6 BULLETIN 211, U. S. DEPARTMENT OF AGRICULTURE. 



When i t is remembered that the months of spring and 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 irrigated fields. 



Exposure. — Differences 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 ne al- 

 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 soil is of drifting sand or so alkaline as to kill plants, 

 or on the flat play as that are subject to occasional inundation, 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 gov- 

 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 



i Ward, R. de C. Climate ... p. 363, New York, 1908. 



