NORTH INDIAN OCEAN 



95 



about 7 percent) ; so, too, in the equatorial belt (lat. 0° to 5° N.) clear 

 across from the African coast to the offing of Sumatra (average about 

 17 percent for high swells, about 7 percent for high seas). On the 

 other hand, it is no more common, in summer, for the swell to run 

 high than for the sea in the Red Sea, in the Gulf of Aden, or in the 

 Gulf of Oman. And the boundaries of the areas within which the 

 summer swell is reported "low" in any chosen frequency differ from 

 those for low seas no more widely than can be charged to the nature 

 of the information from which they have been derived (pis. XVIII 

 and XX). 



Winter. — The alteration that takes place, from summer to winter, 

 in the state of the sea in the northern Indian Ocean, with the change of 

 the monsoons, can fairly be described as "spectacular." Thus the 

 northeast winds of January and February average so much weaker 

 than the southwest winds of summer — and with gales so unusual — 

 that a high sea was not reported at all, for January or February, for 

 about one-half of the unit areas (pi. XXI) and the maximum fre- 

 quency was only 4 percent at any of them, except in the general offing 

 of the Gulf of Aden, where an average wind velocity of 12 to 14 knots, 

 December through February, generates 9-foot seas a little more often 

 (5 to 7 percent). It is also perhaps characteristic that a high sea is 

 reported in 4 percent frequency, in winter, between Ceylon and the 

 northern atolls of the Mai dive Group, where we ourselves met a sharp 

 gale in January 1902. 



It is not astonishing, with high seas so unusual, that the frequency 

 of "low," in late winter, should be more than 40 percent throughout 

 the entire extent of the North Indian Ocean, except for a circumscribed 

 tongue off the East African coast, and more than 60 percent, except 

 in the southwestern part of the Arabian Sea, locally in the offing of 

 the Gulf of Oman, and southwest of Ceylon (pi. XXII). 



The following summary (table 25) illustrates how much smoother 

 during the winter than during the summer (both as to seas and as to 

 swells) those parts of the North Indian Ocean are, where the heights 

 of the waves are ruled by the monsoon wind. 



Table 25. — Average percentage frequencies of low and high seas and swells in the 

 Arabian Sea and in the Bay of Bengal in winter and summer 



