42 MR JOHN AITKEN ON DEW. 



General Remarks. 



We see as a result of these experiments, that in our climate at least, water 

 vapour is almost constantly rising from the ground, and this takes place from 

 fallow land, from grass land, and from roads, even on nights on which there is 

 heavy dew. There seems to be but little doubt that the tide of vapour almost 

 always flows outwards from the earth, and ebbs but rarely, save after it has 

 been condensed to cloud and rain. The question as to whether any surface is 

 in a condition to lose or gain moisture on a dewy night depends on its more 

 or less perfect heat communication with the earth. Those surfaces, such as 

 soil, rock, stones &c, which are in good heat communication with the earth, 

 tend to keep warm, and to lose moisture ; while those surfaces not in good 

 communication with the earth, such as leaves of plants, roofs of sheds, &c, 

 tend to lose their heat, and gain moisture. This is the reason why grass 

 tends to collect true dew, while stones on the ground remain dry. Grass is a 

 bad conductor, and forms a non-conducting layer over the ground, preventing the 

 earth from losing its heat. The inside of this covering is hot by contact with 

 the earth, while its outside is cooled by radiation; and as the grass is a bad 

 conductor, its exposed surface gets cooled by radiation to a lower temperature 

 than the better conducting soil and stones ; hence the appearance of dew on it, 

 while the earth is dry. 



Since vapour is constantly rising from the earth on dewy nights, it follows 

 that any measurements of dew we may make ought not to be added to the 

 rainfall, as the water so collected is in no sense a measure of the moisture 

 returned to the earth at night, nor is it even a proof that any water is then 

 returned. The amount of dew measured is simply a somewhat rough indication 

 of the amount of moisture received by plants and other bodies not in heat com- 

 munication with the ground ; while the ground itself does not receive any, but 

 is rather giving off vapour. 



Dew is most copious during clear weather, and these experiments show us 

 that this condition of weather has a threefold action in the production of dew 

 — first, cloudless skies are necessary at night, in order that radiation may be 

 strong, and the surfaces of bodies cooled low enough to condense the vapour; 

 second, clear skies are necessary in order that a' copious evaporation may take 

 place under a hot sun during the clay ; and third, the same conditions are 

 necessary that the ground may be highly heated by the sun, and a large amount 

 of heat stored up during the day to be spent in evaporating an abundant 

 supply of vapour during the night. 



What are known as radiation fogs arc generally supposed to be due to cold 

 air flowing down, at evening, from higher levels to lower and warmer ones 

 and the mixing of the airs resulting in a foggy condensation. The more 



