70 SEAS AND SWELLS 



often cause a violent surf even on the islands of Ascension and of St. 

 Helena in the South Atlantic (in latitudes of about 8° S. and 15° S., 

 respectively) from December to April; and, in the same way, heavy 

 swells from the storms of the Westerlies in high latitudes of the South 

 Pacific run to the island shores of the Paumotos Group, so that they 

 may be expected finally to intermingle with the swells that are gen- 

 erated by the Southeast Trades. 



A swell would eventually be extinguished by the internal friction 

 of the water and by the resistance of the air that it displaces in its 

 advance, if the ocean were of unlimited extent. But this friction is so 

 small that swells actually tend to run until they strike some coast- 

 line, or until they are beaten down by an opposing wind, which may 

 happen so rapidly that a fresh trade wind has frequently been seen 

 wholly to smooth out a moderate swell under the eye of an observer. 

 And the effect of floating ice is still more spectacular in this regard, 

 as described below (p. 147). 



FORECASTING SEA AND SWELL 



The prediction of the swell and state of the sea a day or more in 

 advance can be accomplished by the proper use of the relationships 

 summarized in tables 4 and 5, provided sufficient data on wind con- 

 ditions are available. The details of the operations necessary to make 

 a forecast are given in Wind waves and swell; principles in forecast- 

 ing published by the United States Hydrographic Office. 1 Such pre- 

 dictions have been found very useful during wartime unloading opera- 

 tions off open beaches, as during the allied invasion of Europe. For 

 practical purposes, it is important to know the size of the largest 

 waves rather than merely the mean of the entire set, and the pre- 

 dictions give approximately the mean of the highest third of the 

 waves. 



1 Superseded by H. O. Pub. No. 604, Techniques for Forecasting Wind Waves and Swell. 



