300 The National Geographic Magazine 



' ' The value that should be given each 

 tenth of an inch increase in pressure 

 measured outwardly from the storm 

 center is of vital importance to the cor- 

 rect working of the system, and is ar- 

 rived at by constructing a number of 

 charts similar to that presented in chart 

 XVI. From an inspection of this figure 

 it is apparent that if the value given 

 each tenth of an inch increase in press- 

 ure be decreased the resultants X t and 

 Y x will decrease in length, and the angle 

 between the vectors X, and Y 2 will de- 

 crease, but the vectors themselves will 

 become unequal — that is, X 2 will increase 

 and Y 2 decrease in length. (The work- 

 ing of the system is dependent on the 

 assumption that the general circulation, 

 as represented by the vectors X 2 and 

 Y 2 , is fairly constant.) If the value 

 given the pressure effect be increased, 

 the resultants X, and Y, will likewise 

 increase, and the vector X 2 will become 

 shorter and Y 2 very much longer, and, at 

 the same time, the angle made by these 

 lines will increase, from which it follows 

 that a value of one centimeter for each 

 tenth of an inch increase in pressure best 

 meets the requirements in this case." 



To determine the future course of a 

 storm in the month of May, for instance, 

 resolve the pressure forces about the 

 storm center into their resultant, then 

 take for the future direction and distance 

 of translation of the storm the resultant 

 between this force and the force illus- 

 trated on chart XVII as the upper-air 

 drift or normal storm track as follows : 



b 



In which the low is central near New 

 Orleans, a b representing the pressure 



resultant, or line along which the low 

 will be forced if acted upon by pressure 

 gradient only, and b c the normal storm 

 track, or the distance and direction of 

 movement of the low as the result of 

 upper-air drift alone, and a c the track 

 that the storm center will follow. Hence, 

 instead of the storm moving due south 

 into the Gulf as the result of pressure, 

 or northeast to southwestern Virginia, 

 as chart XVII shows is the place to 

 which upper-air drift will carry it, it 

 moves due east to Jacksonville, Florida. 



TORNADOES 



The four conditions essential to the 

 formation of tornadoes are usually as 

 follows : (i) A cyclone or area of low 

 pressure, the center of which is to the 

 north or northwest, with a barometric 

 pressure not necessarily much below the 

 normal ; (2) a temperature of about 70 

 degrees on the morning map ; (3) a 

 great humidity, and (4) that the time 

 of year be March 15 to June 1 5. These 

 conditions may and often do exist sepa- 

 rately ; one or two of them may be found 

 coexisting ; but so long as the third be 

 absent, tornadic formation is not likely 

 to occur. 



The number of these storms is not 

 increasing ; the breaking of the virgin 

 soil, the planting or the cutting away of 

 forests, the drainage of land surfaces by 

 tiles, the stringing of thousands of miles 

 of wire, or the laying of iron or steel 

 rails have not materially altered the cli- 

 matic conditions or contributed to the 

 frequency or intensity of tornadoes. As 

 well might one by the casting of a pebble 

 expect to dam the waters of the Missis- 

 sippi as attempt the modification or re- 

 striction by the feeble efforts of man of 

 those tremendous forces of nature that 

 surround our earth and control our 

 storms and climate. To be sure, as 

 towns become more numerous and pop- 

 ulation becomes more dense, greater de- 

 struction will ensue from the same num- 

 ber of storms. 



