38 J.L.MONTEITH 



where h is a transfer coefficient depending on wind speed and surface 

 aerodynamic properties. Ignoring a small difference between the molecular 

 diffusion coefficient for heat and water vapour, latent heat transfer may be 

 written 



XW=h{eA-es)ly (2) 



where Ca, es are vapour pressure in the air and at the surface(mb), and where 

 y ( = 0-66 mb/°C) is a constant required to make the equation dimen- 

 sionally correct. 



The heat balance of the leaf or gauge can be written 



R = XW+C+M ■ (3) 



where R is the net loss of heat by radiation and Mis the decrease of internal 

 heat content (both in cal cm -2 hr"^). Assuming that M is neghgible and 

 that air in contact with the dewed surface is saturated, Tg and Cs can be 

 ehminated from eqs. i, 2 and 3 to give 



A + y 



where r= relative humidity ( ^ i) 



t'^' = saturation vapour pressure 



A = de^ldT 



This formula is independent of the source of dew, to be discussed in 

 section 5. At sunset on a clear night, net radiation loss R increases and 

 saturation deficit e^[i — r) decreases. Condensation begins whenever R 

 exceeds he^{i — r)IA and continues throughout the night if the sky remains 

 clear. If gathering cloud reduces R below the value he j(i — rjjA , dew which 

 has already formed will begin to evaporate. 



For a simple estimate of radiative loss from an isolated horizontal leaf, 

 assume that both upper and lower surfaces behave hke black bodies at 

 temperature T^ emitting radiation at a rate cTa'^ from each surface; that 

 radiation emitted from the ground beneath the leaf is oTa^; and that 

 downward radiation from the atmosphere is Ld = (f>-(^TA^, where ^ is a 

 function of vapour pressure. Net radiative loss is then 



R=2aTA^-aTA*-cf>aTA^={l-cl>)aTA' (5) 



In the British Isles 



<;^=: o-53 + o-o65'v/e^ (Monteith, 1961) (6) 



