MOVEMENTS OF THE ATMOSPHERE 47 



dilation, the winds flowing inward. In the northern hemisphere 

 the course of the winds is counter-clockwise, the deflection being 

 again to the right, while in the southern hemisphere the direction 

 of the circulation is clockwise, the deflection being to the left. 



Whirlwinds and Tornadoes. 



These are due to ascending air currents of a localized character 

 caused by excessive heating of the air at one place. In desert re- 

 gions these whirls often reach a height of 1,000 feet or more, as 

 shown by the whirling columns of dust. Rains of exceptionally 

 heavy character, called cloudbursts, often result from the expan- 

 sion of the air above and the consequent cooling. Tornadoes are 

 whirlwinds of unusual strength and very small diameter. The air 

 pressure at the center of the tornado is sometimes a fourth less than 

 that of the surrounding region, while the velocity of the winds has 

 been estimated at from 400 to 500 miles per hour. Tornadoes are 

 thus of exceptionally destructive character. 



The Influence of Mountains on Winds. The air move- 

 ments are locally modified to a considerable extent by mountains, 

 which give them special characters, significant in their influence on 

 climate. As an example, the focJin of Switzerland may be men- 

 tioned. (Hann-40:5^5.) This is a warm dry wind v/hich blows 

 down from the crest of the Alps with great violence from a south- 

 easterly, southerly, or, less frequently, a southwesterly direction, ac- 

 cording to the trend of the valleys. The main valleys which trend 

 southeast and northwest or south and north on the northern side 

 of the central chain of the Alps are the most exposed to the foehn, 

 while east and west valleys are seldom or never visited bv it. The 

 greatest frequency of these winds is between Geneva and Salzburg, 

 where in the valleys the velocity of the wind, the rise in temper- 

 ature, and the dryness are the greatest. In the upper portions of 

 the valleys of the Rhine, the Linth, the Reuss, and the lower Rhone, 

 the foehn sometimes attains the velocity of a gale. As it recedes 

 from the mountains its violence decreases, and over the main part of 

 the Swiss highland and the Jura, and along the northern frontier of 

 Switzerland, it causes only a slight rise in temperature and corre- 

 sponding decrease in humidity. These winds are most frequent in 

 spring, when the snow disappears before their hot breath, and least 

 so in summer. Ten years' observations at Innsbruck give the aver- 

 age number of foehn wind days for spring 17, summer 5, autumn 

 II. I, and winter 9.5; a total of 42.6 for the year. 



While (40: J7p) the foehn is blowing north of the Alps, there 



