SECT. 2] 



s:mall-scale interactions 



65 



temperature is 22% greater than when air and sea temperatures are equal, and 

 Brown (1953) and Fleagle (1956) have reported effects of similar magnitude. 



When applying drag coefficients to the calculation of surface stresses, the 

 distinction between measured and estimated wind strengths should be re- 

 membered. The values estimated from the appearance of the sea surface are 

 virtually estimates of surface stress or of wind speed close to the surface, and 

 the conversion from Beaufort scale to wind speed at, say, 10 m height should 

 properly depend on the stability of the air (Roll, 1953/54). But, using the 

 customary conversion which neglects any stability effect, the resulting wind 

 speeds are most appropriately used with neutral drag coefficient values. 



F. Effect of Slicks 



The sun glitter measurements of mean-square slope shown in Fig. 9 include 

 a number taken after the application of oil to the sea surface to produce 

 artificial slicks some 700 m square. The oil film suppresses the higher-frequency 

 wave motions, which make a large contribution to the mean-square slope. 



That the supf)ression of the riijples and wavelets has a marked effect on the 

 stress is shown by Van Dorn's (1953) results using the surface-tilt method 

 applied to an artificial pond 250 m long. Stresses measured in this way, with 



4 6 8 10 12 



Wind speed , m/sec 



Fig. 10. Van Dorn's measurements of stress on a model-yacht pond. 

 O Natural water surface • Detergent applied a During heavy rain 



and without a surface film, are shown ip Fig. 10. In this case the surface film 

 was produced by the application of detergent dispensed as a powder at the up- 

 wind end of the pond. It seems probable, as suggested by Munk (1955), that 

 we may take Van Dorn's results as indicating the relative magnitude of the 

 skin-friction and form-drag contributions to the total surface stress. 



The damping of capillary ripples by surface films has been studied by 

 Vines (1961). The natural films present on the surface of the sea as a result of 

 biological activity may be a factor of some importance influencing the state 

 of the sea. 



