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But very often rain falls from these clouds that cling to the 

 mountain sides. The reason for this is easy to understand. As 

 the air comes against the cold mountains so much vapor is con- 

 densed that some of the tiny fog particles grow larger and larger 

 until they become mist particles, which are too heavy to float in 

 the air. They then begin to settle ; and as one strikes against 

 another the two unite, and this continues until perhaps a dozen 

 have joined together so as to form a good sized drop, which is 

 so heavy that it is obliged to fall to the ground as rain. 



Let us now look at our summer storms. These do not form 



7. — A " thunder head " or cumulus cloud. 



about mountain peaks ; yet what has been said about the moun- 

 tains will help us to understand such showers. 



It is a hot summer day. The air is muggy and oppressive, so 

 that the least exertion causes a perspiration, and even in the 

 shade one is uncomfortably hot. Soon great banks of clouds 

 appear (Fig. 7), — the "thunder heads," — and people say "a 

 thunder shower is coming, so that we will soon have relief from 

 this oppressive heat." The clouds draw near, lightning is seen 

 and thunder heard, and from the black base of the cloud, tor- 

 rents of water fall upon the earth. If we could have watched 

 this cloud from the beginning, and followed it on its course, we 



