Forecasting the Weather and Storms 



301 



It is not possible with our present 

 knowledge of the mechanism of storms 

 to forewarn the exact cities and towns 

 that will be visited by tornadoes with- 

 out alarming some towns that will wholly 

 escape injury ; but we know that tor- 

 nadoes are almost entirely confined to 

 the southeastern quadrant of the cy- 

 clone, and that when the thermal, hy- 

 grometric, and other conditions are 

 favorable, the spot 300 to 500 miles 

 southeast from the cyclonic center is in 

 the greatest danger. This fact is well 

 illustrated on chart XVIII, showing the 

 cyclonic system that existed at 8 p. m. 

 on March 27, 1890, the day of the famous 

 Louisville tornado. Two red lines, 

 drawn at right angles to each other, 

 divide the area of the storm into four 

 quarters. Twelve tornadoes, repre- 

 sented by short red lines, mostly in the 

 southeast quarter of the general cyclone, 

 occurred during the afternoon or even- 

 ing of the day. As most of these oc- 

 curred several hours before 8 p. m., 

 the time of the taking of the extensive 

 system of observations on which the 

 chart is based, when the center of the 

 cyclone was 100 or 200 miles farther 

 west, it is apparent that they all oper- 

 ated somewhere within the southeast 

 quarter of the general storm, although 

 exceptions to this rule occasionally 

 occur. 



It is desirable to make clear the dif- 

 ference between the cyclone and the 

 tornado. The majority of the press 

 and many persons who should know 

 better use these terms as synonymous. 

 The cyclone shown on chart XVIII, 

 which is fairly typical of all cyclones, 

 is a horizontally revolving disk of air, 

 covering the whole United States from 

 the Atlantic Ocean westward to and in- 

 cluding the Mississippi Valley, with the 

 air currents from all points flowing 

 spirally inward toward the center, while 

 the tornado is a revolving mass of air 

 of only about 1,000 yards in diameter, 

 and is simply an incident of the cyclone, 



nearly always occurring in its southeast 

 quadrant. The cyclone may cause mod- 

 erate or high winds through a vast ex- 

 panse of territory, while the tornado, 

 with a vertical motion almost unmeas- 

 urable, always leaves a trail of death 

 and destruction in an area infinitesimal 

 in comparison with the area covered by 

 the cyclone. 



The tornado is the most violent of all 

 storms, and is more frequent in the cen- 

 tral valleys of the United States than 

 elsewhere. It has characteristics which 

 distinguish it from the thunder-storm, 

 viz., a pendent, funnel-shaped cloud 

 and a violent, rotary motion in a direc- 

 tion contrary to the movements of the 

 hands of a watch, together with a vio- 

 lent updraft at the center. 



Right here it is well to inquire if a 

 satisfactory reason can be given for the 

 occurrence of these violent agitations of 

 the atmosphere mainly during the 

 spring and early summer, and usually 

 only in the southeast quarter of the cy- 

 clone. In answer it may be said that 

 an hypothesis can be formulated that 

 fairly well satisfies the requirements of 

 the case. First, one must recall the fact 

 that the atmosphere, even at the surface 

 of the earth, is not dense enough to 

 absorb but a small part of the solar 

 heat waves. They therefore reach the 

 earth and warm its surface, but the ab- 

 sorbed heat does not penetrate to any 

 great depth, because the land is a poor 

 conductor, but a good absorber and a 

 good radiator. The land therefore re- 

 tains its absorbed heat near the surface 

 and quickly and freely radiates that 

 which it has absorbed. The atmos- 

 phere, which is a poor conductor, is 

 thus rapidly warmed at the bottom, but 

 the heat is slowly conducted upward, 

 and in the spring of the year the gain- 

 ing intensity of the solar rays and the 

 increasing hours of sunshine warm a 

 thin stratum of air next the earth to an 

 abnormal degree of heat in comparison 

 with the stratum next above, which 



