in the open sea. But these longer and flatter waves achieve sig- 

 nificance and practical importance when entering shallower water 

 or when approaching the shores (surf). 



3) Composite wave patterns. Inter fe rence phenomena. Groups of 

 waves . 



Everchanglng patterns and single outsize waves are striking 

 features of the sea surface roughness, and we have to consider these 

 facts in practical wave forecasts. Just these single waves which 

 often cause heavy breakers in storm areas have to be taken into ac- 

 count for practical purposes. 



The source of outsize waves of this sort is the merging of 

 those waves that advance with different velocities, because longer 

 and faster waves are constantly overtaking the slower running ones. 

 This always happens when there are two or more series of character- 

 istic waves present, and the phenomenon is called interference. More 

 complicated patterns of outsize waves may occur when waves are run- 

 ning in different directions, combining a series of characteristic 

 waves produced by the local wind and "dead sea" (very young swell) 

 running from another direction. This happens under certain meteoro- 

 logical conditions, for example when passing cold fronts with pro- 

 nounced "wind- jumps," or in some sectors of hurricanes, especially 

 in the "eye" where "dead sea" of different directions and locally 

 produced sea interfere. The so-called "cross-sea" is a well known 

 phenomenon, where waves are piled up in single interference patterns 

 to considerable height, often forming irregular dangerous breakers 

 in the fierce sea. But it seems that this tumultuous cross sea is 

 usually dissipated in a rather short time. 



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