Section II 

 FORECASTING WIND WAVES AND SWELL 



In section I the processes have been described 

 which lead to the growth and decay of wind 

 waves. Section II will describe the practical 

 applications of the theory and its applications 

 in making a reliable and useful forecast of wind 

 waves and swell. 



The present approach to wind waves and 

 swell forecasting is based upon the concept that 

 the ocean surface generally can be described by 

 any combination of these parameters. Until 

 recently this approach has been considered suf- 

 ficient; however, new problems are continually 

 arising and new requirements are being placed 

 upon the oceanographer for more complete de- 

 scriptions of the sea surface. Until more ac- 

 curate and undoubtedly more intricate descrip- 

 tions of sea roughness can be made oceanog- 

 raphers and meteorologists must make the most 

 of the present techniques. 



Forecasts of wind waves and swell can be 

 made with a reasonable degree of accuracy if 



adequate and consecutive surface weather maps 

 are available. The factors to be determined 

 from these weather maps are (1) the fetch, or 

 generating area, (2) the wind direction, speed, 

 and duration within the fetch, (3) wind condi- 

 tions which may exist between the fetch and 

 the target, and (4) the decay distance from 

 the fetch to the target. Once these have been 

 determined the technique for preparing wave 

 forecasts is somewhat mechanical. However, it 

 involves careful interpretation and interpola- 

 tion of the data selected from the weather maps 

 and careful use of the numerous graphs. The 

 tables presented below have been compiled in 

 order to assist in developing the proper con- 

 cept of the terms used in preparing wave fore- 

 casts. The terms are grouped according to 

 three general conditions: (1) No decay dis- 

 tance, (2) decay but no secondary wind, and 

 (3) decay and secondary wind. These tables 



Table II. 1. — Summary of quantities to be determined for preparing forecasts of wind waves 

 (No decay or secondary wind area is involved.) 



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