WINDS. 15 



The evaporation of water from an open water surface is not an 

 exact measure of the demands made by the atmosphere upon plants ; 

 yet it is a relative measure and the best we have at present. Experi- 

 ments have shown that the loss of w^ater by plants varies. In south_ 

 eastern Colorado the evaporation from an open water surface is 

 about 50 inches during the growing season, and diminishes to the 

 northward, on account of the decrease in temperature, to about 35 

 inches in northwestern Nebraska. 



The demands for water during critical periods, which may be only 

 a few hours in duration, are often as important as those for the sea- 

 son; in fact, during dry periods the greater part of the injury to 

 crops is often done within a few^ extremely trying hours. These de- 

 mands are frequently excessive and often beyond belief. At Lin- 

 coln, Nebr., August 26, 1909, Profs. Montgomery and Kiesselbach 

 found that a single corn plant standing in a field of corn lost 9J 

 pounds of water in eight and a half hours. August 26 was not 

 nearly so hard a day on the corn as was August 23, when the tem- 

 perature was higher, the wind more than doubled, and the relative 

 humidity only about two-thirds as high. Judging from the record 

 of August 26, the same plant must have lost about 15 pounds in the 

 same length of time on August 23. Even August 23 was not nearly so 

 trying a da}" as some that have occurred in southeastern Nebraska 

 during very dry seasons. What the demands upon plants in still 

 drier regions may be at times we can only imagine. In a large part 

 of the region the demands are much greater than at Lincoln. 



WINDS. 



The semiarid portion of the Great Plains is the windiest extensive 

 area in the United States. There are not many records that fairly 

 represent the ^\ind sweep on the smooth prairies. The following 

 data published by the Weather Bureau are the best available on the 

 subject and are included here as being at least suggestive: 



Average wind velocities on the semiarid plains. 



station. 



March. 



April. 



May. 



June. 



July. 



August. 



Septem- 

 ber. 



Amarillo. Tex 



19.8 



20.2 



18.9 



18.8 



16. 1 



13.4 



16.9 



Dodge Citv, Kans 



12.8 



14.2 



13.7 



13.8 



11.8 



10.9 



11.9 



North Platte, Nebr 



10.8 



12.6 



11.8 



10.9 



9.0 



8.6 



9.3 



Valentine, Nebr 



11.6 



13.4 



12.2 



12.2 



10.3 



9.5 



10.9 



Peoria, 111. (3 years) 



11.0 



11.0 



9.7 



8.6 



7.1 



6.7 



7.9 



Dodge City, Kans., North Platte, Nebr., and Valentine, Nebr., are 

 near the eastern limit of the semiarid area, and are in valleys w^hich 

 apparently must protect them from the full force of the wind, or at 

 92597°— Bui. 215—11 3 



