304 PHYSIOGRAPHY 



up to any considerable height ; but in desert regions they often reach 

 heights of 1,000 feet or more, as shown by the whirling columns 

 of dust. The rise is sometimes so great that the air is expanded 

 and cooled enough to cause condensation of even the small amount 

 of moisture contained in the desert air. Smart showers may then 

 occur. Showers of this sort are likely to be of short duration, but 

 the rainfall is sometimes very heavy. If exceptionally heavy, such 

 rains are known as cloudbursts. In such a storm in the summer of 

 1898, rain enough fell in a few minutes, in the vicinity of Bagdad, 

 in the Mojave Desert of California, to 'occasion serious washouts 

 along the railroad for miles. A cloudburst at Clifton, S. C., June 

 6, 1903, caused the loss of more than 50 lives, and property damage 

 to the estimated extent of $3,500,000. In desert regions the water 

 which starts to fall from the rising and expanding air is sometimes 

 evaporated before it reaches the ground. Such " suspended" 

 showers may be seen often in Arizona in August. 



Tornadoes. When a convection current is very strong, and has 

 very small diameter, the whirl becomes so intense in some cases as to 

 cause great destruction. A whirling storm of this sort is a tornado. 

 Tornadoes, like thunder-storms and whirlwinds, are phenomena 

 of hot weather. They occur in the United States in the warm sea- 

 son, appearing earlier in the south, and later in the north. They 

 are rather less abundant in the later part of the summer than in the 

 earlier part. They are more likely to occur in a cyclone than in 

 an anticyclone. 



The tornado may be looked upon as a concentrated cyclone or 

 strong whirlwind. The atmospheric pressure in the center of the 

 tornado is usually much lower than in the center of a cyclone. In 

 a very strong tornado the pressure at the center may be a fourth 

 less than that of its surroundings. This is one reason why the tor- 

 nado is so destructive. During its passage the pressure may be 

 reduced from the normal amount, 14.7 Ibs. per square inch, or 2,117 

 Ibs. per square foot, to three-fourths of this, or to 11 Ibs. per square 

 inch, or 1,584 Ibs. per square foot. If such a tornado passes over 

 a closed building in which the air pressure is 2,117 Ibs. per square 

 foot, the pressure on the outside becomes 1,584 Ibs. The walls 

 are therefore pushed out with a force of 533 Ibs. per square foot, 



