Atmospherical Phenomena. 261 



rain, or in colder weather, snow descends, when the super-satu- 

 ration of the atmosphere becomes still greater. However va- 

 ried the circumstances may be, relating to the formation of 

 clouds, yet one law lies at the foundation of the whole of them 

 which was first announced by Hutton, viz. wherever two nearly 

 saturated masses of air of unequal temperature become mixed, 

 either a precipitation takes place, or, at all events, the mixed mass 

 of air is relatively moister than either of the separate masses. 



Between the tropics, where all meteorological phenomena oc- 

 cur with great regularity, the phenomena connected with rain 

 are much simpler than in our regions, if local circumstances do 

 not occasion a disturbance. Where the ascending current of 

 air acts with power in the region between the two trade-winds, 

 a great quantity of vapour reaches the upper colder regions of 

 the atmosphere, which is then rapidly condensed and descends 

 as rain. This process takes places more especially when the 

 sun, about the time of its culmination, acts powerfully on the 

 ground. Hence generally the morning and evening are se- 

 rene, and the rain falls in the afternoon. As the sun in its yearly 

 course moves further to the south than to the north, the region 

 moves with it in which the ascending current of air, and con- 

 sequently the rain, is greatest ; when the sun removes from a 

 region, the rain becomes less considerable, and at last fine 

 weather returns. This alternation occurs so regularly, that 

 between the tropics, the year has been divided into two halves, 

 the dry and the wet season. 



In our part of the world, where, in the course of the year, 

 the NE. and the SW. struggle for predominance, the pheno- 

 mena are more complicated, but still may all be referred to a 

 few simple laws, if we keep before our eyes the circumstance, 

 that the SW. is a wind, which, in consequence of its origin, 

 blows above and then sinks to the ground, while the NE. 

 spreads itself from below upwards. If, with this, we further 

 combine the circumstance that the SW. wind, as it comes from 

 warmer regions, brings along with it moist air from the Atlantic 

 Ocean, whereas the cold NE. brings dry air from the interior 

 of the continent, we can easily understand that these two winds 

 must exercise a very unequal influence on the abundance of the 

 precipitation?. Observations made for several consecutive years 



