246 



THE WEATHER 



but a little way below the main cloud, the air whirl may be 

 reaching the ground but is doing little or no damage. When 

 the funnel cloud extends clear to the ground the whirl is usu- 

 ally violent enough to destroy whatever it strikes. 



272. Tornado Effects. The destruction wrought by the 

 tornado is due both (1) to the violence of its rotary winds, which 

 blow 200 to 400 miles or more per hour, and (2) to the explosive 

 effect of the reduced pressure at the center of the whirl. If the 

 air pressure at the center of the funnel rs reduced one-third, 

 the remaining pressure there would be 10 Ib. per sq. in. Let 



FIG. 176. Explosive effect of tornado. 



the core of this funnel pass for an instant just outside a build- 

 ing that is filled with air at the ordinary pressure of 15 Ib. per 

 sq. in. ; instantly there is a force of 5 Ib. per sq. in. on the wall 

 pushing outward toward the center of the funnel. Compute 

 the total pressure that would be exerted on the wall of one side 

 of your schoolroom under those conditions. The difference 

 in air pressure at the center of the funnel and outside of it is 

 not known and may be greater, dr less, than 5 Ib. per sq. in. 

 (see Figs. 176 and 177). Tornadoes have been known to 

 carry heavy stones high in the air; to carry children a mile 

 or more and lodge them unhurt in tree tops; to pluck the feath- 

 ers from chickens, and to drive straws into boards (Fig. 178), 



