30 M.J.BLACKWELL 



If, following Rider (1954b), an attempt is made to use the generalised 

 wind-profile method of eq. 22, it is found that the complex dependence 

 of d on crop height and wind speed makes it impossible to determine 

 Deacon's stabiHty parameter ^ with the accuracy required to give E from 

 the modified aerodynamic equation: 



The existence of errors in readings from the present type of anemometers 

 must also be borne in mind. Even i % errors can produce changes in j8 of 

 the order of 10% and there is no doubt that we must reject any further ideas 

 of using tliis method over tall crops. Simultaneous measurements, with two 

 sets of apparatus used at different heights, again confirm the value of making 

 measurements at the lowest practicable height. It is unfortunate that this 

 requirement causes almost insuperable difficulties in determining the zero- 

 plane displacement and other parameters needed in the calculations. 



In the present writer's opinion, one of the next steps must be to arrange 

 for StabiHty effects to be clearly recognisable so that the function/(Ri) can 

 be investigated and suitably corrected for. In addition, in the simple 

 aerodynamic formulae (eqs. 14 and 15) for E and Q, and in the energy 

 balance control checks (eq. 1 1), we may have to introduce simple ideahsed 

 models for the variation of KhlK-m with the Richardson number. The fact 

 that the use of formulae, vahd only for neutral conditions, leads to results 

 which have even some limited practical value, makes it seem worthwile to 

 introduce an equally simple and idealised stabiUty function f{Ri)- A 

 schematic diagram of the main results from three observational pro- 

 grammes, designed by members of the Cambridge group, is given in 

 Fig. I , together with a hue based on Deacon's eq. 24 and two Holzman-type 

 lines based on the Cambridge data. The latter are : 



f{Ri) ^ i-isiRi (for Ri < 0) (28) 



and ■ f{Ri) = 1 - ilRi (for Ri > o) (29) 



These values of a are of a similar order to those found elsewhere. It is to be 

 expected from recent work on free convection that a fairly sharp transition 

 will occur between the regimes for forced and free convection. 



It is perhaps surprising that Rider and Robinson's (195 1) method of 

 extrapolating 'aerodynamic' fluxes to zero height, or to zero Pdchardson 

 number, has received so httle attention in later years. This method clearly 

 needs careful investigation, but it is interesting that the experimental 



