LOCAL STORMS 



245 



itself downward. It can not be seen below the cloud until the 

 whirl becomes strong enough to form a column of cloud at its 

 core. The centrifugal force of the whirling air creates a 

 partial vacuum at the center (see Art. 620). In this partial 

 vacuum, some of the moisture in the air condenses into cloud, 



FIG. 174. Tornado. 



Two miles or more distant, 

 in Fig. 173. 



Funnel much larger than 



FIG. 175. Waterspout. (Tornado over water.) Funnel is 3500 ft. in 

 length from cloud to sea, and 300 ft. wide in its narrowest portion. A, Base 

 of the Cumulo-Nimbus cloud. B, Rain curtain of a typical thunderstorm. 



but it does not condense in the air of ordinary density outside 

 the whirl. So the funnel cloud does not appear until the air 

 whirl has developed considerable energy, and the stronger the 

 whirl, the farther down the funnel cloud extends, and the wider 

 it becomes (see Figs. 173, 174 and 175). If the funnel extends 



