Observations and Measurements of Ocean Waves 



51 



on the sea surface, energy supplied by wind to waves is distributed more 

 and more over a certain range or wave length with different heights as the 

 sea grows. A spectrum of ocean waves is being formed, with wave components 

 ranging from ripples to large billowing waves in a storm sea. Therefore, it 

 is not possible to describe the wind-generated wave pattern by the previously 

 used quantities which apply only to some kind of a fictitious single wave train. 

 The basic values of wave motion as obtained from visual observations 

 or wave recorders shall be ascribed to "'apparent waves". The apparent wave 

 height H is to designate the height difference between a wave crest and the 

 preceding trough. The apparent wave period T gives the time difference and 

 the apparent wave length L the distance between succeeding crests at a fixed 

 position. As the motion of the sea is not a periodical phenomenon in the 

 mathematic-physical sense, but rather a superposition of a large number of 

 partial waves whose frequencies are in an irrational proportion, the structure 

 of the oscillation is subject to perpetual changes and the above "apparent 

 basic values" give only a rough approximation of the motion of the sea. 

 The true quantities, that is, the amplitude, the period and the height of 

 harmonic wave components, can neither be observed nor directly measured. 

 A spectrum of sea motion can be obtained quantitatively only by means 

 of harmonic analysis. More recent systematic observations of apparent wave 

 quantities in deep water are not very numerous (Seiwell, 1948; Putz, 1952; 

 Neumann, 1952/?, 1953a, b; Waiters, 1953; Darlington, 1954; Roll, 

 1953, 1954), but they stress the importance of such investigations. An example 

 of the frequency distributions obtained in this way is given in Fig. 31. 



2 3 4 5 6 7 8 9 10 11 10 20 30 40 50 60 70 80 90 100 110 120 130 



Fig. 31. Distribution of frequency of apparent wave periods and wave lengths in a heavy 



sea. (After Roll, 1957.) 



The results of visual observations on the high seas differ in some respects 

 from those obtained off-shore by pressure recorders at the sea bottom, but 

 they are similar to the frequency distributions from Radar observations. 



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