126 GENERAL SCIENCE 



the condensation of much moisture. It is to this excessive 

 condensation and consequent heavy rainfall that the inky 

 black cloud accompanying the tornado is usually due. At 

 times more or less of dust and soil is carried up by the rising 

 air currents. The more moisture there may be in the air 

 the more of condensation there will be. The more latent 

 heat there is liberated the greater will be the upward rush 

 within the funnel of the tornado. As the supply of moisture- 

 laden air lessens, the violence of the storm subsides, and the 

 tornado may thus die out because of lack of heat energy. 



The air that constitutes a tornado has a violent whirling 

 motion. As a result of this the interior of the whirl is 

 likely to be very much rarefied, approaching more or less 

 the condition in a vacuum. As the warmed air rises, its 

 ascent becomes a spiral, and in the northern hemisphere 

 the direction of the whirl is to the right around the centre of 

 motion. In dusty roadways on hot days in summer there 

 are often seen illustrations of this air movement in the dust 

 whirls that spring up apparently without any disturbance 

 whatever from passing air currents. In such cases the 

 highly heated air would seem to have broken its way through 

 and upward, and to rise with sufficient energy to carry with 

 it in its whirling motion some of the finer dust particles. 



SUMMARY 



Wind is a term that very properly may be restricted to air currents 

 along the earth's surface of sufficient velocity to be noticeable. Winds 

 are the direct result of inequality of pressure in different parts of the 

 atmosphere, the result of unequal heating and of varying amounts of 

 moisture. 



All air currents, including winds, may be considered as efforts to 

 restore equilibrium in an atmospheric pressure that is all the time being 

 disturbed by solar heating, and by the processes of vaporization and 

 condensation. 



Somewhere off in the general direction toward which the wind is 

 blowing the air of lighter density and lesser pressure is being forced 



