DEW: FACTS AND FALLACIES 39 



and error in using this formula elsewhere will be unimportant in this 

 analysis. 



The maximum or 'potential' rate of dew formation, found by setting 

 r= I in eq. 4 and eA = e^ in eq. 6 is 



W = ARIX{A+y) (4a) 



and is therefore proportional to radiative loss. Table i shows that potential 

 condensation increases with temperature from o to I5°C (because A 

 increases with temperature) but decreases at higher temperatures (because 

 radiation loss decreases with increasing vapour pressure). Over a wide 

 temperature range, potential condensation Hes within ± 10% of 0-067 

 mm/hr. 



Table i 

 Variation of potential condensation with temperature 



The dependence of transfer coefficient on wind speed is needed to fmd 

 condensation when r<i. From measurements on the evaporation from 

 smaU pieces of filter paper, de Vries and Venema (1953) gave 



h = 3-9F0-7 cal cm-2hr-i°C-i 



where V is wind speed in m/sec, and with this value in eq. 4, condensation 

 rates were calculated for varying wind speed and relative humidity (Fig. i). 



Equation 4 sets an upper hmit on leaf dew formation, unhkely to be 

 achieved in practice, even on rare occasions when the atmosphere is 

 saturated, because the approximate value of R in eq. 5 is a dehberate over- 

 estimate. First, in the spectral region 3 to 25 microns, the emissivity of 

 many leaves is 0-90 to 0-95 (Gates and Tantrapom, 1952) so the assumption 

 of black body emissivity overestimates R by 5 to 10%. Second, because 

 Ts< Ta, radiation emitted by the leaf wiU be less than ZaTg'^ by an amount 

 which can be estimated for two special cases : 



Case I : isolated horizontal leaf exposed some distance above a crop 

 canopy which has radiative temperature Tg 



R=2GTs^-aTs^-U 



