Native Vegetation 
i7 
Pig. 6.—Variation in length of frostless seasons for Grand Junction, 
Rocky Ford, Durango, and Steamboat Springs. 
altitude, but located differently topographically. A few examples 
will illustrate this point: Castle Rock (altitude 6,220 feet), has 
an average frostless season of 98 days, Cedaredge (altitude 6,175 
feet), 127 days; Conejos (altitude 7,880 feet), 128 days; Westcliffe 
(altitude 7,864 feet), 106 days; and Pagosa Springs (altitude 7,100 
feet), 85 days. 
Frosts .—The formation of frost and the factors conditioning 
it are quite well known. Plants lose their heat by radiation to 
the surrounding air. In fact, radiation is more rapid from plant 
surfaces than from the surrounding air. If the temperature of the 
plant surface is lowered far enough, the water vapor of the sur- 
