24 
The Colorado Experiment Station 
tem, the rainy months are April and May, with the exception of a 
few localities in Yuma and Phillips counties, where the maximum 
usually comes in June. The Arkansas-Platte Divide and the en¬ 
tire Arkansas Valley receive their maximum rainfall during the 
summer, July showing the greatest monthly amount. 
July and August are the wettest months throughout the San 
Luis Valley. The same months are usually the wettest in the val¬ 
leys of the Las Animas, La Plata, Mancos and McElmo. Over 
practically the whole of the western slope, the spring is wetter 
than the other seasons, and there is quite wide variation as to the 
month that shows the greatest precipitation, although July and 
August are .usually the wettest. In North Park, March and April 
are usually the two rainiest months. April and May are normally 
quite rainy months on the mountains of the eastern slope; this 
period is followed by a relatively dry June, and then July and 
August, with their afternoon showers, show large monthly 
amounts. July is usually the month of greatest rainfall in the Wet 
Mountain Valley. 
The marked differences in the seasonal distribution of precipi¬ 
tation east and west of the Continental Divide are worthy of spec¬ 
ial mention. East of the Divide the greater part of the total precipi¬ 
tation for the year comes during the growing season, when plant 
life can use it. On the other hand, west of the Continental Divide 
only about one-half of the yearly amount comes during the grow¬ 
ing season; here from 30% to 50% of the total for the year falls 
during the four months, June, July, August and September. Al¬ 
though the Western Slope has not the advantage of spring rain¬ 
fall, this is offset in a measure by its greater snowfall, and by the 
presence of a ground cover of snow a good part of the winter sea¬ 
son. 
Increase of Precipitation with Increase of Altitude .—It may 
be stated as a general rule that rainfall increases in amount and 
frequency with elevation above the sea level. There are, of course, 
exceptions which are due to topography. For example, the aver¬ 
age yearly precipitation for Saguache (altitude 7,740 feet) in the 
San Luis Valley is 7.39 inches, while that for Pueblo (altitude 
4,734 feet) on the plains is 11.67 inches. The low precipitation of 
San Luis Valley stations, although at considerable elevations, is 
due to their almost complete enclosure by high mountain ranges. 
There is wide variation in the rate of increase. It may require as 
little as 200 feet or as much as 1,200 feet increase in altitude to 
give an increase of one inch of precipitation. 
Snowfall .—The annual snowfall throughout the state varies as 
widely from year to year as does the annual precipitation as a 
