16 



BULLETIN 67.">, V. S. DKI'AKTMENT OF AGRICULTURE. 



translocation of soil. In practically all regions "wind is sufficiently 

 strong to cause soil particles, not firmly bound by vegetation, to be 

 carried from one place to another and subsequently to be trans] toiled 

 downward by water. On elevated lands enormous quantities of soil 

 are often carried away, not uncommonly causing uniform removal 

 of several inches of the surface soil (PI. I, fig. 2). This is in part 

 due to the sparseness of the vegetative cover, especially of tree 

 growth, and its failure to break the wind. On the more elevated 

 lands the vegetation is usually less dense than at lower altitudes, so 

 that the wind has considerable more effect and its velocity is con- 

 siderably greater, as Table 5 shows. 



Table 5. — Monthly wind movement in miles in the spruce-fir type {elevation 

 10,000 feet) and in the aspen type (elevation 8,500 feet). 



Year. 



Aspen. 



Spruce- 

 fir. 



June 



July 



August 



September. 



Total 



1915 

 1916 

 1915 

 1916 

 1915 

 1916 

 1915 

 1916 



Miles per 

 month. 

 3,0S1 

 3,020 

 3, 055 

 3, 697 

 3, 339 

 3, 198 

 3,008 

 3,080 



Miles per 

 month. 



6,501 

 7,119 

 6,807 

 5,505 

 4,836 

 5,116 

 7, 632 

 6,873 



1915 

 1916 



12, 483 

 12,995 



25,776 

 21,613 



Considering the two locations month by month for the period 

 given, it is evident that the wind movement during the growing sea- 

 son, which is practically the only time when the soil is exposed and 

 subject to wind erosion in the higher type, is approximately 100 per 

 cent greater in the heart of the spruce-fir type at 10,000 feet elevation 

 than in the aspen type 1,500 ttist below. 



In order to show more in detail the periodic behavior of the wind 

 during the season when the soil is exposed and subject to movement 

 by the wind, the maximum and average wind velocities recorded in 

 1916 have been summarized by 10-day periods. The results are given 

 in Table 6. 



Table 6. — Maximum and average wind velocities (miles per hour) summarized 

 by 10-day periods, season 1016. 



Type. 



Wind. 



June. July. August. September. 



1-10 11-20 21-30 1-10 



11-23 



21-30 



1-10 



1 

 11-20 21-30 1-10 



14 10 14 



4.3, 3.7 5.0 

 22 19 34 

 8. 2 6. 7 12. 5 



1 1 



11-20 



12 



3.8 

 16 

 6.6 



21-30 



Aspen, elevation 8,750 feet. 



Spruce-fir, elevation 10,000 

 feet. 



(Maximum 

 I Average... 

 /Maximum 

 (Average... 



13 9 9 

 4. 5 5. 9 4. 1 

 27 20 24 

 10.3 9.:: 9.6 



20 



5.1 

 17 



8.2 



18 



4.2 

 18 



7.3 



17 



5.6 

 12 



7.1 



17 

 5.0 

 21 



9.5 



12 



4.0 

 33 



9.9 



