80 Wind System over the North Atlantic 



uncertain. Recently (Van Dorn, 1953), very precise measurements have 

 been made of the slope of the water surface in a large regular artificial 

 basin. Under such ideal conditions the method gives better results, but 

 the fetch is so short that large waves do not develop. 



Experiments to determine the drag of wind on a free water surface have 

 also been made by Keulegan (1951) and Francis (1951) in wind tunnels. 

 There is evidence that the presence of large waves may not have an 

 important effect on the drag coefficient. Francis (ibid.) suggests that only 

 the very smallest wavelets and ripples act as roughness elements. 



Another method of determining the -wind stress stems from laboratory 

 results in the field of aerodynamics. In these experiments it has been 

 found that equations for the stress in a boundary layer can be computed 

 from a knowledge of the variation of wind velocity \nth altitude over the 

 first 10 m. above the sea. Rossby and Montgomery (1935) apphed these 

 aerodynamic equations to observed %vind profiles. Table 1, adapted from 

 Sverdrup, Johnson, and Fleming (1942, p. 67), gives drag computed from 

 these equations. 



TABLE 1 



Correspondence of Wind Stress (in Dynes per Square Centimeter) 

 TO Wind Velocities Measured Fifteen Meters Above Sea Level 



Wind velocity (m. /sec.) 2 4 6 8 10 12 14 16 18 



Wind stress (dynes/cm.*) . . 0-04 0-16 0-34 1-81 2-83 4-09 5-56 7-25 9-20 



Source: Sverdrup, Johnson, and Fleming (1942, table 67). 



No account of the thermal instability or stability of the air relative to 

 the water is included in these studies. There is need for ingenious and 

 careful experiments and observations on this problem. 



A measure of the wind stress on the sea can be obtained (Sheppard 

 and Omar, 1952) from purely meteorological data, in the following 

 way : 



Let u and v be the horizontal components of the wind along axes x and y, 

 where x is in the direction of the surface ^vind Uq. One then supposes the 

 following equilibrium to hold in the a:-direction : 



I dp Idr^ 

 /w--/ + -^=0. (1) 



pox p dz 



Let us suppose that Vg is the ^/-component of the geostrophic mnd ; then 



^=/>/K-^^); (2) 



