27° BOTANICAL GAZETTE [APRIL 
HUMIDITY 
Unfortunately very little information regarding the relative 
humidity of different parts of Colorado is to be had. The state as 4 
whole, however, has a low relative humidity. Along the eastern 
border of the state the mean annual relative humidity is about 60 pet 
cent.; east of the foothills the mean annual amount is 48-50 pet 
cent.; in the extreme western part of the state the annual relative 
humidity is 46-50 per cent. In the mountains there are rapid varia- 
tions and great extremes of relative humidity.'° The following from 
HANnn (J. c. 289) gives the seasonal march of relative humidity on 
Pike’s Peak, altitude 14,147 feet: winter, 79 per cent.; spring, 81 pet 
cent.; summer, 75 per cent.; autumn, 77 per cent.; year, 78 per cent. 
Over the entire state, the relative humidity is greatest in the winter- 
Because of low relative humidity thin-leaved plants are almost 
unknown in the state, only a very few being able to maintain them- 
selves in the densest shade of steep-walled canyons. 
WIND 
On the western slope the average wind velocity is 5-6 miles pe? 
hour; on the eastern slope 7—7.5 miles per hour; on the northern and 
eastern borders ro miles per hour. In the higher mountains, espe 
cially on exposed ridges and peaks, the wind has a great velocity and 
blows almost steadily the year round. Here the winters are extremely 
windy. Over the whole state, the prevailing direction of the wind 1s 
from the west. Along the eastern base of the mountains there occUt» 
during the winter and early spring, warm west winds, © 
“chinooks.”” ‘These cause the sap to flow and the buds to swell, only 
to be followed by a low temperature which proves destructive to 
vegetation. Mountain and valley winds, which during the day blow 
up the valleys and at night toward the plains, bring about diurn 
variations of humidity, cloudiness, and precipitation. As 4 result 
of the day direction of the winds, the mountains are often visited by 
afternoon showers. 
oT am informed by Professor FRaANcts RAMALEY that records of relative bani 
made at Tolland, Colorado, altitude 8889 feet, during the summer of 1909, do no 
show as great fluctuations as is usually expected in mountain regions. 
