68 



SEAS AND SWELLS 



storm: and the length of the warning will depend on the rate at which 

 the waves outstrip the storm. Swells, for example, with an average 



period of 10 seconds, coming from a tropical hurricane COO miles off 

 shore won Id reach the coast about 24 hours in advance of the storm 

 itself, if the latter were advancing at the rate of 10 knots. But 12- 

 second swells would precede the storm by about 9 hours only, if it were 

 advancing at 15 knots. And we must caution the reader that the 

 approach of a hurricane is not always heralded in this way by swells 

 Coming in advance of it. 



Position of Observer 



Figure 20. — Diagram to show the changes in directions from which swells come, 

 with the advance of the storm center that produces them. 



The direction from which swells come, from hour to hour, may also 

 give some clue to the direction in which the storm center is moving. 

 But the application of this principle is complicated by the fact that 

 the directions of the waves within tropical hurricanes may diverge 

 considerably from the direction of the wind there, as described on 

 page 40, because the latter circles so sharply along the atmospheric 

 pressure gradients in storms of this type, to the left (counterclock- 

 wise) in the Northern Hemisphere and to the right (clockwise) in the 

 Southern Hemisphere, according to the "Law of Storms," with which 

 every navigator is familiar. And while the direction of the swell, in 

 reverse, as observed out in deep water, points toward the storm area, 



