FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 01 



The west-slope limber pine station (F-6) shows scarcely more than 

 half the movement recorded at the control station. The ratio is 

 very low during the summer months when the prevailing winds are 

 of easterly origin. The values of 74 to 83 per cent during some of 

 the winter months are high, when it is considered that the anemom- 

 eter was placed only 18 inches above the ground surface in a fairly 

 dense forest. As the strongest winds blow against the face of this 

 slope, this does not indicate any unusual velocities overhead, but it 

 does indicate extremely severe conditions for reproduction. 



The four north-slope Douglas fir stations (F-7-8, 9, 14, and 15), 

 differing only in the weight of their forest cover, show clearly the 

 ability of the forest to check surface-air movements. The ratios of 

 each station to the control station are quite irregular, on account of 

 the short periods involved, and because of the fact that during the 

 winter months the anemometers at an elevation of only 1 foot may 

 be completely stopped by accumulated snow. This rarely occurs for 

 more than a short period, but of course may obliterate some of the 

 highest wind movements. On the whole, the movement in the forest 

 is seen to be scarcely more than one-third of that in the open; in the 

 partly-cut forest it is two-thirds. The winter values are the high- 

 est, both absolutely and when related to the control station, because 

 this slope is hardly at all sheltered from northwest winds. It is not 

 difficult to believe that the wind that blows over the tops of the 

 trees is here quite as strong as at any similar elevation in the locality. 



The local spruce canyon site (F-3) shows much more wind than 

 would be expected, even with the anemometer 20 feet above the 

 ground and well into the tops of the young spruce trees. The ratios 

 of movements here to those at the control station are perhaps more 

 constant than for any other station. This site is scarcely more than 

 400 feet from the control station, but in a narrow V-shaped canyon 

 bottom, which could not possibly be called "exposed." The fairly 

 good velocities attained here show clearly the importance of the 

 mountain and valley breezes, of which the former particularly is 

 always most noticeable in a canyon bottom. This canyon, moreover, 

 appears to receive its due share of pressure from the stronger west 

 winds of winter. 



The timber-line station (F-16) gives a fair impression of the move- 

 ments in the higher atmosphere. Pikes Peak to the south rises at 

 a sharp angle nearly 3,000 feet higher than this station, and the 

 slope is by no means rounded. The site, however, does not seem to 

 be exempt from the influence of winter winds, although the move- 

 ment in the summer is relatively moderate. West and northwest 

 winds obtain a perfectly free sweep over all topographic features on 

 that side. The resultant velocities at the station, which is unpro- 

 tected by any timber, tax the credulity. The January ratio of 278 

 per cent would mean, when related to the control station averages, 

 a mean velocity of about 20 miles per hour for that month. The 

 following data have been picked from the record of 17 months: 



Velocity, 

 miles per hour. 



Highest 10 days, Dec. 11 to 21, 1916__ 26. 



Highest 5 days, Dec. 21 to 26, 1916 31. 4 



Highest single dav, Jan. 27, 1917 42. 



Highest six hours*, Nov. 30, 1917 44. 9 



Highest single hour, Dec. 3, 1916 49. 



