Cloud formation. ^5 



cipitation. There is not enough buoyancy in 

 its globules when the air is light, as it is when 

 we have a low barometer, to cause the fog to 

 float into the higher regions of the atmosphere. 

 The high clouds, which are called cirrus, 

 under certain conditions drop down to where 

 they begin to melt into ordinary moisture 

 globules, and while this process is going on we 

 have a combined cloud effect which is called 

 cirro-stratus. This form of cloud may be 

 recognized, when looking off toward the hori- 

 zon, by its being formed into long straight 

 bands. It is sometimes called thread-cloud. 

 As it further descends it takes on a different 

 form called the cirro-cumulus, or curl-heap. 

 This is just the opposite in its appearance to 

 the cirro-stratus, as it is broken up into flocks 

 of little clouds separated from each other and 

 in the act of changing to the form of the 

 cumulus, or billowy form of cloud; and this 

 latter jbakes place when it drops to a still lower 

 stratum of warmer air and is there called the 

 cumulo-stratus, which is the form of cloud we 

 most often see in the season of thunderstorms. 

 The lower edge of the cloud is straight, par- 

 allel with the horizon, while the upper part is 

 made up of great billowy masses, having high 

 lights upon their well defined projections and 

 blending into darker shades caused by the 

 shadows in the valleys between the mountains 

 of cloud. 



