The Weather 35 



one mile to two hundred miles ; but the path of devastation 

 seldom reaches thirty miles in length. They occur in all 

 months of the year, but with the greatest frequency in the 

 period from April to July. They are closely associated with 

 hot, oppressive, sultry days, a gentle south wind, and with 

 lofty cirrus clouds flying in the same direction with the wind. 

 These are regarded as significant signs in certain localities. 

 Tornadoes usually occur in the afternoon or early evening. 



The central and eastern part of the United States is the 

 general region in which tornadoes occur. They develop with 

 the greatest frequency in the region along the Missouri and 

 Mississippi rivers, and certain sections of the Ohio valley, 

 but they also appear in northeastern Texas, in Alabama, 

 Michigan, New York, and occasionally in certain inland sections 

 of New England. 



Tornadoes are developed many hundred miles south or south- 

 east of a cyclonic storm center. The whirlwind is promoted 

 by the meeting of several currents of air, as the colder air of 

 the north and the moist, warm air coming from the south, and 

 by a very rapid rise in temperature of the air near the ground. 

 As the warm air with its burden of water vapor is lifted up 

 more or less vertically by the cold-air currents, it expands with 

 the decreased pressure, and acquires in its upward course an 

 increasingly rotary motion. The moisture, condensing as it 

 rises into the upper region, releases heat energy, which aug- 

 ments the force of the rotation and at the same time causes a 

 decrease of pressure at the center. The greater the supply of 

 moisture, the greater will be the amount of heat energy released 

 and consequently the stronger the suction at the center. When 

 the moisture supply is exhausted, the tornado ceases. By the 

 partial vacuum of the spiral, air near the ground is sucked up 

 with almost unimaginable force. The noisy rushing of air 

 up a heated chimney with a forced draft is similar. It is 

 possible that the terrific roaring, which may be heard for miles, 

 is due to this violent rotary motion., 



