TORNADOES. 151 



the immediate track of the tornado cloud, it would not indicate its presence until 

 the crash of the storm was upon the instrument, when of course it would be 

 too late and at best rather injudicious, to weather the fury of the storm for the 

 sake of noting the rise or fall of the mercury. Barometrical observations appear 

 to advantage and are absolutely necessary to a successful consideration of the 

 meteorological conditions of tornadoes from the standpoint of the Weather Map 

 From this panoramic view of the situation a vast extent of country can be most 

 carefully watched from hour to hour, for days, weeks or months. Atmospheric 

 conditions on opposite sides of the probable course of the storm can be watch- 

 ed from their inception and any relation easily detected and analyzed. From a 

 study of the Weather Map it has been found that the formation of what is termed 

 a barometric trough or elongated area of low pressure (where the barometer 

 stands below the normal for thai region at the hour of observation) precedes the 

 occurrence of tornadoes in the Lower Missouri Valley or adjoining States to the 

 south and east. This low pressure area assumes the form of an elipse and 

 generally extends from southwest to northeast between northern Texas and the 

 Upper Lake Region. Such a depression may lie between the Central Missis- 

 sippi Valley and the Lower Lake Region, trending northeastward just south 

 of Michigan and over the Ohio Valley. The major axis of either of these de- 

 pressions is easily estimated, while the minor axis may be stated as generally vary- 

 ing from three to five hundred miles. To the north of the major axis, even 

 to a distance of several hundred miles, the winds are found to proceed from any 

 or all points between northeast and northwest with comparatively low temper- 

 atures accompanied sometimes by a cold rain or even snow. South of the 

 major axis and generally to a greater distance, the winds come from any or all 

 points between southeast and southwest, accompanied by comparatively high 

 temperatures, high humidity and often dashes of quite heavy rain. 



As these conditions continue to prevail there is a growing contrast of temper- 

 ature to the north and south of the major axis, owing to the long continued 

 movement of the atmosphere from opposite directions, such movement eventually 

 affecting the disposition of air in the warmer regions of the extreme south and 

 likewise the colder regions of the extreme north. The contrast of temperature 

 now naturally increases with marked rapidity and the formation of clouds com- 

 mences in earnest. Huge masses of dark and portentous appearance bank up in 

 the northwest and southwest with amazing rapidity and soon the scene becomes 

 one of awful grandeur. The struggle for mastery in the opposing currents is thus 

 indicated by the gathering cloud formations. The condensation of vapor from the 

 extremely humid southerly currents by contact with the augmenting cold of their 

 struggling opponents continues. It increases rapidly. Finally, when the tenacious 

 hold upon a stable equilibrium can no longer be maintained (which is controlled 

 by the rapidity and extent of condensation,) the opposing forces are, as it were, 

 broken asunder, followed by the upward rush of huge volumes of air. The out- 

 ward indication of this event is first shown in the whirling, dashing clouds over 

 the broken surface of the heavy bank of condensed vapor, forming the back- 



