47 



(*the direction of the wind current at tlie very surface is assumed to 

 be 45° to the right of the wind). Angles a and a are the differences 

 between the direction of the surface current and that prevaihng at 

 the given fractions of the frictional depth. The same results were 

 first shown graphically by Ekman in the form of a diagram, a copy 

 of which is shown herewith. As an example of the use of Table 

 VIII, and as further illustrated by Figure 23, if the frictional depth 

 be 50 meters, then at a depth of 10 meters the water particles will 

 flow in a direction 36° to the right of the current on the surface. 

 So it is seen that if the 

 surface velocity and the 

 frictional depth be known, 

 the velocity and the di- 

 rection of the pure drift 

 current throughout the 

 vertical range may be de- 

 termined. Ekman has 

 found that for practical 

 purposes the equation 

 may be simplified to 



7. 6 F 



V sin0 

 (j) is based upon the value 

 of (I equal to 1.025; and 

 W represents the wind 

 velocity in meters per 

 second. It is easy to see 

 from (j) that the greater 

 the wind velocity the 

 deeper downward in an 

 ocean will its effect pene- 

 trate. Also since sin ^ is 

 zero at the Equator and 1 at the pole, it follows that given winds 

 will exert a maximum influence at the Equator and the least effect 

 at the pole. The density of the water is of some importance; a given 

 wind current will be stronger and penetrate to a greater depth in a 

 region of light water than in a region of heavy water. Table IX gives 



Table IX 



Z) = 



(j) 



Fig. 23.— Ekman's diagram in wliich the position and length 

 relatively of the successive arrows represent the direction 

 and velocity, respectively, of the pure drift current, down 

 to the frictional depth, set up as a result of wind and earth 

 rotation alone 



