28 M.J.BLACKWELL 



formula (eq. 14) can be derived with an accuracy of about ± 20% for 

 typically wet seasons in this country, with a fairly small range of stability 

 and for periods of a few hours. 



To give a practical example of the advantage of taking measurements 

 near the ground, Pasquill (1949) showed that the non-dimensional para- 

 meters of eq. 19, which we refer to as k^f[Ri), would have a two-fold 

 variation at a height of 37-5 cm for a corresponding ten-fold variation at 

 150 cm. The variation found at the lower level covered a wide range of 

 natural conditions, from moderate stabihty to marked instability, and is 

 merely another expression of the statement above that fluxes derived from 

 the assumption that f{Ri) equals unity would normally be in error by up to 

 ±20%, even with the most favourable experimental arrangements, for 

 short-term rates of evaporation. 



Returning to the definition of eddy viscosity, we must next re-examine 

 the behaviour of Km with stability, and hence of Ki„. It is clear that, to 

 explain the stabihty dependence found : 



i^,n = (i^m) neutral • /(^O M 



For unstable conditions, the most promising approach has been to follow 

 Deacon (1949) who proposed that wind profdes could be well represented 

 by relationships of the type: 



~ = az^& [a and ^ being independent o£z) (21) 



where j3 is a slowly varying parameter, close to unity, taking values ^>i, 

 I and <i in unstable, neutral and stable conditions respectively. Noting 

 the observed tendency of non-neutral wind profdes to approach the 

 logarithmic form (as for neutral conditions -eq. 2) near the surface. Deacon 

 assumed the boundary condition (n = o at z = z^ to hold in the general 

 case. Integrating eq. 21 and allowing for the profile tendency to approach 

 the logarithmic form as ^ approaches unity, he obtained the generahsed 

 wind profile: 



Since eq. 21 is not an exact relationship, values of ^o derived from eq. 22 

 are found to vary markedly with stability. To overcome this difficulty, it 

 is customary to determine z^^ separately from neutral wind profiles and to 

 regard eq. 22 as determining /3 for various conditions of stability. From the 

 definition of Km and the generalised wind profile : 



