Atmospherical Phenomena, 259 



for several years at a high and a low point, are awanting ; for 

 the measurements made on St Bernard, simultaneously with 

 those made at Geneva, still leave much to be wished for. If I 

 were to trust my own experience, the conclusion would be, thai 

 on an average the upper strata of air are at all events not dryer : 

 nay, even that they are more humid than the lower. If we 

 make the comparison in fine weather, then, assuredly, the up- 

 per regions are generally much dryer than the lower, as has 

 been found the case by most travellers in the mountains of Eu- 

 rope ; but when the sky is clouded so that the hygrometer be- 

 low moves somewhat towards the point of greatest humidity, 

 then the air above is almost saturated, and, without exception, 

 the upper regions of air are much moister than the lower. I 

 must here, however, particularly specify the circumstance, that 

 this difference in the two conditions of weather is caused, not 

 so much by the unequal diminution of the absolute amount of 

 vapour with the height, but much more by an unequal dimi- 

 nution of heat. When I was on the Faulhorn in the year J 832, 

 I had several weeks of the finest weather, and the air was much 

 dryer than at Ziirich ; but in the year 1833, I was almost al- 

 ways surrounded by clouds, and the air was much moister than 

 at Ziirich. Notwithstanding this difference, the amount of va- 

 pour diminished according to the same law in both years ; but 

 on the other hand, the height to which it was necessary to as- 

 cend, in order that the thermometer should sink one degree, was, 

 in the year 1832, double what it was in the following summer. 



Although these remarks will doubtless receive many correc- 

 tions by means of the continued zeal of meteorologists, yet they 

 seem in some measure to correspond to the condition of the at- 

 mosphere in our part of the world. I can be shorter in refe- 

 rence to the cause of these relations in the course of the year. 

 As the warmth augments in winter, the quantity of vapour also 

 increases, until it reaches its greatest amount in July or Au- 

 gust, and then from that time till January it diminishes. The 

 relative humidity is greatest in December, and from that month, 

 till May or June the air becomes dryer, when the hygrometer 

 again moves towards the point of saturation. 



Dew, Hoar-frost, Fog, Clouds, and Rain. — When a space 

 which contains a certain amount of vapour is cooled, it al- 



