778 TABLE 897.— OCEAN CURRENTS 



The permanent currents of the ocean are maintained by differential heat and cooling 

 and by the indirect effects of the wind. They may extend to depths as great as 1,000 m 

 and their speed is usually less than 50 cm/sec. In the Gulf Stream and Kuroshio, speeds 

 may exceed 250 cm/sec. Volume transparents of the large current systems exceed 50 

 million tons/sec. 



Wind-driven currents induced by the drag of the wind are generally shallow, less than 

 100 m, flow with speeds about 2 percent of wind, and deviate about 30° from the wind 

 direction, to the right in the Northern Hemisphere and to the left in the Southern 

 Hemisphere. 



Tidal currents follow elliptical orbits during each tidal cycle. Motion probably extends 

 to the bottom. In restricted coastal channels the currents are reversing and sometimes 

 exceed 250 cm/sec. 



WAVES AT SEA * 



Whenever the wind blows over the water, the surface is formed into waves which grow 

 under the influence of the wind and form a most irregular surface known as a sea. Such 

 waves traveling out from a storm area are called swells. As waves break near the shore 

 surfs are formed. 



Waves may also be formed by earthquakes, fault movements, submarine landslides, or 

 volcanic eruptions beneath the sea. 



The height of a wave, H, is the vertical distance from crest to trough. The length, L, 

 is the horizontal distance between adjacent crests. The wave period, P, is the time interval 

 between passage of successive crests at a fixed point. The velocity, V, of a wave is the 

 speed with which the wave travels along the sea surface. 



The following relations hold for depths greater than one-quarter wavelength with good 

 approximation : 



L = 5 P\ V = 3P 



where the wavelength, L, is in feet, the period, P, in seconds, and the velocity, V, in knots. 

 The waves move along the surface of the water but the water, on the other hand, advances 

 very little — about one percent only of the wave velocity. 

 The height of the sea is determined by three factors : 



Wind velocity, average speed of wind over fetch. 



Fetch, distance over wind blows. 



Wind duration, how long the wind blows. 



Tables 886 and 898 show the wave heights for some conditions. 



* Abstracted from an article prepared for the Encyclopedia Britannica, by Walter Munk, Scripps 

 Institute of Oceanography. Used by permission. 



TABLE 898.— WAVE HEIGHT IN FEET FOR VARIOUS WIND VELOCITIES 



AND FETCHES 



( See also Tables 886, 887, and 895. ) 



SMITHSONIAN PHYSICAL TABLES 



