4 BULLETIN 268, II. S. DEPARTMENT OP 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 in 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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JUDITH BAS/N 

 HUNTLET 

 W/LL/STON 

 DICMA/SON 

 EDGELEY 

 HETTMGER 

 8ELLEFOCRCHE 

 SCOTTSBLOFF 

 NORTH PLATTE 

 AKRON 

 HAYS- 

 GARDEN CITV 

 DALHART 

 AMARILLO 



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 killing 

 frost in the fall. The number of days in 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 in 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 



