THE ATMOSPHERE 125 



the fire may be likened to a high chimney or smoke stack 

 through which is maintained a violent uprush or draft 

 hundreds of feet in height. This will be continued as long 

 as sufficient heat is supplied from the fire below. In some 

 respects this condition resembles that existing in and about 

 the funnel of a tornado whose passage across a country 

 leaves a trail of destruction. Fortunately the width of the 



FIG. 47. The tornado is a storehouse of energy. 



track of the tornado is often scarcely more than fifty or one 

 hundred feet and rarely more than one-half mile. While 

 the velocity of the air whirl about the funnel may at times 

 approach two hundred miles or more an hour, the progress 

 across country may be only forty or fifty miles per hour. 



In the case of the tornado the heat required to maintain 

 the updraft is largely the latent heat (heat of vaporization) 

 which is set free as a result of the condensation of atmos- 

 pheric moisture. (See page 96.) When moisture-laden 

 air feeds into the upward moving air current, it expands 

 very rapidly, with an accompanying fall in temperature and 



