6 BULLETIN 270, U. S. DEPARTMENT OF AGRICULTURE. 
Table IT 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 
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. 
£ : 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. 
INCHES 
[Data (in miles per hour) from the records of the Biophysical 
Laboratory of the Bureau oi Plant Industry. ] 
r ten | _,_ | Aver- 
Year. | Apr. | May. | June.| July. a 
LQ00 2 ace Ce hte ee eee 7 Os be8 | Seawe 
1910! chek Ee ees 8.7| 8.0] 7.0) &¥ 7.4 
Ty GP One Sioet ee 8.8| 8.6| 6.9] 6.1 7.6 
POI IL i ee 7-2) 7:91 2644 88 6.3 
Ci eat See 8.1| 6.5| 6.3] 4.2 6.0 
fo12ss. ees 6.6| 7.1] 4.9] 3.5 5.5 
Average ene a emetes 7.9| 7.6| 6.4] 4.9 | 6.6 
Fic. 2—Graphs showing the average The anemometer used at Williston 
monthly precipitation and evaporation 
during the growing season (April to - Stands about 2 feet from the surface of 
July, inclusive) at Williston, N. Dak, the ground and near the evaporation 
ee se 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 ruiming the small-grain crops. 
