COMBINES WITH 



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TO FORM 



When two systems of waves cross each other at a 

 considerable angle they form a cross sea. In place 

 of long continuous crests, the wave systems meet to 

 form a very irregular surface of unrelated short- 

 crested peaks and hollows. 



When two or more wove trains combine, the result 

 may be a complicated wave form caused by inter- 

 ference. At some points, the waves are in phase (one 

 crest on top of another crest) and the two waves 

 reinforce each other. At other points the waves are 

 out of phase (a crest on top of a trough) and the 

 two waves tend to cancel each other. As a simple 

 case, when two wave trains moving in the same gen- 

 eral direction hove the same height and nearly the 

 same velocity, the result of their combination or inter- 

 ference will be a group of waves roughly twice the 

 height of the original waves with regions in between 

 where the waves disappear. The figure at the left 

 shows graphically how this can happen. 



SURF 



As swell travels across the ocean away from the 

 storm area which generated it, it grows flatter and 

 longer. The waves lose roughly one third of their 

 height each time they travel a distance in miles equal 

 to their length in feet. And their apparent period, 

 wove length, and velocity increase. The waves ore 

 further flattened by an opposing wind. Eventually, 

 the waves, growing flatter and longer, reach shallow 

 water. Here the bottom restricts the motion of the 

 water particles and the wave is slowed down. The 

 wove train is telescoped together, the wave length 

 decreases, and the wave height increases. When the 

 height-length ratio reaches a critical value, the crest 

 topples forward, and the wave breaks. Theoretically 

 the breaking occurs when this ratio reaches 1:7, but 

 observations indicate that it usually occurs when the 

 ratio becomes 1:12 to 1:10. 



Waves can also steepen to the breaking point in 

 deep water by meeting a strong opposing current 

 in a short narrow channel or by the action of strong 

 gusts of wind which add energy rapidly to the wave 

 form causing it to become unstable. Such short-crested 

 waves breaking in deep water ore the familiar white- 

 caps. Whitecops occur only with wind velocities 

 greater than 12 knots. 



