146 WIND CURRENTS AND WIND WAVES 



ship had been established between maximum wave heights and the 

 dimensions of the body. This formula is valid to a distance of 600 to 

 900 nautical miles, at which the maximum wave height characteristic 

 of the open ocean, about 12 m, may be reached. The fact that greater 

 wave heights are rarely observed may be due to the circumstance that the 

 wind systems usually have dimensions smaller than 1000 nautical miles, 

 so that in the open ocean the actual fetch of the wind cannot exceed that 

 distance, or it may be that a longer fetch of the wind tends more toward 

 increasing the length of the waves than toward increasing their height. 



The difference in energy of long-crested and short-crested waves and 

 of high and low waves must be considered in the discussion of what 

 happens to the waves when the wind stops blowing. Owing to their 

 smaller energy per unit area the short-crested waves will be destroyed 

 more rapidly by friction, and the largest of the long-crested waves stand 

 the best chance of surviving for longer periods. It is also probable that 

 the dissipation of energy is more rapid within the steeper waves, for which 

 reasons the steeper and shorter waves will be destroyed more rapidly than 

 the longer and less steep. Thus one should expect that, outside of the 

 region in which the wind blows, long-crested swells will become more and 

 more dominating, and, at considerable distances from the wind areas, 

 only long-crested swells will be present. These conclusions are in good 

 agreement with observed conditions. 



Waves Near the Coast. Breakers 



When the waves approach the coast, a number of things occur. It 

 is conspicuous, first, that the short-crested cross sea mostly disappears at 

 some distance from the coast and that it is mainly the long-crested rollers 

 that reach the beaches. Jeffreys has been able to show that this trans- 

 formation is associated with the change in form and the dissipation of 

 energy that take place when surface waves enter shallow water. One 

 of the characteristic deformations is that originally symmetrical waves 

 become unsymmetrical when the depth decreases, the front of the waves 

 becomes steeper, and, finally, the waves break. This effect is more 

 conspicuous in the case of the short-crested waves, which therefore 

 break at a greater distance from the coast, whereas the long-crested waves 

 can proceed farther without being destroyed. 



Another effect of the decrease of depth is that the wave velocity is 

 reduced. Consequently, a wave that approaches the coast at an angle 

 will be deflected so that it will reach the coast with the wave front nearly 

 parallel to it. The part of the wave that first approaches the coast will 

 be slowed down, but the outer portion of the wave will still advance with 

 a great velocity and the direction of the wave front will be turned. 



Still another effect is related to the fact that when approaching a 

 coast the waves take on a character which is intermediate between surface 



