G2 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE. 



It is seen from these special data, as well as from Table 13, that 

 this station, except for the four summer months, is exposed to wind 

 forces more than twice as strong as those at the control station. 

 Hence these winds are strong enough to move snow and soil and 

 actually blast off the exposed portions of trees as with a sand-blast. 

 There is no doubt in the mind of the writer that here the upper limit 

 of timber growth is set by the mechanical effects of the wind and 

 only indirectly by temperature conditions. The spruce and bristle- 

 cone pine trees which form the upper fringe of the forest are severely 

 exposed during eight months of the year, and during much of this 

 time they are cut off from soil moisture by the freezing of the ground. 

 Although low temperatures greatly modify the drying power of the 

 violent winds, it will be seen later that these winds are by no means 

 incapacitated for doing damage. 



The high spruce station at Wagon Wheel Gap (W-D), though 

 fully exposed in an old burn, shows by no means the wind velocities 

 that have just been described. Its velocities are only a fifth greater 

 than those at the control station, the lowest ratios occurring at 

 midsummer. The immunity to high wind is doubtless character- 

 istic of the vicinity of Wagon Wheel Gap, and not due to any special 

 features of the exposure, for even at an elevation corresponding to 

 F-16 (namely 11,500 feet) no excessive velocities have been re- 

 corded, and at 12,000 feet, on the highest ridge of the locality, the 

 evidences of injury from wind are confined to a small flat at the 

 very top, where the movement of snow is most easily accomplished. 



Turning now to the Douglas fir station in this locality (W-Al), 

 protected by the walls of a basin which drains to the east, it is seen 

 that the annual movement corresponds closely to that of the fully 

 exposed north-slope station at Fremont (F-7-8), but that the winter 

 velocities of the former are much lower and arc counterbalanced by 

 greater movement during the spring and summer. In fact, Station 

 W-Al is the only one in the list which shows higher velocities in 

 summer than in winter, relative to the control station. Only the 

 winter velocities are of any import. It will later be seen that, in 

 combination with low temperatures but a drier atmosphere, these 

 light winds have considerable power for evaporation; but during 

 the winter months there is less of this influence than is shown at the 

 control station. 



Finally, a glimpse may be had of the conditions which charac- 

 terize the lodgepole forest of southern Wyoming (F 11). This 

 station is only partly exposed and receives a, great deal of protection 

 from the forest on the north and west. Hence the relatively high 

 velocities of December and January bespeak a. wvy great disturbance 

 of the atmosphere; and, even though the ratios decline during the 

 latter part of the winter, it is difficult to concede that the exposure oi' 

 trees is any less severe in this locality than at the control station. 

 During the summer the wind movement might best he compared 

 with that of the yellow pine type at Monument. However, it must 

 he remembered that this Lodgepole locality, and every other Lodge- 

 pole locality which was examined in connection with air temperatures, 

 is characterized by verj cold winters, and it will be seen later that, by 

 actual test, this practically nullifies the evaporation at Foxpark. 



It should he broughl to attention thai the data presented in no 

 case refer to wind conditions at the Level of the tree top, and that, 



