swell at a distant 

 coast Is to be Dredlcted. Direct observations of these wind waves 

 are rarely available, but their height and period can be determined 

 from consecutive synoptic weather maps if the relationship between 

 v/lnd and waves is known. 



In the area of wave formation the highest waves present at any 

 time depend upon the wind velocity, the stretch of water over which 

 the wind has blown (the fetch) , the length of time the wind has 

 been blowing ever the fetch (the duration of the wind), and the 

 waves which were present when the wind started blowing (the state 

 of the sea) . These four factors can all be determined If a sequence 

 of weather maps is available showing the meteorological conditions 

 over the oceans at Intervals of, say 12 or ?4 hours. These maps 

 must be based on a sufficient number of ships' observations to make 

 possible the plotting of fairly accurate Isobars from v^fhich winds 

 may be determined. In the tropics wdnd observations must be avail- 

 able from ships or exposed stations on islands. In middle and 

 higher latitudes direct wind observations on ships v/ill serve as 

 checks on wind estimates from the isobars. 



Thus, v.'lth adequate weather maps at one's disposal, an estimate 

 of the wind waves can be made if accurate relationships between wave 

 height and wind velocity, fetch, and duration are known. Such 

 accurate relationships have not been developed in the past because 

 of the inadequacy of observational data on waves, but they can be 



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