Chapter 5 



THE FREQUENCY OF HIGH AND LOW SEAS AND SWELLS 

 IN DIFFERENT REGIONS 



It is not possible, as yet, to picture the average condition of sea or 

 swell for any part of the world in more than a very rough way, for 

 while great numbers of reports of the heights of waves have been 

 received at the Hydrographic Offices of the maritime nations, these 

 have not only been concentrated chiefly along the more travelled 

 routes, but the great majority have been rough estimates only. A 

 criticism, equally serious from the mariner's standpoint, is that the 

 reports available for this study yield no information whatever as to 

 the frequency with which the sea runs higher than 20 feet, as they often 

 do in the Westerlies of both hemispheres during the stormy season, 

 as well as under the southwest monsoon in the North Indian Ocean. 



The charts presented here (pis. I to XXIV), with the accompany- 

 ing discussion, are based upon information received and analysed 

 at the United States Navy Hydrographic Office chiefly for the period 

 from 1932 to 1940. The relative frequencies (stated as percentages of 

 all reports received) with which the sea and swell was reported as 

 "calm," as "low," as "medium," and as "high" were calculated for 

 each 5° square, or similar area, and it is from these percentages that 

 the contours were laid down on plates I through XXIV. The cate- 

 gories are based on wave heights which are different for sea and for 

 swell. "Low," "medium," and "high" seas indicate waves of 1 to 3 feet, 

 3 to 8 feet, and 8 feet or higher, respectively, whereas the same cate- 

 gories for swell indicate waves of 1 to 6 feet, 6 to 12 feet, and 12 feet 

 or higher. This distinction between the measurement of sea and swell 

 complicates any comparison between the relative frequencies of low 

 waves of the two classes, but it is still possible to compare high waves 

 in a rough way. 



Failure to mention "calm" water more than casually in the follow- 

 ing discussion is deliberate, due to the very strong probability that a 

 very old and hence very low swell may be overlooked, and that a sea 

 only a few inches high may be reported as "calm." Actually, it is only 

 when there is no wind at all that there is no sea at all. Likewise, it is 

 unlikely that any considerable part of the open ocean is ever wholly 

 free from a swell, though the latter is often so low and so long that its 

 presence is made visible only if it runs into shoaling water. For these 



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