FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 31 



The following points in Table 2 are worthy of note: (I) In all but 

 the summer months, temperatures in consecutive decades are subject 

 to very radical changes. Even the mean monthly temperatures 

 are very dissimilar in different years, and, in any one year, for ex- 

 ample, April may be cooler than March. Such discrepancies point 

 to the need for a long record to insure even approximate "normals" 

 for any given period, but more particularly in this study to the abso- 

 lute necessity for comparing temperatures of different stations for 

 identical periods only. 



(2) The first decade of June is seen to attain a temperature of 

 almost 50° F., and temperatures above 50° F. prevail from then 

 imtil after the first decade of September. It will later be seen that 

 these ten decades, except the first one, are also free from .the liability of 

 frost. They form, then, a fairly logical as well as convenient basis 

 for separating the growing season from the remainder of the year, 

 at least at this middle elevation. In other localities the winter snow 

 may remain considerably after June 1, and this necessarily means a 

 shortening of the frost-free period for all adjacent areas. 



(3) The highest temperatures occur from the middle to the end 

 of July. Thereafter the probability of cloudiness and rain increases, 

 until about September 1, when a rise in temperature may again occur 

 with clear weather. It mil be noted that in 5 years out of 11, this 

 first decade of September is markedly warmer than the last of August 

 Thus, w^hile much of the temperate United States is feeling its most 

 oppressive heat in August, the mountain area subject to the " summer 

 rainy spell" avoids any excess at that time. The cloudy weather 

 depresses the minima quite as much as or possibly a little more 

 than the maxima, which may be reached, on many rainy days, before 

 the afternoon clouds gather. That the minima are not held up is 

 evidently due to the quick clearing away of the clouds and to the 

 rapid evaporation after each shower. 



It will oe noted that the normally highest decade has a tempera- 

 ture of only 58° F. and that the highest single decade of record showed 

 a temperature of only 63° F. (July, 1910). Detailed examination of 

 the records shows only a few individual days attaining to the height 

 of 66° F., which, according to Baker (1) , is approximately the mean 

 summer temperature required for the proper development of corn. 

 The failure of the mountains to produce agricultural crops is, on this 

 basis alone, quite fully explained. 



(4) The winter temperatures are not extremely low. The total 

 range from summer to winter temperatures is, in fact, much less than 

 in the lowlands of the northern United States, and the winter tem- 

 peratures are actually higher in the mountains. It will later be 

 shown that they are somewhat higher in the Pikes Peak region than 

 elsewhere at corresponding elevations. This fact alone augurs unfav- 

 orably for vegetation, and especially for vegetation which retains its 

 foliage through the winter. When, however, it is noted that the 

 lowest winter temperatures are synchronous with periods of calm 

 (February), and the warmer periods always associated with high 

 westerly descending winds, the full importance of this period begins 

 to be apparent. The table shows that the temperatures do not 

 decrease in normal fashion to about February 1 , but reach practically 

 their lowest point at the end of December, then rise during the Janu- 

 ary windy period, to find a second depression in the calm of February 



