AIR CURRENTS AS AFFECTED PY TOPOGRAPHY. 17 



fog clouds are carried at a speed of about 10 kilometers (G miles) per 

 hour. It is on the surface of the mountain lakes where free traverse 

 exists that we obtain a better conception in regard to the force of 

 these descending air currents. Thus, on Lake Fend Oreille I have 

 observed that the column of air moving out from the valley of the 

 (Hark Fork has often at sunrise a speed of fully 35 kilometers (21 

 miles) per hour. It is blown across the lake a distance of 30 kilometers 

 (18 miles) with undiminished force. It now strikes a rocky, forested 

 shore and is lost to view. The valley of the Clark Fork at the point 

 where it opens out on the lake has a width of between 4.S and 6.4 

 kilometers (3 to 1 miles). The front of the moving column of air where 

 it leaves the lake has a width of not less than 21 kilometers (15 miles). 

 We can estimate the width by observing the track of the wind across 

 the lake. The depth of the air column appears to be under 300 meters 

 (1,000 feet), as fog clouds resting upon the mountain slopes at this 

 height are not carried along. The air current which moves out at the 

 opening in the Co'ur d'Aiene triangle at the point where the Spokane 

 Eiver leaves Lake Cceur d'Aiene is much greater in volume and trav- 

 els at a higher rate of speed. I have felt the effects of this current at 

 a distance cf 80 kilometers (50 miles)' from its point of emergence. It 

 frequently moves with a velocity of 48 kilometers (30 miles) per hour at 

 a distance of 20 kilometers (12.5 miles^ from the above point. These 

 currents of air lower the night temperature over the plains areas that 

 are situated within their sweep and produce many a frosty night during 

 the summer season. Certain conditions are necessary to produce this 

 phenomenon; they are: warm days, clear nights, and a high baromet- 

 rical pressure. 



The deflection of the air currents is a very complicated matter, as 

 might be predicted in a country so rugged and broken. It varies in 

 each separate locality to a greater or less extent, and changes with each 

 year according to the "dominant point"' from which the storms come. 

 There are three principal lines in the Cceur d'Alenes which the 

 deflected currents of air follow to a greater extent than any other, 

 and these storm ways are very well marked by the large quantities of 

 rain and snow that fall throughout their course. They are the valley 

 of the C<eur d'Aiene and that of the Korth Fork of this stream and 

 around the northern apex of the Cceur d'Aiene triangle and the upper 

 portion of the St. Mary valley. 



In the southern part of the territory the advancing clouds from the 

 southwest first encounter the lower portion of the western rim of the 

 Cceur d'Aiene triangle. A portion of the clouds are here driven 

 northward until they reach the gap in the mountains where Lake Cceur 

 d'Aiene is situated. They now enter a low region and are afforded 

 a comparatively easy exit toward the east from the pressure behind. 

 Another portion which has passed over the first mountain barrier is 

 massed against the much higher ridges trending north and south 



