100 Generation, Growth and Propagation of Waves 



with long crests and almost straight fronts. This is a well-developed swell, 

 propagating in its original direction independently of the wind. 



In the area of generation the ratio between wave height H and wave 

 length A is about 1:20, and occasionally 1:10; according to Count von 

 Larisch (1925), this ratio is for swell 1:30 to 1:100; great wave lengths 

 and small heights are predominant and round trochoidal profiles are their 

 characteristic shape (see Table 8). The stereophotogrammetric picture one 

 sees is a mixture of waves of the most varied wave lengths, which last only 

 a short time, on account of the different velocities of its components. The 

 long waves, with their greater velocity, rapidly leave the wind area and the 

 long waves with small height dominate entirely in the swell. Owing to the 

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

 rapidly by friction and by turbulence. Photogrammetrical pictures have 

 confirmed that waves with great wave length are also present in the generation 

 area. In the storm area they become inconspicuous because of the usually 

 steeper and chopped waves. 



A wave group can be represented by the superposition of two normal 

 wave trains of nearly equal wave length. The phenomena of the reappearance 

 of particularly high wave crests (see p. 13) can also be explained in this 

 manner. Such a superposition is graphically illustrated in Fig. 6. It shows 

 the wave composed of "wave groups" alternating with smooth surfaces, 

 where the amplitude of the wave decreases to zero. Figure 44 reproduces the 



5 r 48' 5"49' I 5 h 50' 5 h 5l' 5*52' 5 h 53' 5 h 54' 5 h 55' 5 h 56' 



Fig. 44. Registration of waves on a "Petravic" pendulum. "Meteor", 30 March 1925. 



records of a "Petravic" pendulum which registered the approaching waves 

 aboard the "Meteor" while the engines were stopped. The wave groups present 

 can easily be represented by the interference of two wave systems of nearly 

 the same wavelength (see also Fig. 22). 



The simple theory of interferences cannot explain all phenomena relative 

 to wave groups, for instance, the question how it is possible that a field of 

 waves continues to travel after the generating force has ceased to exist. 



Swell is particularly characteristic in the trade wind regions, where in the 

 winter months of the respective hemispheres it grows to considerable pro- 

 portions. It always comes from the poles and originates in the storm areas 

 which cross from west to east the higher mid-latitudes. When a large 

 storm area remains for some time over a region at sea or travels slowly 

 eastward, enormous wind-seas develop mostly on the backward side of the 

 depressions. These waves are very long; consequently, they travel with great 

 velocity towards lower latitudes, and will reach eventually the polar boundary 



