THUNDERS TORMS 



247 



B 



The whole area of low pressure moves in a definite 

 direction. In the northern United States this direction 

 is easterly; the cyclones begin in western Canada. The ve- 

 locity of the eastward movement is about 30 miles an hour. 



Cyclones must not be confounded with tornadoes (cf. 

 278), which are smaller masses of very rapidly revolving 

 air, and which do great damage. True cyclones bring 

 the cold waves that sweep down upon us at more or less 

 regular intervals, both ^ 



in summer and in win- ^^ / \ 



ter, dissipating the ex- / \ 



hausted air, and giving / ^ 



vigor to our people. 



The curved path taken by 

 the wind in approaching a 

 "low" area is simply the 

 result of two motions : one is 

 the straight-line motion of 

 the wind, by which it seeks 

 the "low"; the other is the 

 rotation of the earth from 

 west to east. The path of a 

 person on a revolving bridge 

 (Fig. 223) can be made to 



illustrate the effect of these two motions. Suppose that the bridge is 

 standing still, and that you walk toward a street car at the opposite 

 end; your path is a straight line. But suppose that the bridge turns, 

 while you continue to walk toward the car. Your path on the bridge 

 will change its direction at every point : it will be a curve. 



277. Thunderstorms. Thunderstorms are smaller 

 disturbances in the great cyclones; the frictional electricity 

 (lightning) that comes with them (cf. 147) is generated 

 by the rapid condensation of w^ater vapor. The cause of 



Fig. 223. 



If you walk across a revolving bridge, keeping 



your direction toward some object on the 



opposite shore, your path will be 



curved, somewhat like AC. 



