IX 



Climates of the World 151 



usual direction of motion of the storm-centre are represented 

 on Plate VII. 



209. Whirlwinds. Eddies of ascending air which are 

 of small diameter compared with their height, and move 

 rapidly forward over the Earth's surface, are called Whirl- 

 winds. They are often set up by the sudden heating of 

 the lower layers of the atmosphere. The dreaded Simoom 

 of the Sahara is a whirlwind which raises great gyrating 

 clouds of sand, and sweeps forward with irresistible force, dark- 

 ening the sky at midday. The Tornado of North America is 

 even more destructive. It is most often formed in the south- 

 east side of a slowly moving cyclone, and usually acquires its 

 full force suddenly in sultry summer afternoons. The origin 

 of a tornado has given rise to much controversy, but is 

 usually believed to be the rapid heating by the Sun of a lofty 

 column of air fully charged with water-vapour. The heated 

 air expands upward and rotates as it rises, drawing the 

 surface air in from all sides. The water-vapour, condensing 

 as the air cools in ascending, adds to the heat-energy ( 159) 

 of the whirl, and helps to produce a tremendous reduction 

 of pressure in the centre. Surface winds rush into this 

 partial vacuum, and whirl with terrific violence up the 

 central hollow as if it were a furnace chimney. In conse- 

 quence of their force the tornado cuts a clean path through 

 forests or towns that lie in its path. The breadth of the 

 zone of destruction is seldom more than a quarter of a 

 mile. Houses are not simply knocked down but burst up 

 when a tornado passes over them. The low pressure of 

 the centre creates a partial vacuum, and the air inside a 

 house consequently expands so rapidly, that the roof is 

 blown off and the walls thrown outward. Sheep and fowls 

 when caught up are completely plucked of wool or feathers 

 by the fierce whirls of wind before they are dropped. After 

 about an hour the heated vapour-laden air that originates 

 the tornado is dispersed, and as the whirl travels at the 

 rate of 30 miles an hour the track of destruction is usually 

 30 miles long, although instances of papers being carried 

 45 miles are on record. Tornadoes are most common in 

 the United States east of 100 W. ; but it is only in a 



