176 GENERAL SCIENCE 



and the nimbus. In the daytime of fair summer weather 

 ascending convection currents are produced in the atmos- 

 phere. The air expands and cools as it rises. If the cool- 

 ing continues until -the dew point is reached, some of the 

 vapor is condensed, forming great masses of fleecy clouds 

 which appear like the lightest wool in the sunshine. 

 These clouds are very irregular in shape, and their tops 

 are constantly changing. Their bases are more regular 



FIG. 147. Cumulus Clouds. 



and float in the air at the height of 1000 feet to 2500 

 feet. These clouds are called cumulus clouds (Figure 

 147). They are usually seen from the middle of the fore- 

 noon to the middle of the afternoon, when they dissolve 

 and disappear. 



Cirrus Clouds. Quite commonly an advancing cyclone 

 is preceded by plumelike strips of pale, whitish cloud (Fig- 

 ure 148), often five or ten miles high. These clouds are 

 formed in the currents of air that flow out and forward 

 from the upper part of the storm and they move quite 



