WIND CURRENTS AND WIND WAVES 



141 



where L' = 27r//c' represents the length of the crest. This formula 

 defines a series of waves that travel at a speed somewhat greater than 

 that of the long-crested waves, the crest itself forming a wave line at right 

 angles to the direction of progress: c' = c(l + LV-t/'^)^ (fig. 37). 

 Since such a wave can also travel without altering form, it therefore 

 belongs to the group of waves which are theoretically possible. It is 

 interesting to observe that, according to Jeffreys, the first waves which 

 are generated by the weakest winds must be long-crested, but, when the 

 wind increases in velocity, short-crested waves can be formed. The 

 explanation is that the turbulence of the wind is characterized not only 

 by random motion in the direction of the wind, but also by random motion 

 at right angles to the wind. At higher wind speeds, the turbulent veloci- 



Fig. 37. Schematic picture of short-crested waves. 



ties at right angles to the wind may be great enough to break the originally 

 long-crested waves and to create new short-crested waves. These results 

 help greatly toward understanding the irregular appearance of the waves 

 in strong winds. 



In table 15, p. 136, only wave velocity, wave length, and wave period 

 are listed, and no information is given as to the wave height. A large 

 number of measurements of wave heights have been made by different 

 methods which, although uncertain, are considered more accurate than 

 the measurements of wave lengths. The wave height can be found if a 

 location on board ship can be selected at which the tops of the waves 

 appear level with the horizon. The wave height is then equal to the eye 

 height of the observer above the water line. Another method is based 

 on the records of a delicate barometer, and still another that gives very 

 accurate results makes use of photogrammetric measurements. 



The greatest wave heights observed in most oceans are about 12 m. 

 Cornish gives a very vivid description of waves of that height during a 

 storm which he experienced in the Bay of Bisca}^ in December, 1911. 



