WIND CURRENTS AND WIND WAVES 133 



Every wind system, whether stationary or moving, will create currents 

 associated with the redistribution of mass due to wind transport. It is 

 furthermore possible that within a moving wind system the distribution 

 of mass does not become adjusted to the wind conditions, and that 

 actual piling up or removal of mass may occur such as takes place in 

 partly landlocked seas. If this is true, slope currents (p. 108) reaching 

 from the surface to the bottom develop, but they are of a local character 

 and are soon dissipated. One may thus expect that superimposed on the 

 general currents will be irregular currents due to changing winds and, 

 furthermore, eddies that are characteristic of the currents themselves 

 and independent of wind action. A synoptic picture of the actual 

 currents can therefore be expected to be highly complicated. 



Origin of Wind Waves 



It is evident to the most casual observer that surface waves are 

 created by wind, but only recently a successful physical explanation of 

 the process has been presented by H. Jeffreys. Jeffreys avails himself 

 of the fact that in a turbulent flow of air, eddies are formed on the lee side 

 of obstacles. Thus, when the wind blows over a sequence of waves, 

 eddies will be formed on the lee side of the waves, for which reason the 

 pressure of the wind will be greater on the windward slopes than on the 

 slopes that are sheltered by the crests. This condition can prevail, 

 however, only if the waves travel at a velocity which is smaller than the 

 speed of the wind. On the basis of these arguments, Jeffreys finds that 

 waves may increase only if 



c(w - cy ^ ^"^^"J '''^ . (VII, 23) 



Here W is the velocity of the wind, c is the velocity of the waves, v is the 

 kinematic viscosity of the water, g is the acceleration of gravity, p and 

 p' are the densities of the water and the air, respectively, and s is a 

 nondimensional numerical coefficient that Jeffreys calls the ''sheltering 

 coefficient." It should be observed that in his reasoning Jeffreys takes 

 into account both the turbulent character of the wind and the viscosity of 

 the water. His theory therefore must be expected to give results in 

 better agreement with actual conditions than earlier theories based on 

 the concepts of classical hydrodynamics, which neglect turbulence and 

 viscosity. 



The term on the right-hand side of the equation (VII, 23) is always 

 positive. The product on the left-hand side must therefore always be 

 positive and can exceed the right-hand term only if the wave velocity 

 differs sufficiently both from zero and from the wind velocity. For any 

 given wind velocity, there can be only a limited range of possible wave 

 velocities. At a given wind velocity the right-hand side of (VII, 23) 



