142 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



happen to be is 1 in 625,000, and " not worth worrying about." 1 

 The late Prof. Cleveland Abbe concluded that even in the so-called 

 ''tornado States" the probability of tornado destruction is less than 

 that of lightning- or fire. 2 



DISTRIBUTION OF TORNADOES IN PLACE AND TIME. 



The real home of the tornado is over the great lowlands east and 

 west of the central and upper Mississippi and of the lower Mis- 

 souri valleys, and, to a less marked degree, over some of the South- 

 ern States. Tornadoes are rare west of the one hundredth meridian, 

 and very rare or unknown in the mountain areas. They have been 

 reported from all States east of the Plains, but decrease markedly 

 in frequency toward the north. They are rare in the Appalachian 

 Mountains, and also infrequent along the Atlantic and Gulf coasts. 

 The widespread impression that tornadoes are increasing in number 

 in the United States is without foundation of fact. Tornadoes are 

 reported with greater accuracy than formerly, and they are likely 

 to do more damage than they used to do because the country is more 

 densely populated. 



Tornadoes may appear in any month and at almost any hour of 

 the day or night. Like thunderstorms, however, they distinctly pre- 

 fer the warmer months, and the hours closely following the warmest 

 part of the day. Thus spring and early summer (April to July) and 

 3 to 5 p. m. are their favorite times. 



TORNADO WEATHER TYPES. 



Tornadoes have much in common with thunderstorms. In fact, 

 they are, in reality, special local developments, of greater violence, 

 in connection with severe thunderstorms. The general conditions 

 which produce these two phenomena are, to a large extent, identical. 

 The essential difference comes in the formation of the vorticular 

 whirl in the tornado. Thus, like the largest and most severe Ameri- 

 can thunderstorms, tornadoes occur as attendants of the parent 

 cyclones of which they are the offspring. They are born, in the 

 large majority of cases, in the area of warm, damp southerly winds 

 flowing northward from the Gulf of Mexico in front of a general 

 cyclonic storm. This storm is usually more or less elliptical or 

 V-shaped, its major axis extending north to south or northeast to 

 southwest from the Great Lakes, across the central lowlands well into 

 the Southern States. The " wind-shift line " or " critical axis " is 

 usually well marked. North and west of the wind-shift line northerly 

 to westerly winds are blowing with relatively low temperatures, and 



1 M. W. Harrington, "About the Weather," p. 104 (N. Y., 1899). 



s Cleveland Abbe, "Tornado Frequency per Unit Area," Monthly Weather Review, vol. 

 xxv, p. 250. (Washington, D. C, June, 1897.) 



