MOVEMENTS OF THE ATMOSPHERE' 49 



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The foehn winds are explained by the development of South- 

 erly winds which flow toward a region of low pressure in western 

 Europe, because of the advance of barometric depressions or storm 

 centers from the Atlantic. The air of the Alpine valleys is likewise 

 drawn toward this region, and to replace it the air rushes in from 

 the summits, the wall of the Alps preventing any direct supply, and 

 keeping the air calm on the south side of the Alps. Air carried up 

 and expanding is cooled at the rate of 1° C. for every 100 meters, 

 this loss of heat being the thermal equivalent of the work done by 

 the air in increasing its volume against the pressure of the sur- 

 rounding air. Conversely, air drawn down from a region of less to 

 one of greater pressure will be compressed, and at the same time 

 warmed at the rate of 1° C. for every 100 meters (or more ex- 

 actly 0.99° C. for every 100 m.). Since the average rate of de- 

 crease of temperature with increase in altitude is in winter 0.45° C. 

 for every 100 meters, the air coming down from the mountains 

 increases in temperature 0.54° -C. in every 100 meters. This would 

 give 13.5° C. for a descent of 2,500 meters. The increase in tem- 

 perature readily explains the fall in relative humidity, since the 

 capacity of the air for moisture is greatly increased with but slight 

 actual additions. Foehn winds of less magnitude sometimes occur 

 on the south side of the Alps when the conditions are reversed. 

 Similar winds have been recorded from the western coast of Green- 

 land, where a warm, dry easterly or southeasterly wind blows across 

 the ice-covered interior of Greenland and down onto the fiords of 

 the western coast, raising the temperature on the average 12° to 

 20° C. above the mean in winter, and about 11° in spring and 

 autumn. Iceland, New Zealand, Japan, and other countries have 

 foehn winds, and the chinook wind, which blows eastward from the 

 Rocky Mountains of .North America in Wyoming, Montana, Al- 

 berta, and the Saskatchewan country, has a similar origin. The 

 siroccos of Sicily (Palermo), the Algerian coast, the north coast of 

 Spain, etc., are other examples of foehn winds, but the sirocco of 

 Italy and the Dalmatian coast is a damp, muggy south or southeast 

 wind in striking contrast with the cold, dry northerly winds of 

 that region. These winds, the bora, rushing down from the high 

 plateau of the Istrian and Dalmatian coasts and then down the 

 southerly slope of the Caucasus to the Black Sea (at Novors- 

 siisk, Russia), are ice-cold blasts which apparently are not warmed 

 by their descent. This is only apparent, however, since the initial 

 temperature of these winds is so low that, in spite of the increase 

 during the descent, they reach the warm coast of the Adriatic or 

 the Black Sea as a cold blast. The bora occurs only where the 



