DEWrFACTS AND FALLACIES 49 



Equation 17 is valid only when there are no latent heat fluxes, but a similar 

 expression can be obtained for an isothermal saturated atmosphere, assum- 

 ing equahty o£ Kw and Kh, and using eqs. 13 and 16 to give 



^,/^,^ /^^^^Vt^^(f^/y)^ (18) 



Frankenberger (1955) determined Kh at 30 m for wind speed (at 10 m) in 

 classes o-i m/sec, etc., and from Priestley's tabulation of piCj, a value of 

 0-04 c.g.s. units is appropriate for moist soils. Using these data, RjG values 

 calculated from eq. 17 are in good agreement with field measurements at 

 Rothamsted (Monteith, 1958), and the same data were used to compute the 

 values of WajWu in Table 5. 



Table 5 



Variation of dewfall/ distillation ratio with wind speed F (m/sec at 10 m) : saturated air, 



temperature io°C (eq. 17) 



The predicted variation of the dewfall/distillation ratio with wind speed has 

 not been directly tested, but is consistent with measurements over very 

 short grass referred to the wind speed at 2 m (Monteith, 1956). When the 

 wind dropped to 0-5 m/sec or less, dewfall was a negUgible fraction of 

 distillation. With wind about 2 m/sec, dewfall was maximum at about 

 0-03 mm/hr, and the value of Waj Wu estimated from radiation and 

 temperature data was 1-5, consistent with a wind of 4-5 m/sec at 10 m. 

 Dewfall/distillation ratios will normally be much less than the theo- 

 retical value of eq. 1 8 because it is uncommon for the air above a crop to be 

 saturated at night without fog. Both leaf temperature Tl and the dew-point 

 temperature Td of the surrounding air decrease from mid-afternoon 

 onwards, whether dew is forming or not. When the wind is very light, 

 Tl falls more rapidly than Td\ and at sunset, or even before, condensation 

 begins when Tl= Td- The removal of water vapour as dew helps to keep 

 the air slightly unsaturated (relative humidity 96-99%) but, after further 

 cooling of the air by radiation, saturation is reached and fog appears. With 

 stronger wind, theoretically more favourable for dewfall, Tl approaches 

 Td more slowly and condensation may not begin until several hours after 

 sunset. Radiative cooling of the air is neghgible ; relative humidity near 

 the ground remains between 96 and 99%, increasing slowly throughout the 



