4 BULLETIX 268, U. S. DEPARTMENT OE AGRICULTURE. 
Table I gives the maximum, minimum, and average annual pre- 
cipitation and the seasonal maximum, minimum, and average pre- 
cipitation and evaporation of the Great Plains area. By seasonal is 
meant the period between the average time of seeding and the average 
time of harvest. 
Seasonal variations in climatic factors have been found in these 
investigations to be more . important hi determining crop production 
than have differences in methods of tillage. This is shown by the fact 
that at some stations in some years climatic conditions have been such 
that all methods have resulted in practical failures of the crops. In 
other years all methods have given fair returns. 
Figure 2 shows the earliest and latest dates of the last "killing frost 
in the spring, the earliest and latest dates of tne first killing frost in 
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Fig. 2.— Diagram showing the average frost-free periods and the earliest and the latest dates at 
which the last killing frost in the spring and the first killing frost in the fall has occurred at four- 
teen stations in the Great Plains area. 
the fall, and the average length of the frost-free period at each station. 
Each hatched horizontal bar represents the period between the aver- 
age dates of the last killing frost in the spring and the first Mlling 
frost in the fall. The number of days hi the average frost-free 
period for the station is shown in the figures above the bar. The 
solid-line curve at the left shows the earliest date at which the frost- 
free period has begun. The broken-line curve at the left represents 
the latest date at which the last killing frost of the spring has occurred. 
The solid-line curve at the right represents the earliest date and the 
broken-line curve at the right the latest date at which the first killing; 
frost of the fall has occurred. 
This diagram shows clearly the increase hi the length of the frost- 
free period from the north to the south. The shortest average period 
is 100 days, at Hettinger, and the longest 194 days, at Amarillo. The 
