h5 



the factor of proportionality C^q "being a dimensionless height- dependent 

 quantity, termed resistance, drag, shear-stress, or friction coefficient. 

 The problem of determining the surface stress t^ is then reduced to ascer- 

 taining reliable values of C^q- 



Estimates of the drag coefficient have been based largely on indirect 

 evidence. The following five methods have been used so far: 



In air: (l) Wind profile method : Under adiabatic conditions the drag co- 

 efficient can be easily calculated from the log profile. In fact, the drag 

 coefficient is a function of the roughness length Zq and also of the height z. 

 With a diabatic wind profile, the additional knowledge of the vertical heat 

 flux is necessary. So far the wind profile method was used in about 22 

 studies in the field and in the laboratory and supplied quite useful results. 



(2) Geostrophic departure method : The covariance - pu'w' of 

 the turbulent fluctuations in the horizontal and vertical wind components is 

 recorded and supplies an estimate for the turbulent Reynolds stress. This is 

 a rather direct approach. Unfortunately, it needs a fixed or stabilized plat- 

 form as well as sensing elements of sufficiently rapid response. Therefore, 

 we have as yet only four or five studies of this kind. The data show a 

 considerable scatter. 



In water: (3) Sea surface tilt method : If an enclosed body of water is 

 available and the wind has blown for a sufficiently long period as to assure 

 steady state conditions, then the surface wind stress is assumed just to 

 balance the hydrostatic forces due to the tilt of the surface. The surface 

 slope provides an estimate for the wind stress or the drag coefficient. 

 Quite a number of studies (l8) were made up to now, partly in the field, 

 partly in the laboratory. The necessary accuracy (lO"') could mostly not 

 be achieved with small wind speeds. Disturbing effects as stratification in 

 the water, horizontal density gradients, near-shore effects due to waves, 

 nonsteady state etc., may make the result uncertain. 



At the water 



surface: (h) Surface film method : An insoluble monolayer is applied to 



the water surface . Its contraction under the action of the wind provides 



a measure of the wind stress. This seems to be essentially a laboratory 



method. Only one paper (Vines, 1959) has become known so far. T.-f*^- 



I would like to present the results obtained in a somewhat condensed we^c^ 

 form by showing a diagram (Figure 7) containing all the empirical relation- ^'^^^, 

 ships suggested between C-^q and u-^^q. There is, of course, a substantial 

 scatter in the single measurements which are not reproduced here . Looking 

 at these different results we are in a similar position as we were before 

 with regard to the roughness length. There is no satisfactory agreement 



