6 BULLETIN 211, U. S. DEPARTMENT OF AGRICULTURE. 
When it is remembered that the months of spring ae early summer. 
are usually quite dry, as well as cold at night, the late starting of the 
native plants is explained. At high elevations the growing season 
is short, and above 8,500 feet frosts are recorded for almost every 
month in the year. 
Wind motion.—Wind motion is an important climatic factor 
throughout the State. The air is nearly always dry and frequently 
very dry, and the wind blows much of the time. The spring is 
apt to be particularly windy, and the most violent sand storms are 
usually accompanied by low humidity and consequent rapid evapora- 
tion. Many young seedlings are dried out or cut off by the sand 
during these windstorms, and much damage is done to cultivated 
crops even in the urigated fields. 
Exposure.—Difierences in exposure to the sun’s rays, arising from 
the direction of slope of all hills and mountain sides, cause striking 
differences in the climate of stations at the same level and near 
together, with the consequent differences in vegetation. This - 
effect is readily seen when traversing a mountain canyon that runs 
east or west. The north-facing slope is always occupied by a plant - 
association entirely different from that of the south-facing slope at 
similar altitudes above the bottom of the canyon. 
Vegetation.—Notwithstanding the various unfavorable climatic 
conditions that plants must be able to endure, there is a covering of 
vegetation of some kind practically all over the State except locally in 
spots where the soul is of drifting sand or so alkaline as to kill plants, 
or on the flat playas that are subject to occasional mundation, or on 
exposed rocky surfaces where there is little or no soil. This vegeta- 
tion is frequently very scanty and scattered, often scrubby and spiny, 
showing in many ways its adaptation to a scanty supply of water. 
Many of the plants are valueless as forage, but many times more are 
good for this purpose, and when examined in detail the wonder grows 
that so many and not so few are usable by stock at one time or 
another. 
‘Finally, it is clear that man, whether by reforestation or deforest- 
ation, by flooding a desert or by draining a swamp, can produce no 
important or extended modifications of natural climate. This is goy- 
erned by factors beyond human control.” ? 
There seems to be no doubt of the correctness of this generaliza- 
tion. But it is possible to materially improve or impair the living 
conditions for humanity in a given region by the management of 
those industries that man carries on which are dependent upon the 
adaptation of these industries to the existing climate of that region. 
The truth of this statement is recognized without question in a humid 
1 Ward, R.deC. Climate .. . p. 363, New York, 1908. 
