20 G. I. TAYLOE ON EDDY MOTION IN THE ATMOSPHERE. 



represents the observed velocity and direction of strong winds at different heights. 

 In each of the figures the curve on the right represents deviations from the gradient 

 direction, which is shown as a vertical line. The curve on the left represents wind 

 velocity at different heights. 



It will be seen that there is good agreement between the two sets of curves. 

 Strong winds have been chosen for the comparison in preference to light winds, 

 because it is less likely that heat-convection currents will persist through such a 

 distance before mixing takes place, as to prevent the resistance, due to eddy motion, 

 from obeying the ordinary laws of viscosity. The observed curves for light winds, 

 however, agree as well with the theoretical curves as those for strong winds. 



Besides the various points of resemblance already noticed between theory and 

 observation, an inspection of the curves in figs. 4 and 5 reveals yet another. 

 Above the height at which the gradient direction is attained the wind goes on veering 

 slightly up to a certain height, when it begins to return again to the gradient 

 direction. The wind is again blowing along the gradient direction at a height 

 slightly less than twice the height at which it first attained it. Nearly all the curves 

 in Mr. DOI?SON'S paper have this characteristic sinuosity, but they are not the only 

 ones which show it. Mr. J. S. DINES, in his Third Report to the Advisory Committee 

 for Aeronautics (1912), has published a number of curves which exhibit the same 

 sort of sinuosity. The theoretical curve, fig 4, has the same characteristic. The 

 successive heights H u H'j, H'^, ... at which the wind blows exactly along the gradient 

 direction are given by the solutions of equation (18). 



o 



We have already obtained the first solution, namely BB^ = -- + a. 



The others are ETL\ = +a + ,r, BH", = + + 27r and . 



4 4 



The ratio of the first two is 1 = 'fj 7?r ) + a 



When a = 20 degrees this is equal to 2' 16. 



In the case of strong winds it will be seen from fig. 5 that the observed values of 

 H, and R\ are 900 metres and 1750 metres. Hence the observed value of H^/H! is 

 1'95. The good agreement between the observed and calculated values of H'/H is 

 possibly a coincidence, but it is interesting to notice that, on theoretical grounds, 

 we should expect a sinuosity in the curve representing the direction of the wind 

 at various heights when it blows under the action of a constant pressure gradient, 

 and that such a sinuosity is actually observed. 



The close agreement between theory and observation is evidence that the 

 assumptions made in the theory are correct. In particular the eddy motion does not 

 diminish much in the first 900 metres. 



