bright path across the rippled surrounding surface which, with its 

 rough wave motion, appears darker. 



With a very weak wind, when the first small wavelets grow up 

 in the initial state of wave formation, the energy dissipation ap- 

 parently is determined only by viscosity or molectaar friction (if 

 the water is not disturbed by strong currents, for example, strong 

 tidal ciu"rents or other turbulent motion). Since surface tension 

 is of decisive influence on small wavelets (besides gravity and 

 viscosity), a minimum wind velocity for the generation of initial 

 waves is found (Neumann [14]). This limit is a wind speed of about 

 70 cm/sec, and the first ripples that will be generated are those 

 with a wave length of 1.75 cm. If the wind speed is 90 cm/sec the 

 wave length of the ripples has increased to 5»2 cm, provided we dis- 

 regard capillary waves which are generated together with the longer 

 (gravity) waves, (The wave length of these capillary waves at a 

 wind velocity of 90 cm/sec would be about 0.57 cm.) These "initial 

 waves" grow in such a way, that their steepness increases with in- 

 creasing wind speed. At a wind velocity of about I23 cm/sec these 

 primary waves attain the steepest form possible for stable waves. 

 The ratio of wave height to wave length is in this case H/x = I/7, 

 which is the maximum steepness computed by Michell based on the 

 theory of Stokes. The wave length of the ripples in this state of 

 development generated by a wind of I23 cm/sec velocity is about 

 10,5 cm. ♦ 



If the wind becomes a little stronger, the pointed crests of 



* In shallow water layers, where the bottom friction is an influenc- 

 ing factor, the wind velocity for generating waves of maximum 

 steepness is higher than in the case of "deep" water, and the 

 wave length of the ripples with maximum steepness is shorter 

 than in the case where the waves "do not feel" the bottom [14], 



39 



