76 FREQUENCY OF WAVE CONDITIONS 



High swells similarly average about twice as frequent (about 7 

 percent) as high seas do (about 3 percent) for the North Atlantic in 

 August from latitude 40° N. southward, the contrast in this respect 

 being especially interesting in the downwind parts of the two Trade 

 Belts off South America, where a high swell has been reported locally 

 in August with frequency as high as 19 to 23 percent, but high seas 

 with only 9 percent at most. 



On the other hand, the mid-latitudinal belt where the waves are 

 reported as "low"' more than half the time in summer is much more ex- 

 tensive for swells than for seas (cf. contours for 60 percent low, pis. 

 II and IV), while the swell is also more often 1ow t along the West 

 African coast between the offings of Cape Verde and of Cape Palmas 

 during that month (GO to 84 percent) than the sea is (42 to 62 per- 

 cent). The greater frequency with which the category "low" is re- 

 ported for swells than for seas is probably due to the fact that it in- 

 cludes a wider range of heights for the first of these classes of waves 

 than for the second, as explained on page 71. But it is likely that 

 the discrepancies between the frequency distributions for high seas 

 and for high swells chiefly reflect the fact that, while the former are 

 the direct product of whatever wind may be blowing at the time, the 

 swell that is encountered on any given occasion is likely to be the 

 cumulative product of seas generated by stronger winds alternating 

 with weaker winds, or even with calms. 



We may also point out that high swells are perhaps a better index 

 to the effects of hurricanes in the western side of the Atlantic than 

 high seas are, to judge from the fact that the former are reported de- 

 cidedly more often (6 to 12 percent) than the latter (1 to 3 percent) 

 to the north of the Virgin Islands, as well as in Bahaman waters; high 

 swells are also more frequent (6 to 7 percent) than high seas (1 to 2 

 percent) out from the coasts of the Carol inas. 



Closer analysis of the local differences between the frequency dis- 

 tributions of summer swells and seas of different heights over the 

 North Atlantic would require a much more detailed comparison be- 

 tween the waves and the character of the wind, and especially with the 

 frequency of gales, than we have been in a position to undertake. 



The state of the swell in August may be summarized as follows for 

 enclosed seas on the western side of the Atlantic : 



a. Gulf of St. Lawrence. — Swell lower than 5 to 6 feet for more than 

 three- fourths of the time (82 percent), and very seldom high (0 per- 

 cent), as might be expected from the fact that the maximum effec- 

 tive fetch within the Gulf is not more than about 180 miles, no matter 

 what the direction of the wind may be. 



b. Gulf of Mexico. — Swells low over the entire area of the Gulf for 

 at least 80 percent of the time, and for more than 90 percent of the 



