of the sea, and a definite roughness of the sea surface. Thus they 

 contribute essentially to the resistance coefficient and therefore 

 to the effective horizontal stress, which acts at the rough air-sea 

 interface. If longer waves emerge at this rough interface the total 

 horizontal stress will do a net amount of work even if the longer 

 waves travel faster than the wind (see equation (6) with (4a), or 

 A. in (9)). Thus, a certain "waviness" of the air-sea interface 

 will be maintained against dissipation as long as the wind velocity 

 remains constant. 



However, the mechanism governing the tangential transfer of 



energy to waves which may travel with ff > v is not yet completely 



2 2 

 explained. If the surface mass transport velocity v 6 (r from 



Stokes' theory is taken into account, (equation (42)), a transfer 

 of energy from wind to waves would be possible even if the waves 

 move faster than the wind (equation (6) and A. in (9)). This idea, 

 first used by Sverdrup-Munk [l] seems plausible, and is fit to over- 

 come the difficulties. But if the difference between the wind velo- 

 city and the horizontal component of particle velocity is introduced 

 in the expression for T^, and if the variation in shear due to the 

 variation of wind velocity and due to the variation of the water 

 particle velocity at the wavy surface is considered, apparently no 

 energy is added even on the basis of Stokes' theory, if the waves 

 move with 6" 5?. v. The result of an analysis of Schaaf and Sauer [15] 

 would limit the growth of the waves to the region where the wave 

 velocity is less than the wind velocity. From the expression for 

 A^ given by these authors, it follows that no energy is added by 

 tangential shear stress, if the wave velocity exceeds about 75% of 



86 



