50 ELEMENTS OF GENERAL SCIENCE 



If the conditions are such as to cause very vigorous move- 

 ments of the air, and if there is a good deal of water in the 

 lower air, the clouds may grow until they become heavy and 

 dark and rain begins to fall. The fall of rain is accompanied 

 by lightning and thunder, as w r ell as by considerable wind. 

 After the storm has passed, the temperature is cooler 

 because the warm air has flowed away upward and has 

 been replaced by cooler air from above. 



FIG. 31. A tornado cloud 



Note the "funnel" reaching from the upper clouds to the earth. Mount Morris. 

 Illinois, May 18, 1898 



Thunderstorms usually occur in the southerly part of a cy- 

 clone during warm weather and in the warmer part of the day, 

 but they may occur even in winter. They move eastward 

 across the country at a rate of from twenty to fifty miles 

 per hour and may affect a region many miles wide or be 

 restricted to a width of a fraction of a mile. They also vary 

 greatly in severity, but are not commonly destructive. 



50. Tornadoes. In the United States, particularly in the 

 central part, there are occasional storms of very great severity 

 over a limited area. These storms are called tornadoes (sect 42). 

 Like the thunderstorms, they are due to the excessive heating 



