294 HOW RAIN IS FORMED. 



l)rocess goes on until some portion of the ascending air liavS become 

 cooled to the point of condensation. No sooner does it attain this than 

 a small taft ofcumulns clond appears on the top of the ascending cur- 

 rent, and the movement which was invisible before now becomes visi- 

 ble. In a calm atmosphere each tuft of cloud has a flat base, which 

 marks the height at which condensation begins, but it is really only 

 the top of an ascending column of air. No sooner is this cloud formed 

 thau the ascent becomes more rapid, because tlie cooling wliich checked 

 its further ascent now takes place at a much slower rate, and therefore 

 the cloud grows rapidly. 



On a summer afternoon Avheu the air is warm and very damp such 

 cumulus cloud ascends sometimes to very great lieights and develops 

 into a thunder-cloud, condensing into rain. Eain differs fr.im fog and 

 cloud only in the size of the water drops. In fog and cloud these are so 

 minute that they remain suspended in the air. But as the cloud be- 

 comes denser a number of them coalesce to form a raindrop, which is 

 large' enough to overcome the friction of the air. It then begins to fall, 

 and having to traverse an enormous thickness of clond below it grows 

 larger and larger by taking up more and more of the clond corpuscules, 

 so that when finally it falls below the cloud it may have a considerable 

 size. 



Such then is the mode in which rain is formed in an ordinary sum- 

 mer shower; and the more prolonged rain-fall of stormy wet weather is 

 the result of a similar process, viz, the ascent and dynamic cooling of 

 the moist atmosphere. But in this case the movement is on a far larger 

 scale, being shared by the whole mass of the atmosphere, it may be, 

 over hundreds or thousands of square miles; and to understand this 

 movement we shall have to travel somewhat farther afield, and to in- 

 quire into the general circulation of the great atmospheric currents set 

 in movement by the sun's action in the tropics, and modified by the 

 earth's diurnal rotation and the distribution of the continents and 

 oceans on its surface. 



Before however entering on this subject, which will require some 

 l)reliminary explanation, and in which we shall bave to take account 

 both of ascending and descending currents on a large scale, I will 

 draw your attention to another and simpler case, in which both these 

 classes of movements are prominently illustrated, and in which they 

 exhibit their characteristic features in a very striking manner. 



In the valleys of the Alps, more especially those to the north of the 

 central chain, in Switzerland and the Tyrol, there blows from time to 

 time a strong, warm, dry wind, known as the Fohn. It blows down the 

 valleys from the central chain, melting the snows on its northern face, 

 and although there is more or less clear sky overhead, all the southern 

 slopes of the mountains are thickly clouded, and heavy rain falls on 

 the lower spurs and the adjacent plain, replaced by snow at the higher 

 levels up to the passes and the crest of the range. Cloudy weather also 



