SECT. 21 



SMALL-SCALE INTERACTIONS 



57 



where Uc is the corrected wind speed and h the height of the anemometer above 

 the axis of roll. 



3. Drag Coefficients of the Sea Surface 



A. From Wind-Profiles 



Values of the drag coefficient Cio = rl{puio^) (the subscript 10 denoting that 

 the wind speed is for 10 m height) are collected together in Fig. 6 from the 

 results under neutral or near-neutral conditions obtained in those studies made 

 with the more satisfactory exposures. The broken line for an aerodynamically 

 smooth surface is that indicated by equation (11) from laboratory results for 



0.003 



0.002 - 



a> 0.001 - 



0.1 S 



0.01 

 0.001 



wind speed, m/sec 



Fig. 6. Drag coefficients of the sea surface for near-neutral conditions. 



From wind-profiles By eddy -correlation method 



+ Montgomery ^ Mcllroy 



Johnson ^ Vinogradova 



A Roll 



• Deacon, Sheppard and Webb 



O Takahashi 



A Brocks ; North Sea 



n Brocks ; Baltic 



the flow over polished flat plates. Also in the laboratory, Vines (1959) has shown 

 that a liquid surface at low air speeds has the same drag coefficient as a polished 

 plate. On the atmospheric scale there is some evidence (Deacon, 1953) that 

 the wind-profile over such surfaces as very smooth wet mud and level fresh 

 snow conform quite closely with the laboratory results. So for the unruffled sea 

 surfaces at speeds below about 1.5 m/sec (Francis, 1951 ; Van Dorn, 1953) the 

 drag coefficient should be close to that indicated by the broken line in Fig. 6. 

 Montgomery's (1936) results were obtained over water 6 m deep in Buzzard's 

 Bay with on-shore winds. They suggest a drag coefficient close to the smooth 

 surface value up to 6 m/sec but subsequent work suggests that some of his 

 values may be somewhat too small. Van Dorn's (1953) observations of the 

 surface of a pond showed that incipient rippling occurs with wind speeds at 

 25 cm height between 1.1 and 2 m/sec (10 m speeds 1.5 to 3 m/sec) : at slightly 



