34 WAVE DIMENSIONS 



are made on smaller, hence younger, waves, and that gales so com- 

 monly change direction before the velocities of the waves they produce 

 have risen as high as that of the wind. And it has long been known 

 that as soon as the wind does slacken, the waves outstrip it in their 

 continued advance, so much so, that the waves of an old swell often 

 run at velocities as high as 30 to 40 knots, and sometimes even as high 

 as 60 knots (as indicated by their periods) even during a flat calm. 



The nearest approach to a working rule that we dare offer for the 

 velocities of waves, as compared to that of the wind, is the following: 

 Storm seas that have risen nearly, but not quite, to their maximum 

 heights for the wind in question, are usually traveling at a velocity a 

 little lower than that of the w T ind, if the latter is still blowing strongly ; 

 waves may outstrip the wind slightly, even while the latter is near 

 its peak strength, if it has been acting on them for a long period; and 

 the waves invariably run faster than a dying wind. 



The waves, produced by a storm in the offing, often give warning 

 of its approach before the wind has begun to blow up where the 

 observer is stationed. The reason is that atmospheric disturbances 

 often advance at rates much lower than the velocities of the winds 

 within them, or than the rates at w r hich the resulting waves advance, 

 as is illustrated by the fact that only 60 out of a group of 264 gales 

 were found to have traveled faster than about 31 knots from the 

 Atlantic toward the coasts of Great Britain (Cornish, 1934, p. 28). 

 The coastwise inhabitants of many parts of the world are, in fact, 

 well acquainted w T ith large waves as forerunners of storms — they were 

 known at one time (perhaps locally) as "death waves" on the west 

 coast of Ireland, and perhaps still are (Kriimmel, 1911, p. 92). This 

 general phenomenon is of practical importance in those parts of the 

 world, in particular, where tropical cyclones are to be expected during 

 the "hurricane" or "typhoon" season, for when heavy swells develop 

 there, for which the w T ind then blowing is not responsible, the chances 

 are that they are coming from an atmospheric disturbance of this 

 sort. This applies in the West Indian-Gulf of Mexico region and 

 off the southeast coast of the United States from July through Octo- 

 ber; in the southwestern tropical Pacific from December through 

 March and into April; in the Philippine region and the China Sea 

 from June through October; in the Arabian Sea from March through 

 June and from September through December; in the Bay of Bengal 

 from May through December; and in the southern Indian Ocean from 

 November through May. These are the months when hurricanes or 

 typhoons occur most often, not the extreme dates for them. 



We have seen it stated that the velocities of these forerunning swells, 

 as calculated from their periods or from their lengths, give the 

 velocities of the winds within the approaching hurricanes, on the 



