6 The Colorado Experiment Station 
CLIMATIC FACTORS* 
TEMPERATURE OF AIR 
Mean Annual Temperature .—The Arkansas Valley up as far 
as Canon City, the plains east of the Arkansas-Platte Divide, a 
strip along the eastern foothills from below Denver to Boulder, 
and the lower Grand and Uncompaghre sections, have a mean 
annual temperature of 50°f and above. An average yearly temper¬ 
ature of between 45° and 50° is experienced by the northern Great 
Plains, the Arkansas-Platte Divide and a belt along the eastern 
foothills up to about 7,000 or 7,500 feet. Much of the sagebrush 
country below 7,500 feet in the southwestern counties and in west¬ 
ern Dolores and San Miguel counties, and also narrow belts run¬ 
ning up the Grand, Gunnison and Uncompaghre rivers, have a 
mean annual temperature of between 45° and 50°. The foothills 
proper along the eastern mountain front, the entire level of San 
Luis Valley, thousands of square miles in Moffat, Routt and Riou 
Blanco counties, and extensive areas in the southwest and middle- 
west valleys below about 9,000 feet, have a mean annual tempera¬ 
ture of 40° to 50°. Yearly average temperatures of 35° to 
40° prevail in North Park, Middle Park, and South Park. Most 
stations above 10,000 feet have a mean annual temperature below 
35°. 
Mean Summer J Temperature. —Fig. 2 shows the mean sum¬ 
mer temperatures for Colorado. The summer temperature is of 
more significance in its relation to agriculture than that for the 
year. The 70° isotherm follows very closely the 5,000-foot con¬ 
tour line. The effect of the Arkansas-Platte Divide is shown in 
the eastward deflection of the isotherms for 70° and 65°. The 65° 
summer isotherm and the 45° annual isotherm touch similar points 
quite generally. That portion of the State with a mean summer 
temperature between 60° and 65° ranges in altitude from about 
6,500 feet to 8,500 feet, and within this belt a type of agriculture 
which is sometimes called “high-altitude agriculture” is prac¬ 
ticed. The summer mean for the San Luis Valley is near 60°; the 
same temperature prevails throughout Middle Park. North Park 
has a summer mean between 55° and 60°. It is well to keep in 
"“Throughout the bulletin, the climatological data are from the records of the Colo¬ 
rado Section of the United States Department of Agriculture, Weather Bureau, and 
from Bulletin 182, by Robert E. Trimble, of the Colorado Agricultural Experiment Sta¬ 
tion. 
f All temperatures throughout are Fahrenheit, unless otherwise indicated. 
JThe “summer” months here considered are June, July and August. 
