SECT. 2] 



SMALL-SCALE INTERACTIONS 



83 



0.0015 



0.001 



0.0005 - 



0.1 



0.05 



-I 



Fig. 18. Coefficient (for height 8 m) in the bulk evaporation formula (8), derived from the 

 theoretical values of Fig. 17 and the drag coefficient relationship (22) shown in Fig. 6. 

 The right-hand scale gives the coefficient for p—1.2x 10~3, pressure 1000 mb, in the 

 equivalent formula E = coeff. x wsl^s "" ^s)- 



(a) A=7.8; (b) A=11.5; (c) A = 27.5, in Sverdrup's (1937) theory; (d) Sheppard's 

 theory. 



M, The Bowen Ratio 



In computing evaporation on the basis of the heat balance (for accounts of 

 which see Sverdrup et at., 1942, pp. 100-125, or Sverdrup, 1951), it is necessary 

 to be able to derive the Bowen ratio, jS, which is the ratio of vertical flux of heat 

 to that of latent heat, i.e. 



^ = HjLE, 



where L is the latent heat of vaporization of water. As first deduced by Bowen 

 in 1926, ^ may be estimated from the sea-air temperature and humidity 

 differences using the relationship 



^ = B{ds-da)l{qs-qa). 



In a turbulent region with equal transfer coefficients for heat and water vapour, 

 the value of B = CplL would apply. In actual circumstances, where there is also 

 a layer of predominantly molecular transfer to be included, the relationship is 



B = {r„irE){c^iL). 



As seen above, the indication from the theories, at least in near-neutral condi- 

 tions, is that FhITe is close to 0.98 ; so that taking account of the surface film 

 of molecular transfer the value of the Bowen ratio calculated on the usual 

 assumption of B = CplL is only slightly changed. 



