144 



UNIVERSITY OF COLORADO STUDIES 



altitude plants by means of which they are able to complete a season's 

 growth in a few weeks is to be accounted for by increased chemical 

 intensity of sunlight. Where there is little moisture in the air and little 

 dust the sunlight is intense as soon as it reaches the plants in early 

 morning and again in late afternoon. Hence in effect the day is longer 

 for these plants than for the plants growing down on the plains. 1 



Rainfall. — No data concerning rainfall are at hand. It is, however, 

 probable that the annual precipitation is about 760 mm. (30 inches) . 

 This estimate is made after comparison with a number of rainfall 

 records from various parts of Colorado. Snows are heavy and drifts 

 remain in protected places until late June. The presence of certain 

 particular species forming patches of vegetation here and there through 

 the forest is only understood when it is known that snowdrifts regularly 

 lie in those situations. A large patch of Sibbaldia just beyond the 

 moraine east of Redrock Lake is able to live there because of the immense 

 snowdrift which furnishes moisture during the spring and early summer. 

 This high ground would otherwise be quite dry (see Fig. 11). 



Relative Humidity. — Very little can be said in regard to relative 

 humidity. As is well known, this varies greatly, even during a few 

 minutes, in high altitudes. Probably it is somewhat lower than in the 

 foothills and plains. 2 The general lower temperature of the higher 

 points, as Redrock Lake, and the greater rainfall, more than compensate 



1 Hann, Handbook of Climatology (Ward's Translation), pp. 235, 236. 



'This is particularly true during the day. The following data of August 8, 190S, illustrate the point. 

 Stapp's Lake, where the observations were made, is about three miles north of Redrock Lake at an altitude of 

 9,400 feet. 



Highest relative humidity 



Lowest relative humidity 



Range for the day of 24 hours 



Mean relative humidity for the 24 hours. 



Mean relative humidity, hourly observations from 6 A. M 

 to 6 P. M 



Stapp's 

 9,400 ft. 



100 

 28 

 72 

 64 



While the mean relative humidity for the entire day was not very different for the two stations the daily 

 range was much greater at the mountain station; the day was drier and the night more moist. August 8, 1908, 

 was a typical hot summer day in Boulder with very little wind and no thunder shower. Conditions at Stapp's 

 Lake were similar. The highest temperature recorded at Boulder was 89 degrees and at Stapp's Lake 72 



degrees. 



