DEWrFACTS AND FALLACIES 43 



which has an area of 5 x 32 cm and a tliickness of 2-5 cm, dimensions which 

 fail to satisfy condition (a). The emissivity of the paint is unknown but is 

 unhkely to be less than 0-9, satisfying condition (b). Assuming a specific 

 heat of 0*4 cal/g, heat capacity is 0-4 cal/cm^, satisfying condition (c) for 

 periods exceeding a few hours. The relation between amounts of dew on a 

 Duvdevani gauge and on underlying vegetation have not been studied 

 systematically, but in a short series of observations by Angus (1958) the 

 mean gauge deposit was o-oi mm/hr compared with o-oo6 mm/hr on short 

 grass. As the fdter-paper method used to absorb moisture from the grass 

 inevitably underestimates condensation, this agreement is good. The 

 duration of dew on a Duvdevani gauge and on crops was examined by Shaw 

 (undated report) who found that the correlation coefficient r^ was highest 

 when the gauge was exposed at the same height as the crop canopy and 

 when the canopy was completely closed. Over grass, r2 = o-93 with the 

 gauge at the surface, decreasing to o-8o at a height of 50 cm and to 0-74 

 at 100 cm. With the gauge at the crop surface, r2 = o-98 for soybean and 

 0-96 for maize. These results suggest that observations of the duration of 

 dew on a correctly exposed Duvdevani gauge, or on any simple surface 

 satisfying the first three design criteria, may be useful in plant disease studies ; 

 but there is obviously no advantage in using such a gauge unless it is self- 

 recording. 



Maximum dew deposits measured with various dew gauges were 

 reviewed by Hofmaim and by Jones (1956). Values in Table 3 support the 

 conclusion from Table i that potential condensation is virtually independent 

 of cHmate. The largest values are equivalent to about 7 hours maximum 

 condensation from a saturated atmosphere but the real period of dew forma- 

 tion was probably 10 to 12 hours with a correspondingly lower rate of 

 condensation when the atmosphere was unsaturated. 



Table 3 

 Maximum dew on artificial surfaces (mm/night) 



