590 



Principles of Weather Forecasting. 



ing so is thrown into a rapidly whirling movement in the 

 same manner that water runs into whirls in discharging 

 through the bottom of a wash-bowl. When the volumes of 

 air which must change places are large and the stratum 

 of cold air deep, there comes ultimately to be developed 

 an enormous rotary velocity which gives to the air an ex- 

 tremely destructive power. 



FIG. 275. Diagram of the path of a tornado. 



751. Explosive Violence of Tornadoes. At the center of 

 a tornado cloud the rapidly whirling motion reduces the 

 air pressure at the center of the funnel so much as to pro- 

 duce a high vacuum, and when a building lies in the path 

 of the funnel the vacuum surrounds it so suddenly that 

 often the great pressure of air within the building will 

 throw the walls outward or lift the roof off before the air 

 has time to escape into the vacuum formed by the tornado. 



