GRAIN-SORGHUM EXPERIMENTS EN OKLAHOMA. 
11 
TEMPERATURE. 
The daily range in temperature is wide. In summer the days are 
warm to hot. usually followed by cool nights. The data on mean, 
maximum, and minimum temperatures, by months, for the growing 
season April to September' during the 8-year period from 1914 to 
1921. inclusive, are ^iven in Table 4. 
The frost-free period is of considerable moment in experimental 
work when the time of seeding is a factor under observation. Data 
on the date of last killing frost in the spring and the first killing 
frost in the fall are available for 14 years at Woodward, and these 
dates, together with the length of the frost-free period, are shown 
graphically in Figure 3. A map of the southern Great Plains area 
is shown in Figure 4. on which the average annual precipitation and 
the length of the frost-free period are indicated. 
J#A/. FEB. MfiP. PPF. MAY JUNE JULY YfUG. SEPT. OCT. NOV. DEC. 
/&ZO sSS 
/^// 
/97 0/9YS 
/98^P?YS 
2O90&Y, 
- 
/72 PAYS 
/7<? £Y?YS 
/98 DYPYS 
206 Z>/?X 
/94 &&Y3 
/90 £F?YS 
223 &Y?YS 
23/ £Y?YS 
2/0 &/?Y3 
206 D#YS 
20/ £#YS 
Y9P/Z/L. 9 
OCTOBER. 27 
Fig. 3.— Diagram showing the annual and average frcst free period for the 13 vears from 1909 1 
inclusive, at Woodward, Okla. (Data from the records of the United States Weather Bureau.) 
EVAPORATION". 
The loss of moisture by evaporation in this region is great. The 
principal factors which influence evaporation are humidity, wind, 
temperature, and precipitation. Evaporation is naturally most 
rapid under conditions of low humidity, strong wind, high tem- 
perature, and low rainfall. 
