THE TORNADOES OF THE UNITED STATES. 1 



By Prof. RoiiERT DeC Ward, 

 Harvard, University, Cambridge, Mass. 



[With 1 plate.] 

 NATURE OF A TORNADO. 



The relation of a tornado to human life and property depends 

 upon its nature. What it does is determined by what it is. Briefly 

 stated, a tornado is a very intense, progressive whirl, of small diam- 

 eter, with inflowing winds, which increase tremendously in velocity 

 as they near the center, developing there a counter-clockwise, vorticu- 

 lar, ascensional movement the violence of which exceeds that of any 

 other known storm. From the violently agitated main cloud-mass 

 above there usually hangs a writhing, funnel-shaped cloud, swinging 

 to and fro, rising and descending. With a frightful roar comes the 

 whirl, advancing almost always toward the northeast with the speed 

 of a fast train (20 to 40 miles an hour or more), its wind velocities 

 exceeding 100, 200, and probably sometimes 300 or more miles an 

 hour; its path of destruction usually less than a quarter of a mile 

 wide; its total life a matter of perhaps an hour or so. It is as 

 ephemeral as it is intense. 



Fortunately for man, tornadoes are short-lived, have a very narrow 

 path of destruction, and are by no means equally intense throughout 

 their course. Their funnel cloud, which indicates the region of 

 maximum velocity of the whirling winds, ascends and descends 

 irregularly. Where it descends, the destruction is greatest; where it 

 rises, there are zones of greater safety. The whirl may be so far 

 above the ground that it does no injury whatever. It may descend 

 low enough to tear roofs and chimneys to pieces. It may come down 

 to the ground and leave nothing standing. 



DAMAGE AND LOSS OF LIFE IN TORNADOES. 



The central low-pressure core of the tornado is surrounded by 

 radially inflowing winds of moderate strength, and then, closer to the 



1 Reprinted by permission from Nature, vol. 101, July 18, 1918, pp. ?.0r>-.°.00 ; abridged 

 by the author from Quarterly Journal of the Royal Meteorological Society, vol. 03, No. 

 183, July, 1917. 



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