16 
BULLETIN" 675, U. S. DEPARTMENT OE AGEICULTUEE. 
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 transported 
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 (PL 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). 
Month. 
Year. 
Aspen. 
Spruce- 
fir. 
/ 1915 
\ 1916 
/ 1915 
\ 1916 
/ 1915 
\ 1916 
/ 1915 
\ 1916 
Miles per 
month. 
3,081 
3,020 
3,055 
3,697 
3, 339 
3,198 
3,008 
3,080 
Miles per 
month. 
6,501 
July \ 
7,119 
6,807 
5, 50o 
4,836 
5,116 
7,632 
6,8/3 
Total 
/ 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 it^t 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 1916. 
Type. 
June. 
July. 
August. 
September. 
Wind. 
) 11-2) 21-30 1-10 11-20 21-30 1-10 11-20 21-30 
Aspen, elevation 8,750 feet . { Jf^JjJ 111 
Spruce-fir, elevation 10,000 (Maximum 
feet. leverage... 
13 
4.5 
27 I 26 | 
10. 3 9. 3 
5.9, 4.1 
24 
20 
5.1 
17 
8.2 
18 
4.2 
18 
7.3 
17 
5.6 
12 
7.1 
17 I 14 10 14 
5.0 4.3 3.7 5.0 
21 l 22 19 34 
9.5 8.2 6.7 12.5 
12 
3.8 
16 
6.6 
12 
4.0 
33 
