46 



DEACON AND WEBB 



[chap. 3 



curve). These and other studies at different heights over land surfaces show 

 that, for near adiabatic or unstable lapse rates of temperature, it can be taken as 

 an approximate rule that the highest frequency which need be catered for is given 



4 6 10 20 40 60 100 200 400 



cycles /hour 



Fig. 1. Atmospheric eddy spectra. (After Panofsky and Deland, 1959, Fig. 15.) 



O ■ Spectrum of vertical velocity (w) 



X Cospectrum of w and horizontal wind speed. 



The ordinates are spectral intensities in m^ sec~2 per unit range of In (frequency). 



by u/z, i.e. the mean wind speed divided by the observation height. The height 

 dependence is a result of the observed fact that the scale of the eddy motion 

 (vertical component) increases in proportion to height above the boundary. 



The eddy-correlation method of flux measurement has been treated at 

 greater length than is appropriate here by Swinbank (1951) and also in Priest- 

 ley's (1959) book on transfer. Types of instrumentation have been described by 

 Swinbank {loc. cit.), Cramer and Record (1953), Krechmer (1954) and Mcllroy 

 (1955). Mostly hot-wire anemometers have been used to sense the eddy velocity 

 components and fine wire resistance elements the temperature fluctuations : 

 with these, photographically recording mirror galvanometers are commonly 

 employed. 



Over land the application of the eddy-correlation technique has already led 

 to considerable advances in knowledge on the transfers of heat, water vapour 

 and momentum and to the discovery of relationships of wide application. As 

 yet only a few measurements have been made over the sea but, as will be seen 

 later, even these few are helpful in resolving discrepancies arising from the 

 other methods. More extensive application of the eddy-correlation method 

 should be rewarding. 



B. Profile Methods 



To be able to deduce the vertical flux of S from the vertical gradient requires 

 a knowledge of the eddy transfer coefficient, Ks, defined by 



Fs = pKs dsjdz. (2) 



