239 



because the dry crystals do not hang' to cacli other ; such snow does not 

 hang on or ball easily nor does it make good sleighing. It lies so loose 

 that it is easily moved by the wind, like sand, and causes snow-drifts, 

 while the wet snow falling at higher temperatures, which has begun to 

 thaw and contains much air inclosed, packs tight and shrinks quickly. 



The first snows of the season soon melt away, except in the highest alti- 

 tudes, because the temperature of the soil and atmosphere are still too 

 high for it to remain. As soon as the soil is cooled down to freezing 

 point the snow remains. If the temperature rises above freezing point 

 the snow begins to thaw superficially, part of the water evaporates, part 

 seeps through the snow and saturates it with water, or else the water 

 penetrates to the soil and softens this gradually. The snow thus shrinks 

 and settles until the temperature sinks again to freezing point, when 

 snow and water freeze together. 



Thawing, evaporation, settling of the snow, depend on temperature, 

 the relative humidity of atmosphere, clear or cloudy sky, and on the di- 

 rection of the winds. 



It would be supposed that winters of much snow-fall would bring the 

 greatest number of snow-slides. This is not the case in the Alps, on 

 account of the dry southern wind (Foehn) prevailing in such winters^ 

 which evaporates with great rapidity much of the snow. If, in addition, 

 the soil was not frozen before the snow-fall and is capable of taking up 

 the snow water, snow-slides are made still less frequent. 



Note. — The existence of a " FoeliD " iu our own mountain region is perhaps 

 interesting enough to justify the following extract from a paper by Prof. W. M. 

 Davis, of Harvard, especially as it may suggest explanations of other climatic charac- 

 teristics of the Rocky Mountain region : 



''First in Switzerland and afterwards in other mountainous countries, the atten- 

 tion of meteorologists was called to the occurrence, especially in winter time, of a 

 warm, or even hot, dry wind, blowing briskly down the valleys from the high, cold 

 passes. The Swiss name for such a wind is " Foehn," said to be derived from the old 

 Latin name Favonius. Various local names are used in other countries, but with the 

 present understanding of the origin of the wind, all examples of it may be included 

 under the Swiss term, which has now become of generic value. When the Foehn blows, 

 it is common to see a bank of dark clouds over the pass at the head of the valley from 

 which the wind descends. Under its effects the snow-tields melt away, and the 

 streams rise in freshets. 



''The origin of the wind should be looked for, not on the farther side of the mount- 

 ains, whence it blows, but in the direction towards which it flows. Its warmth and 

 dryness were first properly, but, as will be seen, not fully, explained as follows : ^Yhen 

 a current of air, moving on its oblique path towards a center of low pressure, encoun- 

 ters a transverse mountain range, and is forced to ascend over it, the air expands and 

 is thereby cooled; in consequence of the cooling, its vapor is condensed into cloud, 

 and soon begins to fall as rain, so that on reaching the summit of the range the air 

 contains less vapor, although it is very moist and cloudy ; its fall in temperature has 

 decreased its absolute humidity, while increasing its relative humidity. It must be 

 further noted that on account of the release of the energy before emx)loyed in main- 

 taining the rain in the state of vapor, the cooling of the ascending current is consid- 

 erably retarded ; the rate of cooling in an ascending mass of saturated air being only 

 from one-third to one-half as fast as in non-saturated air. As soon as the current begins 



