CLIMATOLOGY 



75 



crop may be seriously injured by a low temperature for only 

 one night, without any regard to the average. Conse- 

 quently the whole record should be examined carefully so 

 that one may be assured that this has not been sufficiently 

 low during any twenty-four hours to do permanent damage. 

 The time of the last killing frost in the spring and the first 

 killing frost in the fall indicates the length of the growing 

 season for plants susceptible to slight cold or frosts. The 

 average temperature of one of the early spring months and 

 one of the late fall months may be such as to indicate that 

 both of these months are suitable to plant growth, yet one 

 low temperature might occur near the end of the spring 

 month or near the beginning of the fall month that would 

 eliminate both from the growing season. The temperature 

 records that interest the agriculturist are the maximum and 

 the minimum that occur in any twenty-four-hour period 

 of the month, and the dates of the last killing frost in the 

 spring and the first in the fall. 



Table Showing Temperature Data for a Ten-Year Period in 

 Duluth, St. Louis County, Minnesota, 1900-1910* 



*Compiled from the records of the United States Weather Bureau. 



Rainfall. Cycles of rainfall are similar to those of tem- 

 perature, although there is no apparent relation between 

 the two. A series of wet years will be followed by dry ones, 



