240 



its descent on the leeward slope of the range, it is warmed by compression, but until 

 all its cloud is evaporated it warms as slowly as it cooled before ; however, by reason 

 of having lost some vapor that fell on the windward slope as rain, the cloud mass to 

 be evaporated in descent is less than the total cloud mass formed in ascent ; the de- 

 scending current soon becomes clear, and then warms at the relatively rapid rate 

 proper to non-saturated air, and as a consequence of warmiug faster than it cooled 

 it must reach the valley bottom as a warmer wind than it Avas on starting to asceud 

 the other side of the mountains. Having lost some of its humidity and gained in 

 temperature, it must be relatively dry ; it is a Foehu. 



^' Only one element remains to be added to the explanation, but it is a significant 

 one. Thus far the production of the Foehn dopends on the evolution of ' latent heat,' 

 while the wind is rising and raining on the farther side of the range ; now it appears 

 from more precise observations that the winter Foehn is often felt in the northern valleys 

 of the Alps a day, or even more, before any rain falls on the southern slope ; and there- 

 fore, although the rain is an aid when it begins, it must in these cases be preceded by 

 some other cause not dependent on the ascent of air towards the ]3asses and the conden- 

 sation of vapor on its way. The solution of the difficulty is as follows : The average rate 

 of variation of temperature in the atmosphere is closely one degree Fahrenheit to 300 

 feet of descent. Inasmuch as the air is less active than the ground in changing its 

 temperature, this rate will be increased in the summer season and decreased in the 

 winter ; in winter the rise of temperature encountered in descending through the air 

 is generally less than the gain of temperature given to a descending mass of air by 

 reason of its compTession. If at such a time the air in a valley be withdrawn by 

 flowing away in answer to the call of an area of low pressure, and its place be taken 

 by air descending from the passes to windward, this fact of descent Avill require that 

 the new supply of air shall be warmer than that which has moved away; it is nec- 

 essarily very dry, because it gains capacity for vapor as its temperature rises, with- 

 out gaining the vapor to satisfy its capacity. This is the first cause of the Fcehn, 

 and explains in good part why it is more pronounced in winter than in summer. 

 When the wind over the pass is well established, it may be joined by currents of air 

 rising from the further slope ; these soon become cloudy and yield rain, and then the 

 second cause of the Foehn is in operation, as already explained. 



''The Chinook wind of the northwest, along the eastern foot of the Cordillerau 

 ranges, is described by G. M. Dawson as ' a strong westerly wind becoming at times 

 almost a gale. It is extremely dry, and, as compared with the general winter tem- 

 perature, warm. Such winds occur at regular intervals during the winter, and are 

 also not infrequent in the summer; but being cool as compared with the average 

 summer temperature, are in consequence then not commonly recognized by the same 

 name. When the ground is covered with snow the effect of the winds in its removal 

 is marvelous, as, owing to the extremely dessicated condition of the air, the snow 

 may be said to vanish rather than to melt, the moisture being licked up as fast as it 

 is produced.' (Science, 1886, vii, 33). 



" Professor Loomis notes the following case of rapid temperature changes at Denver, 

 Colo., on the plains just east of the front range of the Rocky Mountains : 'An 

 area of low pressure passed over San Francisco January 14, about 4 p. m. During 

 the following night the center passed near Salt Lake City, and at 4 p. m., January 

 15, the center was near Leavenworth, having traveled about 1,400 miles in twenty- 

 four hours. It was this storm which brought the air from the west side of the Rocky 

 Mountains over to Denver. The vapor contained in this air would be mostly precipi- 

 tated on the west side of the Rocky Mountains so that it would descend on the cast 

 side deprived of its moisture and with a temperature above that which prevailed in 

 the Salt Lake basin, on account of the latent heat liberated in the condensation of 

 the vapor. After the center of low pressure passed Denver, the northeast wind 

 returned and brought back the cold air which had constantly prevailed at stations 

 not very distant. Thus wo see that in winter, during periods of extreme cold on the 



