ledge it seems very difficult to estimate with the necessary de- 

 gree of accuracy the value of the single components of wind force 

 \mder different conditions, especially when considering the compli- 

 cated sea surface pattern with its complex wave motion. Further 

 dlfficiaties arise in the calculation of the work done by the wind 

 on the wave motion, when the actual wind speed at the sea surface 

 or the actual velocity difference between the motion of water particles 

 and the wind is sought. 



Under these circumstances it seems more expedient at present, 

 to make use of empirical relationships between the "effective wind 

 stress" at the rough sea surface and the wind velocity. The re- 

 sultant action of tangential stresses and normal pressures may be 

 estimated by certain observations and related to the average wind 

 speed at "anemometer-height." "Anemometer-height" is defined as a 

 height of about 10 m above the sea surface, where the vertical in- 

 crease of wind velocity with height is relatively small. 



The second question which becomes quite Important in this prob- 

 lem is the question of energy dissipation with turbulent wave motion. 

 The energy dissipation may be due to viscosity or molecular friction, 

 in the absence of turbulence. But in the case of ocean waves, the 

 energy dissipation by eddy viscosity (which Ekman calls turbulent- 

 friction or virtual friction) has to be taken into account. This 

 turbulence in the uppermost layers of the sea seems to be caused 

 mainly by the breaking of larger and smaller waves, associated with 

 whirling and stirring up of the water masses (eddying) due to un- 

 stable wave motion. If there are any other more efficient causes 

 which lead to an additional eddying of the sea surface water, that 

 is, to an additional txirbulent motion, the wave motion or wave- 



37 ' 



