248 THE WEATHER 



the condensation is an ascending column of warm air 

 loaded with moisture. When this has produced a large 

 cloud (cf. 269), rain falls, and cools the ascending col- 

 umn below the temperature of the warm, moist air that 

 rises around it. The central, ascending column thus 

 becomes a cool, descending one, which draws down into 

 it the wet ascending currents, and precipitates their 

 moisture in violent rains. The thunderstorm has a center 

 of high pressure and low temperature; its circumference 

 has a low pressure and a high temperature. 



278. Tornadoes. Tornadoes, or "twisters," are small 

 cyclones, as distinguished from the great, but not violent, 

 cyclonic storms (cf. 276). But while the tornado is of 

 small size, it is very violent and destructive. The wind 

 sometimes reaches a velocity of 200 miles, or more, an 

 hour, and no objects on the earth can resist it. Heavy 

 boulders are carried away hundreds of feet, and locomo- 

 tives are lifted from the rails. So great is the force of the 

 wind that frail objects, such as twigs and straws, are 

 driven into oak wood. 



A tornado, as seen approaching, is a funnel-shaped cloud with the 

 small end downward. The lower end dangles along the ground in an 

 irregular way, touching it here and there. The path of the tornado 

 may be 50 feet to half a mile wide. The whole storm moves north- 

 easterly, in the northern hemisphere, at a rate of 25 to 40 miles an hour; 

 it generally exhausts itself in an hour or so. 



In the great deserts, large whirlwinds carrying columns of sand 

 continue for hours at a time. Waterspouts are tornadoes over bodies 

 of water. Waterspouts at sea precipitate fresh water. This fact 

 shows that while a small column of water may be pushed up by atmos- 

 pheric pressure (cf. 40) toward the low pressure region of the water- 

 spout, most of the water in the waterspout comes from the air itself. 



