Native Vegetation 
35 
quently come in late summer. There is a strip along the eastern 
border of the Great Plains with an annual precipitation approach¬ 
ing 20 inches, an amount slightly higher than for the Plains as a 
whole, and it is within this strip that we find elements of the 
prairie-grass formation of the states to the east, and here too 
“dry-farming” is carried on with slightly more advantage than in 
the western part of the grass-steppe area. The Plains experience 
marked variations from the average precipitation (see Fig. 9), that 
is, there may be very wet years and very dry ones, or a series of 
these. 
The average temperature for the year over the Great Plains 
is mostly above 50°, the range being from 45° to 56°. Its western 
limit is approximately the 45° isotherm. However, the factor heat 
is not the critical one in determining grass-steppe. Water out¬ 
weighs it in influence. A limited area of grass-steppe in South 
Park exists under a mean temperature for the year of about 35° 
to 40°. The summer mean temperature of the Great Plains ranges 
from 65° to 75°, by far the greater portion being above 70°. The 
highest temperatures come in July and August. The daily ranges 
are high, which means warm days and cool nights. The average 
length of the frostless season is from 125 to 150 days in the north¬ 
ern two-thirds of the Great Plains of Colorado, and on the high 
land running east of Trinidad, and from 150 to 163 days through¬ 
out the Arkansas Valley. There is considerable variation in the 
length of the frostless season from year to year, (see Fig. 6). The 
average date of the last spring frost is from May 10 to May 30, 
although many stations have had frosts as late as May 25 to 30, and 
some few as late as June 15 to 25. The average date of the first 
fall frost ranges from August 29 to October 20. As a rule, the first 
frosts in the fall come near the first part of October. 
The relative humidity is generally low, in the neighborhood 
of 50% to 55%. This low relative humidity, associated with a 
high percentage of cloudless days, high temperatures, and consid¬ 
erable wind movement, causes a heavy evaporation from the soil 
surface. Here, again, is a climatic condition with which the dry 
farmer has to contend. The rate of evaporation is somewhat more 
rapid in the southern part of the Great Plains than in the north¬ 
ern, hence 15 inches of rainfall in northern Colorado is worth more 
in crop production than the same amount in the southern localities 
at the same elevation, and with similar soil conditions. 
The percentage of possible sunshine is 65 or above. 
Relation to Agriculture .—The grass-steppe of Colorado is a 
type of vegetation indicative of a low rainfall. Within its area, 
