20 BOTANICAL SURVEY OF THE COGUE D'ALENE MOUNTAINS. 



duration may occur any time during the dry season and are especially 

 abundant in the North Fork basin. They seem to be mainly a part of 

 the great evaporation which ascends from these moisture-laden ravines 

 during the day, and meeting with a colder current condenses and falls 

 back. 



Frosts are liable to occur at any time during the growing season in 

 the bottoms of the valleys within the C<eur d'Aleue triangle. Wet 

 summers produce more frosty nights than dry. The reason for this is 

 that the high barometer which follows a storm brings with it a cold, 

 dry condition of the atmosphere. The frosts are most severe at the 

 mouths of the canyons. Elevations of 50 to 100 meters (160 to 350 

 feet) above the floors of the valleys and the bench lands along the 

 streams are very nearly free from summer frosts and have in general 

 a higher temperature. This patent tact is not generally recognized as 

 yet by the farmers of the region. The valleys would be far more frosty 

 but for the fog which forms above them on clear nights. In the upper 

 St. Mary valley at elevations of 800 meters (2,000 feet), where my 

 opportunities for such observations were of the best, I found that after 

 sundown the temperature would descend steadily until 4° C. (39.2° 

 F.) was reached. It then became stationary and the fog began to 

 form. After 1 a. m. the temperature began to rise, the thermometer 

 indicating 5° to 7° C. (41° to 44.6° F.) at sunrise. Needless to say, the 

 amount of dew which fell was very great. 



The highest temperature recorded by me is 36.5° C. (97.7° F.) at Mul- 

 lan on August 2, elevation 970 meters (3,200 feet). This is an unusually 

 high temperature for this locality. The hot wave was followed by an 

 equally unusual depression of temperature, producing frosts and severe 

 freezing throughout the C<eur d'Alene valleys until August 14. Our 

 lowest record for this period is — 3.2° (J. (+26.2° F.) at Wolf Lodge on 

 August 14, altitude 750 meters (2,400 feet). 



There is no permanent snow line on any of the C<eur d'Alene Moun- 

 tains. Summers which follow unusually severe winters with heavysnow- 

 fall may witness a bank of snow remaining on the north side of some 

 of the ridges throughout the season at elevations above 1,550 meters 

 (5,100 feet). Generally, however, even the highest ridges are free from 

 snow by the 1st of August. Exceptions to this occur on the north 

 side of Stevens Peak, on the northeast slopes of the high rocky ridges 

 some 10 or 12 kilometers (0.2 to 6.3 miles) east from Sunset Peak, and 

 on the northern slopes of the ridges south from Wiessner Peak. In 

 the last locality I found old snow in September with the fresh snow of 

 the season covering it. This is due here, as well as in the other places 

 mentioned, to the great drifts which are blown over the crests of the 

 ridges and accumulate on the northern slope rather than to the ele- 

 vated position they occupy. When the summer thaw begins great 

 masses of snow are loosened and fall into the chasms below, where 

 the summer's sun can not reach them with sufficient force. This is 

 the case at the foot of the precipices on the northern slopes of Stevens 



