124 CHARACTER OF SURF 



rapidly increased, so as to form rollers which toppled over on the 

 beach at regular intervals with a noise like thunder. Sometimes this 

 surf increases suddenly during perfect calms, to as great a force and 

 fury as when a gale of wind is blowing.'' A moderately heavy surf 

 also develops, similarly, from time to time on the coast of Peru in 

 calm weather, as described many years ago by Humboldt, who ob- 

 served breakers 10 to 14 feet high at Callao, on such occasions. 39 And 

 we can assure the reader that it is no less spectacular a sight now than 

 it was then, as one looks out over the sea, to watch the swells, so small 

 as to be imperceptible offshore, being reborn, as it were, over the shoal- 

 ing bottom in glassy calm weather, and then increasing in height, 

 without apparent cause, until they break. 



Two examples in the development of breakers of moderate size from 

 a swell so low offshore that the photographs give no hint of its ex- 

 istence to seaward of the breaker line are pictured in figure 30. Nor is 

 it unusual for the breakers that form in this way to be of the same 

 general order of magnitude as the seas are that break out over 

 deep water in windy weather, i. e., as high as 6 to 20 feet, and some- 

 times even higher. The surf that so constantly pounds the exposed 

 faces of coral reefs and islands in the western tropical Pacific and 

 Indian oceans, even in calm weather, are cases in point. 



The individual breakers that compose a surf always vary consid- 

 erably in height as they succeed one another. Thus it has been esti- 

 mated that the heights of the breakers on the Californian beaches 

 usually vary between two-thirds and four-thirds of their mean heights, 

 which probably applies to breakers from swells in general. And our 

 own observations suggest that the variation is often wider still for 

 breakers caused by storm seas. These variations are due in part to 

 the fact that the individual members of any train of waves always 

 differ more or less both in height and in initial steepness, one from 

 the next, partly because it is a common event in a storm for a train 

 of waves to come considerably larger than the common run (p. 00), 

 and also because two or more waves, or series of waves, ofen unite be- 

 fore they reach the surf zone, whether advancing in the same direction 

 or coming from different directions, so that a breaker or series of 

 breakers much larger than the others may come at intervals. One of 

 15 feet, observed at Long Branch, New Jersey, February 8, 1944, when 

 the average height during a 10-minute interval was only 4 feet, may 

 have had this origin. And while there is no foundation for the old 

 notion that every seventh or every ninth breaker is invariably the 

 largest, experienced surfmen are well acquainted with the fact that 

 an unpredictable single huge breaker, or a series of such, may develop 

 on days when the general run are low, or of only moderate heights, 



^Uuuiboldt, A. von. 1S5.H. Koanins, Stuttgnrt vol. 4, p. 229. 



