DEW : FACTS AND FALLACIES 47 



dence with the categorical statement that : 'Dew is nowhere until it appears 

 on the surface (of plant leaves) : it therefore neither rises nor falls.' It was 

 legitimate to adopt this standpoint in section 2 where the rate of dew forma- 

 tion was calculated without reference to initial sources of water vapour, 

 but their differentiation is important ecologically. 



The immediate source for condensation on plant leaves is water vapour 

 in the surrounding air, but for continuous condensation throughout the 

 night this vapour must be replaced from the free atmosphere, from the 

 soil, or from both. Distinguishing terms are 'dewfall', for the downward 

 flux of water vapour from the atmosphere; and 'distillation', for the 

 transfer of water vapour from relatively warm soil to cooler leaves. Both 

 dewfall and distillation are diffusion phenomena with fluxes which can be 

 represented formally by the product of a vapour pressure gradient and a 

 transfer coefficient Kw (cm^/sec). For dewfall, Kw is a coefficient of turbu- 

 lent diffusion, determined by wind shear and atmospheric stability. Several 

 metres above the ground on clear nights its order of magnitude is 10^. In 

 contrast, characteristic values o£ Kw for distillation on short grass are close 

 to the value for molecular diffusion, 0-24 cm^/sec (Monteith, 1956). 

 Although respective transfer coefficients differ by four orders of magnitude, 

 dewfall and distillation fluxes are often comparable, and their theoretical 

 ratio may be estimated from the surface heat budget. 



In Fig. 2, showing relevant fluxes, the wavy line represents a closed crop 

 canopy at a lower temperature than the underlying soil. Sensible heat 

 transfer C is now split into two components, Ca from the atmosphere, and 

 Cu from the soil. Similarly, the latent heat flux has components XWd 

 (dewfall) and XWu (distillation). Corresponding temperature and vapour 

 pressure profiles are shown in papers by Long (1958) and Penman and 

 Long (i960). Within the soil, the main mechanism for heat transfer is 

 conduction at a rate G, and latent heat transfer, normally a small fraction 

 of G, wiU be neglected. When the moisture content of the soil is suffi- 

 ciently high for the soil atmosphere to be assumed saturated (see Fig. 3), 

 and when the free atmosphere is also saturated, the ratios XWalCa and 

 XWulCu depend on temperature alone, and at surface temperature Tg 



XWulCu = XWalCa = ^{Ts)ly (13) 



The heat balance for the crop canopy is 



R=Wa+Cd+lVu+Cu (14) 



and for the soil surface is 



G=Wu+Cu (15) 



