FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 57 



rious character on the eastern slope of the Rocky Mountains, because 

 in descending the slope they increase in temperature and in desiccat- 

 ing power. These winds not only fail to bring moisture with them 

 or to precipitate any on the eastern slope, but they sometimes melt 

 and always evaporate the snow which may have fallen in advance of 

 their occurrence. Vegetation, then, is not only exposed, but the soil 

 is denied the protection of a snow blanket which mi^ht retard its 



pre< 

 mulation is frequently moved from exposed slopes to easterly slopes 



£ 



freezing, and the soil-building humus of the previous season's accu- 



or canyon bottoms. 



There is not the slightest doubt that this combination of conditions, 

 in which anticyclonic winds are the chief element, comprises a very 

 serious obstacle to forest growth in the Pikes Peak region, and that 

 its analysis in relation to forest types may be of prime importance. 

 Elsewhere along the eastern slope of the Rockies similar conditions 

 may prevail, though it is doubted whether any locality can show such 

 a powerful combination existing as a normal winter condition. 

 Further back from the plains, at higher elevations, and on the west- 

 ern slope of the main range the westerly winter winds combine far less 

 effectually for the destruction of vegetation and soils. 



Considering the winter periods during which evaporation, as well as 

 wind movement, has been measured, it is found that evaporation is 

 not always rapid when wind velocities are high. When the mean 

 temperatures are below freezing the amount of evaporation is so 

 strongly influenced by the length of thawing periods that at this 

 season, more perhaps than at' others, the temperature may be said 

 to affect the rate of evaporation more strongly than does the wind. 

 In order to show, therefore, that high winds do cause excessive evapo- 

 ration it is necessary only to show that they do not have a depressing 

 effect on temperatures. This has been done by comparing the wind 

 movements and temperatures for the 78 decades of record dining 

 December, January, and February. It has already- been shown in 

 Table 2 that the average mean temperature for these three months 

 is essentially uniform. The comparison indicates that with mean 

 wind velocities of 100 miles per day the mean temperature is 23.5°; 

 for velocities of 150 miles, 25.0°; and for velocities ot 200 miles, 26.1°. 

 No doubt this power of winter winds to increase the temperature 

 would be much more apparent if individual days were considered, as 

 the decade which witnesses a very strong anticyclone may also cover 

 the cyclone and the very low temperatures which often follow snow- 

 fall. It is, however, very certainly the tendency for higher tempera- 

 tures to accompany the higher wind velocities, even though excep- 

 tions to this rule are very numerous. 



The important thing is, then, that sometime during each winter 

 wind movements occur which approximate velocities of 10 miles per 

 hour over a period of a decade, and that these winds are always of 

 westerly origin. Such velocities do not appear excessive, and, 

 indeed, examination of the automatic record shows that excessive 

 velocities seldom occur. A maximum of about 27 miles per hour is 

 the highest record that thas been specifically noted, and over a period 

 of 12 or 24 hours an average of 20 miles per hour is unusual. Even 

 excessive movement for a day or two at a time is not likely to induce 



