] Understanding the Weather 



following an extremely hot day. The overheating of the surface 

 air when the atmosphere is fairly quiet brings about an unstable 

 condition, and the storm is the outcome. 



"Cold Front" Thunderstorms: The "cold front" thunderstorm is 

 frequently more severe. It may form a nearly continuous line 

 hundreds of miles long where cool air from the west or north meets 

 hot, moist currents. As a result, the vapor in the air turns rapidly 

 into water, and with the heavy downpour of rain great electrical 

 disturbances take place. Such a storm is often accompanied by 

 wind squalls and hail. Though it may break at any time of day 

 or night, the most likely time is in the afternoon. 



Winter Thunderstorms: Winter thunderstorms, which usually 

 c5me at night, almost always announce the arrival of a sharp 

 change in the weather. Toward the close of a cold spell, when a 

 warm wind blows over a region, thunderstorms may occur when- 

 ever there is a great contrast between the cold and warm air masses. 

 Or again, where warm air currents are being displaced by cold air, 

 thunderstorms may give warning of the coming cold wave. 



Storms of Violence 



TORNADOES SEVERAL HUNDRED MILES AN HOUR 



The thunderstorm is a relative of the dreaded tornado and 

 often accompanies it. The tornado has a distinctive feature: It 

 always includes a funnel-shaped whirling cloud. This terrifying 

 spiral, green gray to yellowish black, moves at a rate of thirty to 

 fifty miles an hour, and within the tornado itself the wind moves 

 at more than a hundred miles an hour! 



In fact, the speed of a tornado has been estimated up to several 

 hundred miles an hour; but since recording instruments are 

 destroyed in such a storm, no exact records are available. Fortu- 

 nately, tornadoes do not occur everywhere, and even in regions 

 where they are apt to strike they are infrequent. 



