Sutton — Some OhservaUons of Dew at Kimhcrlcy. 267 



strictly true that a clear sky is essential to the condeiisatiou of moisture in 

 the form of dew. Dew may appear under a clouded sky, as plenty of 

 English authors since the time of Wells — notably Harvey' and Tyndall^ — to 

 say nothing of foreign ones, liave pointed out. A clear sky is only favour- 

 able to dew-making when other conditions remain the same. That is to say, 

 let the temperature of the air and of the dew-point at sunset be respectively 

 the same on any two evenings; then if one of the evenings be clear 

 and the other cloudy, the former will show, to begin witli, the more 

 rapid rate of dew-formation. But the point to be taken into account 

 is that in nine cases out of ten the dew-point is higher on the cloudy 

 night than it is on the clear night, and hence that the temperature 

 of the air has not so far to fall to the saturation-point, so that, although 

 the clouds do considerably check the radiation of heat from the Earth's 

 surface, yet, on the otlier liand, no great intensity of radiation is required in 

 order that the lower air may cool sufficiently to allow its excess of moisture 

 to be condensed.^ 



This fact is very well exemplified by the observations of dew made at 

 Kimberley.'' It is not in the clear, bright, calm nights of the Kimberley 

 winter that the most dew (or frost) is deposited, but rather in the relatively 

 more clouded autumn. My own observations go to show that on a clear, 

 damp night a great deal of dew is made in a short time, but that the energy 

 of the dew-making process soon diminishes, largely, perhaps, because the 

 high and increasing relative humidity of the layer of air in contact with the 

 surface hinders a continuous rapid fall of temperature, and partly, perhaps, 

 because of the high specific heat of water. On a night when there are clouds, 

 however, the rate of condensation may be less rapid ; yet there are times 

 when as much dew is deposited in the long run, in spite of the clouds, as in 

 the former case when the sky is clear. 



1 have not yet been fortunate enough to observe the rapid alternations 

 of condensation and evaporation, as clear sky has alternated with clouds, 

 which seem to have been observed by others. Nor have I ever seen any 

 pronounced rise in the reading of a radiation thermometer lying on the 

 grass (and concomitant evaporation of dew) which could be ascribed solely 



' G. Harvey, ia Ai-t. " Meteorology," Ency. Met., 1845. 



2 J. Tyndall, " Heat a Mode of Motion," 1880, p. 496. 



^ It is a curious circumstance that a phenomenon so much within the province of meteorologists 

 as de-w is should have been so often wrongly explained by them, whereas the physicists seem 

 somehow to have got the explanation nearly always right : — e.g., W. H. Besant, "A Treatise on 

 Hydromechanics," 1877, p. 116 ; also Poynting and Thomson, " Heat," 1904, p. 218. 



^ These are mainly observations of dew proper, and not the " dew " which proceeds directly from 

 growing plants. 



•I u2 



