WINDS. 



15 



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

 exact measure of the demands made by the atmos[)liere u|)0!i ])l;iiits: 

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

 ments have shown that the loss of water by plants varies. Tn soutli_ 

 eastern Colorado the evaporation from an open water surface is 

 about 50 inches durhii:; the growing season, and diminishes to the 

 northward, on account of the decrease in temperature, to ;i])()u( 35 

 inches in northwestern Nebraska. 



The demands for water during critical periods, which ma}?^ be only 

 a few hours in duration, are often as impoT-tant as those foi- tlie sea- 

 son; in fact, during dry periods the greater part of tlic 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 tli;i,t a single corn ])lant standing in .i iicid of coi'ii lost 9-^ 

 pounds of water in eight and a half hours. August 20 was not 

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

 ])erature was higher, the wind more than doubled, and the relative 

 humidity oidy about two-thirds as high. Judging fiom 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 nearh' so 

 trjdng a da}^ as some that have occurred in southeastern Ne})raska 

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

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

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



WINDS. 



The semiarid ])ortion of the Great Plains is the windiest extensive 

 area in the United States. There are not man}^ records that fairly 

 rei)resent the wind 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 .sentiarid plains. 



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

 near the eastern limit of the semiarid area, and tire in valleys which 

 apparently must [)rotect them from the full force of the wind, or at 



9259/ 



-Bui. 215—11- 



