24 



UNIVERSITY OF COLORADO STUDIES 



or thirty degrees during the first hour that it blows. 1 Continued east 

 winds both day and night bring rain or snow. West winds are almost 

 invariably dry and are not accompanied by rain except slight thunder- 

 showers. 



Relative Humidity. — Evaporation is rapid, the relative humidity 

 being low. Records for Boulder taken with a Draper hygrometer on 

 the University campus are given in Tables II, III and IV. These show 

 a somewhat low relative humidity as compared with stations in the 

 eastern and central United States, but a much higher degree of saturation 

 than exists in the great basin or the desert southwest. The readings at 

 Boulder are higher than those of the weather bureau at Denver. This 



TABLE V 



Record of Cloudiness at Boulder; Averages for Eleven Years Ending 

 December 31, 1907* 



* Kindly furnished by Hon. S. A. Giffin, voluntary observer at Boulder. 



is partly accounted for by the location of the instrument which is placed 

 in a shelter only a few feet above sod, while weather bureau observa- 

 tions are taken from a roof high above the ground. However, Boulder 

 should have a higher relative humidity because of its greater rainfall 

 than Denver. In comparison with the Mississippi valley and the 

 states to the east evaporation is rapid. South slopes of mesas and 

 foothills furnish rather severe conditions for plant growth and so they 

 are much more poorly clad with timber than north slopes. Since dew 

 falls but seldom, perhaps four or five times in a month during summer, 

 it follows that evaporation continues during the night as well as the 

 day. In Table II the mean relative humidity at Boulder during June, 



1 See Hann, Handbook of Climatology, p. 360. 



