120 



WIND CURRENTS AND WIND WAVES 



From wind measurements at different distances above the sea surface 

 conducted by Wlist, Rossby and Montgomery concluded that at moderate 

 wind velocities the sea surface has the character of a rough surface with 

 roughness length 0.6 cm. From a general theory of the wind profile in the 

 lower part of the atmosphere, Rossby later developed formulas that 

 permit computation of zo from the angle between the surface wind and 

 the gradient wind, or from the ratio between surface wind velocity and 

 gradient wind velocity. When applying these formulas to observed 

 values from the ocean, Rossby found that at moderate and strong winds 

 the roughness length is independent of the wind velocity and is equal to 

 0.6 cm. With this value of zq, one obtains from formula (VII, 7) 



TO = 2.6 X 10-'PaW,\, 



(VII, 8) 



where pa is the density of the air and "FF15 is the wind velocity 15 m above 

 the sea surface. 



As early as 1905, Ekman derived a similar expression by an entirely 

 different approach, which will be explained on p. 122. Ekman's con- 

 clusions have been confirmed by the recent studies of Palmen and Laurila, 

 but all results apply to the stresses exerted by moderate or strong winds. 



At low wind velocities, somewhat different conditions appear to 

 prevail. From his analyses, Rossby concludes that at low wind velocities 

 the sea surface has the character of a smooth surface, because the wind 

 profiles indicate the existence of a laminar sublayer. At low wind veloci- 

 ties the stress would therefore be computed from equation (VII, 4), 

 leading to values which are about one third of the values obtained on the 

 assumption that the surface is rough. 



Table 14 



STRESS OF THE WIND (G/CM/SEC^) CORRESPONDING TO STATED 

 WIND VELOCITIES (IN M/SEC) AT A HEIGHT OF 15 M AND ASSUM- 

 ING THE SURFACE TO BE SMOOTH OR ROUGH (ROUGHNESS 

 LENGTH, 0.6 CM) 



Table 14 shows the values of the stress corresponding to a smooth 

 surface or a rough surface characterized by Zo = 0.6 cm. It is evident 

 that, if the sea surface can be considered smooth at wind velocities below 

 6 or 7 m/sec, this limit is of little significance, but the above conclusions 

 need to be confirmed by many more data. The problem of the stress 

 of the wind still deserves great attention; it is particularly desirable to 



