HEIGHT 25 



The stormier latitudes of all oceans experience about equally severe 

 gales at one time or another; hence it is not astonishing to find that the 

 largest storm waves that have actually been measured so far have 

 been of about the same heights in the North Atlantic as in the South 

 Atlantic and in the Southern Ocean. 



Single high waves and groups of high waves. — Successive waves al- 

 ways differ considerably in height, whether the general run is high 

 or low at the time, and from time to time a wave comes that is con- 

 siderably higher than the common run. Thus a 6- to 8-foot wave is 

 not unusual when the common run is only 4 or 5 feet, nor are storm 

 waves more than 30 feet high uncommon, even when the average is 

 only 18 to 20 feet while occasional single waves of 50 feet, or even 

 higher, have been observed not uncommonly, as just noted. This is 

 partly because longer and hence faster running waves are constantly 

 overtaking and combining with the slower running ones. This hap- 

 pens when there are two series of waves present, and the phenomenon 

 is called interference. And the chief source for outsize waves of this 

 sort is the union of those that advance from different directions, a 

 frequent event in stormy weather. When this happens, the joint 

 waves may be much higher than those that precede them or that follow 

 them; and there is no way to predict the coming of a wave of this 

 sort. Likewise, when a trough of one series of waves coincides with 

 the crest of another, the resulting wave is considerably lower than 

 most. 



During a gale, a ship may also encounter groups of waves, from 

 time to time, that are much larger than the usual run; these may be 

 the product of the more violent squalls with which every gale is 

 punctuated. During the early stages of a blow, when the waves are 

 still so steep that the sea is breaking, a sharp squall often lowers 

 the heights of the waves by temporarily cutting off their tops bodily, 

 as noted elsewhere (p. 19). But the effect of a squall later in the 

 gale, when the waves are relatively longer, is to increase the size of 

 the particular group on which it acts. And, since every gale of any 

 severity is interspersed at irregular intervals by squalls of brief 

 duration, the wave pattern is correspondingly interspersed by groups 

 of considerably larger crests. Cases frequently quoted are a 4-minute 

 squall during a moderate gale in the North Atlantic that was accom- 

 panied by waves about 7 feet higher than the ordinary run (p. 20) ; 

 and another 3 minutes in duration with waves 6 feet higher. The 

 number of individual waves that may combine to form a train of this 

 sort depends on the area covered by the squall responsible for them 

 and on the rate at which it is advancing as a whole, not on the velocity 

 of the wind within it. A squall, for example, advancing at a rate of 

 20 nautical miles per hour and occupying 4 minutes in its passage (a 



