Sutton — Some Observations of Dew at Kimherley. 271 



intense on the clear nights. But it by no means follows that this average 

 result stands for a general truth. On the contrary, it is only when the 

 relative humidity is not high that the fall of temperature on a clear night 

 greatly exceeds the fall under clouds. And certainly the temperature may 

 often fall to the dew-point under a clear sky, whereas under clouds it may 

 not be able to do so. On the other hand, when the relative humidity is 

 high, there is no greater virtue in a clear sky than in clouds, and dew will 

 form under either, because the radiation is pretty much the same under 

 either. For instance, the average fall of temperature at Kimberley during 

 1900, from midnight to the following minimum, when the relative humidity 

 ■wa,s 90 per cent, or more, was : 



On clear nights, . . 4'9° 



On very cloudy nights, . 4*3° 



This result depends upon the fact that when the relative humidity is 

 high, the radiation of heat is pretty nearly as much retarded by the invisible 

 moisture as by the clouds. The only advantage that a clear sky has at any 

 time is that under it the dew will, as a rule, lecjin to form more quickly. 

 Therefore, dew-making is not a function of the clearness of the sky merely, 

 but rather of the dampness of the air and the length of the night. 



The wind is an important factor in preventing the formation of dew; 

 but because of its prevailing lightness at Kimberley, it is not so important 

 here as it is in most other places. For that reason it has not been brought 

 into this discussion. 



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