242 , THE WEATHER 



produced only when rain forms in the air currents of a cumulus 

 cloud (cumulo-nimbus). It is thought that the electricity 

 in the cloud exists on the surface of the water droplets. When 

 many small droplets join together into fewer and larger drop 

 the total surface is much reduced and the intensity of the 

 electric charge increased. In the rising air currents of a 

 cumulo-nimbus cloud the larger drops may again be broken 

 into spray or smaller drops, and the process repeated, until a 

 discharge occurs. The discharge is the lightning flash, and 

 may occur (1) between a cloud and the earth, (2) between two 

 clouds, or (3) between parts of the same cloud. Flashes to 

 earth are the only dangerous class. Tall objects like trees, 

 chimneys, spires, etc., are most likely to be struck, and any 

 object much taller than its surroundings should be avoided 

 during a thunderstorm. 



A distant thunderstorm at night often presents a beautiful 

 display if the sky in its direction is otherwise clear. The 

 so-called HEAT LIGHTNING at night is usually the reflection on 

 nearer clouds of the lightning of a distant storm. 



THUNDER is a series of sound waves in the air caused by the 

 explosive effect of the intense heating along the path of the 

 lightning flash. Thunder can sometimes be heard 15 or 20 

 miles. 



Thunder often occurs in portions of the general storms of the 

 colder months, but is not then accompanied by the other 

 features of the summer thunder shower. 



269. Thunderstorms; Frequency. Thunderstorms in the 

 United States (Fig. 171) occur most often in the southeast 

 half of a low-pressure area (Art. 280, Fig. 180), or along the 

 line of wind shift in elliptical "lows" (Art. 285, Fig. 186). 

 Their appearance and behavior vary somewhat in different 

 regions. 



270. The Hail Storm. Hail seems to occur only in thunder- 

 storms having ascending air currents of more than average 

 strength. Hail is usually made up of alternate layers of clear 

 ice and cloudy ice or snow. Its formation is not fully under- 



