THE WEATHER 51 



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 move- 

 ment of the air in such a storm resembles that of water in 

 a whirlpool (fig. 31). 



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 thunder- 

 storm moves. The condensation of water and the presence 

 of dust and dirt which have been blown up by the whirl 

 make the storm visible as a conical or funnel-shaped cloud 

 which is whirling rapidly. 



The path of a tornado is never wide and may not be more 

 than 100 or 200 feet in width. Within this path the destruc- 

 tion is often so complete that all buildings, fences, trees, etc. 

 are leveled to the earth or carried long distances. 



The velocity of the wind within the central whirl is very 

 great, but it has not been possible to measure it. From the char- 

 acter of the results which are produced, it is estimated that the 

 wind velocity may be as high as five hundred miles per hour. 

 The storm at Mount Morris, Illinois, in 1898, and the one 

 which passed from the vicinity of St. Louis, Missouri, across 

 Illinois and Indiana in 1917, of which pictures are shown, 

 carried off heavy objects, such as stoves and pianos, destroyed 

 buildings to the foundation (fig. 32), drove light sticks and 

 straws through or into boards and posts, plucked the feathers 

 from living chickens, and did many other peculiar things. 



Fortunately such storms cover such a small area and occur 

 so rarely that the chance of any one place being visited by 

 one of them is not great. Tornadoes sometimes occur at sea 

 and are then known as waterspouts. 



51. General circulation of air on the earth. The surface of 

 the earth may be divided into zones, or belts, characterized 

 by different sorts of winds. The belt which we have been 

 discussing is called the belt of westerly winds, because the 



