DEW: FACTS AND FALLACIES 41 



(dewfall) increased during the night as relative humidity rose slowly 

 towards 100%, and on one night reached an absolute maximum of 0-035 

 mm/hr shortly before dawn. Allowing for simultaneous distillation of 

 moisture from the soil to the grass (which could not be measured separately), 

 total condensation may have reached 0-05 mm/hr, close to the theoretical 

 Hmit. The maximum dewfall for a whole night was 0-13 mm. Dewfall 

 on agricultural crops was measured at Rothamsted from 1957 to i960 

 using a field balance designed by Morris (1959). The machine gives a con- 

 tinuous record of the change in weight of a block of soil 56 x 56 inches 

 square and 24 inches deep, carrying the same crop as the field in which 

 it is buried. Rumiing at normal sensitivity, the scale is too coarse for con- 

 densation to be measured hourly, but Table 2 gives maximum nightly 

 totals in milhmetres with an accuracy of about ± 0-02, 



Table 2 

 Maximum dewfall on crops at Rothamsted (mm/night) 



Spring wheat 20 May, 1957 0-26 



Sugar beet 8 August, 1958 0-47 



Grass (60 cm) i August, i960 0-20 



These figures, and many others in the hterature, fall below the theoretical 

 hmit and are consistent with the predictions from Fig. i when relative 

 humidity is a few per cent below saturation. On the other hand. House, 

 Rider and Tugwell (i960), using a surface energy balance computer, 

 observed a maximum dewfall rate of 0-097 nim/hr over short grass. The 

 contemporary radiation loss was 6 cal cm"^ hr~^ and air temperature was 

 I5°C, giving a potential condensation rate of 0-065 mm/hr. The observed 

 rate is energetically impossible. 



Very high dewfall figures from two independent sets of observations in 

 the United States are often quoted in the literature (e.g. by Went, 1955). 

 First, Thornthwaite and Holzman (1941), using profiles of wind speed and 

 vapour pressure measured to a height of 25 ft over a meadow, calculated 

 maximum dewfall totals of o-8 to 1-2 mm/night, barely possible energeti- 

 cally, and a ten-month total of 45 mm (compared with 300 mm evapora- 

 tion). Subsequent experience showed that the Thomthwaite-Holzman 

 formula is invahdated by vertical temperature gradients unless observations 

 are made within a few feet of the ground. AppHed to the mean gradient 

 over 25 ft, the formula may seriously overestimate vapour transfer on a 

 clear, calm night. 



