16 BOTANICAL SURVEY OP THE CCEUR D'ALENE MOUNTAINS. 



whatquarter the wind blows near the earth's surface. When one of these 

 storms especially distinguished from the more common form by its two 

 strong air currents moving' in opposite directions is about to occur, the 

 first indication of its approach is afforded by the cloud formations 

 about the peaks which rise above 1 ? 600 meters (5,250 feet). There are 

 seen heavy masses of grayish-colored clouds rolling from the north 

 along the mountain summits. The lower limits of these clouds are 

 pretty sharply defined. At very high elevations dark clouds are mov- 

 ing slowly from the south. Near the surface of the earth the air is 

 calm. Suddenly the lower stratum of clouds descends to the earth, 

 accompanied by a fierce northerly wind, the upper are much accelerated 

 in their northward couive and apparently sink lower, and blinding 

 masses of snow begin to fall. 



The northern current in these cases seldom lasts more than two or 

 three days, after which the upper appears to prevail; at least it reaches 

 the lowest levels and blows thenceforth more or less continuously from 

 a southerly direction. 



Occasional breaks in the lower cloud masses reveal now and then 

 small clouds forming high up and drifting from the north. This would 

 indicate a reversal of the air currents. 



There are no two localities in the Cceur d'Alenes separated by 3 or 4 

 kilometers (2 or 2.5 miles), or even less, which experience exactly 

 the same climatic conditions though the elevations may in all cases 

 be the same. This statement is not intended to carry with it the 

 implication that the precipitation and the mean annual temperature 

 vary within these narrow limits. The variation consists principally in 

 the unequal distribution of the daily temperature— that is to say, the 

 nights or the days may differ in temperature conditions over such lim- 

 ited areas as here indicated. This circumstance is due to several causes, 

 among which may be noted, first, the general trend of the neighboring 

 ridges, which deflect the air currents in various directions; second, the 

 distance each point is removed from the western rim of the mountains — 

 for, as a rule, the farther east any place in the Cceur d'Alenes is situated 

 the lower appears to be the mean annual temperature. The nightly 

 interchange of air which takes place between the summits of the ridges 

 and the bottom of the valleys or the open plains regions is also a pow- 

 erful factor in causing local climatic variations. This interchange of 

 air is more marked during the spring, summer, and autumn months 

 than during the winter. When clear nights prevail there is a down- 

 ward flow of cold air from the crests of the ridges and an upward flow 

 from the valleys. The downward flow follows the canyons and vaileys, 

 the upward movement follows the slope. In the inclosed mountain 

 basins it is difficult to estimate the force of this interchange. The 

 downward flow of air is much obstructed by the forest, as is also the 

 upward movement. Fog often forms during the night and is borne 

 along on the downward current. Where no trees obstruct the way the 



