102 Generation, Growth and Propagation of Waves 



of storms according to the maps of the Atlantic Ocean of the German Seewarte 

 is given in per cent of all observations. In the equatorial area of calms there 

 is generally a strong north-westerly swell in the northern winter and spring, 

 which is a consequence of the winter gales in the northern hemisphere, whereas 

 in summer the swell is more from the south and the south-west, which are 

 caused by winterly storms in the high latitudes of the southern hemisphere. 



The behaviour of swells on the European coast has been observed by 

 Cornish (1910); he found that wave lengths of a very long period (12-20 sec) 

 are not exceptional; once he found an average period of 22-5 sec, which would 

 correspond to a wave length of more than 1200 m in the open ocean. Thorade 

 once observed, from gauge recordings in Heligoland, waves with a period 

 of about 15 sec. 



There is as yet no absolute certainty as to whether the large waves of the 

 swell are already present in the storm area, and concealed there by the 

 amplitudes of the stronger wind-sea; according to Poisson and Cauchy's 

 theory, it is not right to deduct the wavelength of the wave in the generating 

 area from the length and period of the waves in the swell; it was found that 

 the wave length increases considerably as the waves travel (p. 68, Fig. 36). 



The relation between a swell and the storm-waves which generated it, 

 has been investigated more or less successfully in some special cases. Krummel 

 reports a few interesting cases in the southern part of the Indian Ocean 

 (Paris) and in the southern Atlantic Ocean (Gassenmayer) in which it is 

 quite probable that there really was a connection between these two 

 phenomena. He investigated more thoroughly in a synoptical way several 

 cases in the North Atlantic Ocean by drawing charts of the state of the 

 waves based on the diary of the German Seewarte. Observations were 

 made during the international wave week held from 14th to 19th November, 

 1938, in the nothern Atlantic Ocean. A synoptical presentation of the ob- 

 servations for each day at 12 G.M.T. was given by Keyser. Figure 45 gives 

 a simplified example of these charts; it shows the distribution of the swells 

 for the entire northern Atlantic Ocean for the 15th of November. This chart 

 shows also the distribution of pressure at the ocean surface; however, the 

 winds have been omitted, in order not to clutter up the picture. They can 

 be derived with sufficient accuracy from the pressure distribution. The direc- 

 tion and the force of the swell according to the Douglas scale is given for 

 each separate position of the ships. An attempt was made to draw from 

 these values the wave fronts of the existing swell. One wave-train spreads 

 from the western North Atlantic to the north-east, thereby increasing in force. 

 A second wave-train covers the eastern part of the ocean and spreads to the 

 south-east almost perpendicularly to the first one. The first wave-train is 

 probably the result of an extensive low-pressure region, which on November 

 14th travelled rapidly eastward from northern North America and which 

 was on 15th November with its centre above the Davis Strait. The second 



