240 ° 
its descent on the leeward siope 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 warming faster than it cooled 
it must reach the valley bottom as a warmer wind than it was on starting to ascend 
the ether side of the mountains. Having lost some of its humidity and gained in 
temperature, it must be relatively dry ; it isa Fehn- 
‘‘ Only one element remains to be aaaea to the explanation, but it is a significant 
one. Thus far the production of the Fahn dapends 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 Fehn isoften feltin the northern valleys 
of the Alpsa day, oreven 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 passes and the conden- 
sation of vapor on its way. The solution of the difficulty is as follows: The average rate 
of variation of teniperature 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 wass of air by 
reason of its compression. Ifat such a time the air in avalley be withdrawn by 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 will 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 Feehn, 
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 ee cloudy and yield rain, and then the 
second cause of the Fcehn is in operation, as already explained. 
“The Chinook wind of the northwest, along the eastern foot of the Cordilleran 
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- 
peratare, 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 
suinmer 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 snew 
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 brenght the air from the west side of the Rocky 
Mountains over to Denver. The vapoy contained in this air would be mostly precipi- 
tated on the west side of the Rocky Mountains so that it would descend on the east 
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 concensation 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 we see that in winter, during periods of extreme cold on the 

