196 



ELEMENTARY SCIENCE 



earth and cause effects which are familiar. Heat lightning 

 is the reflection on clouds of lightning which occurs below 

 the horizon. Thunder is caused by vibration of the air 

 due to the passage of lightning through it. 



FIG. 71. Cumulus clouds. 



FIG. 72. Cumulo-nimbus clouds. 



FIG. 73. Cirrus clouds. FIG. 74. Cirro-stratus clouds. 



After Cloud Chart, Hydrographic Office, Department of Navy. 



Kinds of Clouds. Cumulus clouds are the kind that 

 seem piled up in the sky, with rounded, swelling contours 

 above, and an almost horizontal base (see Fig. 71). They 

 are formed by rising water vapor, and their level bases 

 indicate the height at which this vapor condenses to such 

 a point that it interferes with the light, so that it becomes 

 visible as cloud. Rain-clouds are the nimbus kind; they 



