SWELL PERSISTENCE 69 



it may no longer do so by the time the swells reach the coast, because 

 of the refraction to which they are subject as they advance over a 

 shoaling bottom (p. 155). 



Perhaps it is hardly necessary to caution the reader that the familiar 

 phenomenon of a swell reaching the coast ahead of a storm has no 

 bearing on the question whether the velocities of the swells are higher 

 than that of their parent seas and winds, but simply means that the 

 swells often outrun the storm centers, or wind systems as a whole, 

 even when these are moving in the same direction. In fact, swells 

 often reach the coast from storms that pass by offshore, altogether. 



THE PERSISTENCE OF SWELLS 



The identification of the regions from which swells originated, 

 through the study of synoptic weather maps, combined with vessels' 

 log books, has proven in many instances that swells may run for 

 hundreds, or even for thousands, of miles, unless they are beaten 

 down by contrary winds. Thus, a swell from the northwest, originat- 

 ing from severe gales in the Gulf Stream region south of the New- 

 foundland Banks, has been reported not only in the Trade Wind Belt 

 at a distance of 1,500 miles, but even within 270 sea miles of the coast 

 of Sierre Leone, i. e., at a distance of at least 2,500 miles from its birth- 

 place. Very heavy swells have, again, been experienced on the south 

 coast of England at a distance of at least 1,680 sea miles from the 

 storm center that almost certainly gave them birth, while in December 

 1880 the whole eastern half of the northeast Atlantic experienced a 

 swell, spreading from an area west of the Azores, following winds 

 of hurricane force that had blown there 2 days before (Krummel, 1911, 

 pp. 87, 88, fig. 21). It has also been known for many years that swells 

 from the winter gales of the West Wind Belts of high latitudes, north 

 and south, are common in the equatorial belt of the Atlantic. Indeed, 

 they reach the coast of Morocco so regularly from barometric depres- 

 sions between Ireland and Iceland, some 1,600 miles away, that swells 

 of 3 feet (1 meter) or higher were recorded at Casablanca and at 

 Rabat on every day when observations were made, from January 1943 

 to April 1945. Their average heights ranged from a little more than 

 3 feet (1.2 to 1.3 meters) during July and August to about 10 feet 

 (3.1 to 3.2 meters) in January, with a maximum of about 26 feet 

 (recorded for March), 24 while their recorded periods ranged up to 17 

 seconds as they approach the shore. And the French Department of 

 Public Works of the Moroccan Protectorate of Rabat has found it 

 possible to predict their arrival about 70 percent of the time. 25 They 



24 Observations by Institut Scientifique Cherifien. French Morocco, received through 

 Commander C. J. Fish, U. S. N. R. 



26 Cited from Maimer, H. A., 1930, The Sea, New York, p. 189. 



