Ig10] ROBBINS—VEGETATION IN COLORADO ~— 263 
INCREASE OF PRECIPITATION WITH INCREASE OF ALTITUDE.?—It 
may be stated as a general rule that rainfall increases in amount and 
frequency with an increase in altitude. There are numerous excep- 
tions which are due to topography. It has been pointed out by 
Hann (/. c. 305) that high mountains possess a zone of maximum 
precipitation above which there isa decrease. So far as the records for 
high altitudes i in Colorado go, they indicate no such maximum zone, 
there being an increase to the summits. 
It is well known that the increase in the amount and frequency of 
the precipitation in passing from lower to higher altitudes has its 
influence upon the vegetation. ‘This influence finds its best expression 
in the arborescent vegetation. The forests on the lower slopes of the 
mountains are chiefly xerophytic, while those of the higher altitudes 
are predominantly mesophytic. 
Table III gives an idea of the rate of increase of precipitation with 
an increase of altitude as it occurs in Colorado. Hence it indicates 
as well the frequency of vegetational changes, which are due in the 
main to precipitation amounts. 
TABLE III 
RATE OF INCREASE OF PRECIPITATION WITH INCREASE OF ALTITUDE 
Number a, 
: pate . 
Station ea og Fag ade of on a 
inch of ceecipitatiint 
Colorado — ek een Coy 6046 14-41 377 
mee DBloraing. 5... .- 0). 10246 25-59 ce bs 
Soneiealromaaae Soo oye 14108 28.65 
: = Op ea eer ee 5272 14.02 1599 
o Springs. . . Pee ee 4 15-44 
Peri re ae 9-19 ‘6g 
usd ido 28 Nee tase 9500 16.00 
os Springs... 5.52. < 5823 13-20 59? 
comiiieny, es CO ee ee 9483 19.38 
* West of continental divide. 
From the above table it will be seen that the rate of increase of 
Precipitation is less at higher altitudes than at lower; also that on the 
western slope the rate of increase is more rapid than on the eastern 
slope. 
? Hann, Handbook of climatology, p. 301. 
