Chapter 1 

 THE PHYSICAL NATURE OF WIND WAVES 



It is difficult to frame a definition, in everyday terms, that will cover 

 all the types of phenomena that are commonly named ''waves." As 

 applied to the surface of the sea, however, they may be defined as suc- 

 cessive ridges with intervening troughs or valleys which, in the case of 

 wind waves, advance in undulatory motion. Our knowledge of the 

 processes by which the waves of the sea are generated and subsequently 

 developed is still far from complete, in spite of all the attention that 

 has been devoted to the subject. There are two reasons for this. In 

 the first place, it is extremely difficult to take accurate measurements of 

 waves or to analyze their complex contours at any given moment from 

 a ship at sea, or even from the shore. In the second place, the theories 

 that aim to harmonize such observations as have been made with infor- 

 mation of other sorts are still in a fluid state, largely because it has 

 been necessary to assume in most theoretical discussions that waves are 

 produced under ideal conditions such as never exist in the open sea. 

 (For recent summaries of wave theory, as applied to wind waves at 

 sea, we refer the reader to the following publications : Kriimmel, 1911 ; 

 Gaillard, 1901; Thorade, 1931; O'Brien and others, 1942; and Sver- 

 drup, Johnson, and Fleming, 1942.) And difficulties of this same sort 

 also complicate the results of laboratory experiments on waves. Con- 

 sequently, it is not astonishing that various discrepancies still exist 

 between the characteristics of wave action as deduced by theory and as 

 observed at sea or from the beach. Nevertheless, theory checked 

 against empirical observations has advanced to a point where it is our 

 most reliable guide to the dimensions of the waves that are to be ex- 

 pected under a given set of circumstances. After all, water is very 

 nearly a perfect fluid, and conditions in the open ocean, therefore, do 

 at least approximate the ideal state assumed in classical hydro- 

 dynamics; then too, the underlying principles are known for the par- 

 ticular departures from this ideal state that most commonly occur at 

 sea, as for example when waves run from deep water into shoal. 



Wind waves present themselves to the onlooker as a series of irregu- 

 lar crests separated by intervening troughs which advance across the 

 surface of the sea one after another in unending succession from hori- 

 zon to horizon. Depending on the state of the wind locally or on the 

 distribution of wind systems elsewhere, they may range in size from 



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