BULLETIN 270,, U. S. DEPARTMENT OF AGRICULTURE. 



Table II shows that for no month during the six years does the pre- 

 cipitation exceed the evaporation from a free water surface. The 

 average monthly precipitation for the six years, is shown to be 

 greatest during June, 3.08 inches. The average evaporation during 

 June for the same period of years is 6.65 inches, or more than twice 

 the rainfall. The average rainfall for the four months (April to July) 



during the six years is 8.43 inches, and 

 ^ X ^ ^ the evaporation for the same period of 



^ | ^ ^ time is nearly three times as great, or 



24.03 inches. 



Figure 2 presents graphs showing the 

 average monthly precipitation and evap- 

 oration in inches for the four months 

 from April to July for the six years from 

 1909 to 1914, inclusive. 





f/ 



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Fig. 2, 



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/ 



/ 



/ 



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The average monthly wind velocity 

 for the six years from 1909 to 1914, in- 

 clusive, is shown in Table III. 



Table III. — Average wind velocity at the Williston 

 substation, by months, for the growing season, 

 1909 to 1914, inclusive. 



[Data (in miles per hour) from the records of the Biophysical 

 Laboratory of the Bureau oi Plant Industry.] 



Year. 



Apr. 



May. 



June. 



July. 



Aver- 

 age. 



1909 







7.0 

 7.0 

 6.9 

 6.1 

 6.3 

 4.9 



5.8 

 5.7 

 6.1 

 3.8 

 4.2 

 3.5 





1910 



8.7 

 8.8 

 7.2 

 8.1 

 6.6 



8.0 

 8.6 

 7.9 

 6.5 

 7.1 



7.4 



1911 



7.6 



1912 



6.3 



1913 



6.0 



1914 



5.5 









7.9 



7.6 



6.4 



4.9 



6.6 







Graphs showing the average 

 monthly precipitation and evaporation 

 during the growing season (April to 

 July, inclusive) at Williston, N. Dak., 

 1909 to 1914, inclusive. 



The anemometer used at Williston 

 stands about 2 feet from the surface of 

 the ground and near the evaporation 

 tank. Table III shows that the aver- 

 age wind velocity from April to July is 6 miles per hour. It also 

 may be seen that generally there is a decrease in the velocity of the 

 wind each month from April to July. In seasons of normal rainfall 

 protracted hot winds are not common. In seasons of unusually low 

 precipitation hot winds may prevail, as was the case in 1910, when 

 successive hot winds occurred for a period of about four days in June, 

 practically ruining the small-grain crops. 



