upwind and downwind run occurs for waves of frequency 0.100 cps and 0,092 

 cps, respectively. The phase speeds of these waves are 30 and 33 knots, 

 respectively. It is an interesting coincidence that the average wind speeds 

 from the plane were 30 knots upwind and 33 knots downwind. While it is tempting 

 try to relate these seemingly coupled facts, it is more prudent to exercise restraint. 

 As mentioned previously, the wind speed is not known to anywhere near the necessary 

 accuracy„ Besides, while we have of necessity assumed a geophysically constant 

 wind field, there can be no question that the actual wind is variable. One only need 

 look at a typical power spectrum of horizontal wind speeds to obtain a feeling for this 

 variability. Small changes in waves and wave growth parameters, such as just inferred, 

 are a fact of nature. For these reasons the present work can only provide a type of 

 averaged look at the processes of wave generation. 



In comparing the observed OC 's with those predicted by Phillips' theory, 

 (Section 2.0), it is necessary to estimate the three dimensional spectrum of atmospheric 

 pressure fluctuations, P (k, <r ), since it can be shown (Hasselmann, 1960) that 



Recent work by Priestley (1965) provides the essentials for such an estimateo The 

 applicability of Priestley's results was first realized and used by Snyder and Cox, 

 (1966) to partially confirm Phillips (1957) theory. However, their results are restricted 

 to one wave length and to one direction. The general evaluation of OC (suggested by 

 Snyder in a personal communication) in terms of frequency and direction is given in 

 detail by Barnett (1966) and only the results of this evaluation are presented here. 

 The spectrum P(k,(r) is given by 



-*■ 00 -». -* -* -* 



P(k, o- ) = 1 / d 77 d T R(t7 , t ) cos (k . 77 + tr t ) 



(2 tt) 3 -CO 



where tJ" is a horizontal distance vector, t is a lag time, and R (77" , t ) is the 

 correlation function for the static pressure fluctuations. Priestley's work essentially 

 allows one to make an empirical representation of the integrand. Direct integration 

 then yields P (k, a ) in closed form. One problem arises in that the expression for 

 P (k, IT ) has a turbulence scaling factor in it. From Priestley's data, taken for low 

 wind speeds over closely mowed grass, this scaling factor was found by Snyder & Cox 

 to be proportional to approximately the fourth power of the wind speed. With such a 

 power law it is possible to extrapolate to higher wind speeds, provided one assumes 

 that the nature of the turbulent pressure fluctuations was similar during his experiment 

 and ours. However, first estimates of OC using such a representation for the scale 

 factor gave values that were lower than observation by a factor of approximately 50. 

 The fact that a difference existed was no real surprise, but it was a bit of a shock to 

 see such a large difference. A small amount of reflection on the matter, however, 



40 



