Generation, Growth and Propagation of Waves 



101 



of the tradewind region, which they penetrate sometimes. Strong swells have 

 been observed to cross the whole region of the tradewinds and penetrate 

 into the equatorial calm zone, and even into the tradewind area of the other 

 hemisphere. This occurs in the North Atlantic Ocean, where the swell generated 

 by the north-westerly storms of the moderate latitudes travels across the 

 zone of the north-east tradewind and the equatorial regions of calm air 

 into the region of south-east tradewind. For instance on Ascension Island 

 (8°S. lat.) and even on St. Helena (16°S. lat.), there is sometimes a strong 

 surf (the famous "rollers") from a swell generated about 4000 nautical miles 

 distance. If the storm waves have an original wave length of 1 50 m and keep 

 up their original velocity of 15 m/sec, they can travel the distance of 4000 miles 

 within 137 h = 5 days and 17 h. This time, however, will be shorter, as the 

 wave velocity increases with the increase of the wave length. 



In the same way, the imposing swell causing the constant surf at the coast 

 of Guinea (the Kalema) originates from a region near Tristan de Cunha 

 in the moderate latitudes of the southern hemisphere. It consists of very 

 long waves, whose average period in a series of 60 waves was 15 1 sec with 

 extreme values of 6 and 24 sec. The waves, therefore, had the enormous 

 length of 350 m and travelled with a velocity of 23 | m/sec; they only need 

 1 day for 1100 miles and only 2-3 days to reach the coast of French Guinea. 



The swell in the trade-wind region of the North Atlantic Ocean and the 

 surf on St. Helena are correlated to the north-westerly storms of the North 

 Atlantic Ocean, as shown by the comparison of their monthly variations. 



Table 13. Yearly number of breakers at St. Helena and number of storms 



in North Atlantic Ocean 



Table 13 gives the official computation of the rollers on St. Helena during 

 20 years from 1856 to 1875 and the frequencies of the north-westerly storms 

 in the North Atlantic Ocean north of 25° N. lat. registered by Toynbee 

 (Kruemmel 1911, vol II, p. 116); the same table includes the yearly 

 course of storms in the region 30°-50°N., 70°-30°W. in the North 

 Atlantic. The maximum frequency of the rollers respectively of the north- 

 westerly storms has here been given the value 100; the general distribution 



