White caps are formed by the breaking of relatively short 

 waves which often appear as "riders" on longer waves (fig. 5B) . 

 Such short waves may grow so rapidly that their steepness reaches 

 the critical value H/L = 1/7 and they break. If interference 

 occurs long waves may attain this steepness and break. 



EMPIRICAL KNOWLEDGE OF WIKD WA7ES AKD SWELL 

 Measurements of Waves and Swell 



Wind waves are defined as waves which are growing in height 

 under the influence of the wind. 



Swell consists of wind -generated vra.ves which have advanced in- 

 to regions of weaker winds or calms and are decreasing in -height. 



So far, the discussion of surface wsves has dealt mainly with 

 waves which appear as rhythmic and regular deformations of the 

 surface. Because of interference, the formation of short-crested 

 waves, and the breaking of waves there is, however, little 

 regularity in the appearance of the sea surface, particularly when 

 a strong wind blows. Although individual waves can be recognized 

 and their heights, periods, ■ lengths, and velocities measured, such 

 measurements are extremely difficult and comparatively inaccurate. 

 The lengths of most v/aves and the heights of low v/aves are likely 

 to be underestimated, vtfhlle the heights of large waves are general- 

 ly overestimated. Wave heights above 55 feet are extremely rare, 

 yet the literature contains many reports of waves exceeding 80 feet 

 in height. Such errors are probably due to the complexity of the 

 sea surface and the movement of the ships from which measurements 

 are made. 



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