Formation of Thunder 



589 



and another region of dividi-dly colder winds blowing 

 from the west and north of west; and it is along the meet- 

 ing of these two systems of winds that thunder showers 

 tend specially to form, and in advance of it that the tor- 

 nadoes have their birth. 



FIG. 274. Diagram showing the progressive development of thunder 



storms. 



750. Formation of Tornadoes. The most satisfactory ex- 

 planation of the formation of tornadoes is represented in 

 the lower portion of Fig. 273 ? which is a cross-section of 

 the lower portion of the atmosphere at right angles to the 

 line dividing the two systems of winds shown in the upper 

 portion of the same diagram. 



It is supposed that, under these conditions, the cold west 

 and northwest winds at times over-run the moist warm and 

 lighter southerly stratum, thus producing a condition of 

 unstable equilibrium. When such conditions have been 

 developed the warm air, at some point, is supposed to 

 break up through the over-running colder layer, as shown 

 in the lower right-hand corner of the diagram, and in do- 



