42 ELEMENTS OF GENERAL SCIENCE 



move eastward across the country at a rate of from 20 to 

 50 miles per hour and may affect a region many miles wide, 

 or they may be restricted to a width of a fraction of a mile. 

 They also vary greatly in severity but are not commonly 

 destructive. 



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

 central part, there are occasional storms of very great severity 

 over a limited area. These are frequently called cyclones, but 

 the correct name is tornadoes. Like the thunderstorms, they 



FIG. 29. A tornado cloud 



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

 Illinois, May 18, 1898 



are due to the excessive heating of lower air, which is also 

 nearly saturated with water vapor. They differ from the 

 thunderstorms in that the ascending current is very strong 

 and has a rotary motion. The movement of the air in such 

 a storm resembles that of water in a whirlpool (fig. 29). 



A tornado commonly occurs within the southeast quarter of 

 a cyclone and moves toward the northeast, the direction of the 

 prevailing wind, at about the same rate as a thunderstorm 

 moves. The condensation of water and the presence of dirt 

 and dust which have been blown up by the whirl make the 



