82 Wind System over the North Atlantic 



of the drag coefficient y^ {y^ = TjPaU^, where p^ is the density of air, and u 

 is the wind velocity at 6 m.) by these different methods. To date, values of 

 the drag coefficient range from 2 x 10~* to lO^^. The largest of these were 

 determined in strong winds by the slope-of-the-sea-surface method, the 

 smallest values under Ught winds from shipboard. Fig. 51 shows various 

 values of the drag coefficient. In spite of the intensive ejQFort that has been 

 made to measure the stress of the wind on the sea, there is so much scatter 

 in the various determinations that the stress is still not well known. It is 

 hard to imagine an object of study more important to the physical oceano- 

 graphy of ocean currents (see Montgomery, 19366). 



Finally, mention should be made of a fundamental difficulty in pre- 

 paring charts of the mean wind stress over the ocean : because of the non- 

 linear relation between surface wind and surface wind stress, the use of 

 mean wind charts for the computation of mean stress charts is not satis- 

 factory, especially in temperate and polar latitudes, where there are major 

 fluctuations in the observed wind field every few days. The proper way to 

 produce a mean stress chart would be the laborious one of first constructing 

 daily stress charts, and then obtaining vectorial mean stress from them. 

 Actually, the present state of the theory of wind-driven currents does not 

 justify such refinement. An approximate method, proposed by Reid 

 (1948 a), is based upon the use of charts of mean wind plus charts of mean 

 variability. 



