THE OCCURRENCE OF FROST 345 



Table 55. — Spring Frost Data for Selected Stations^^" 



period, in approximately half the years the last frost will occur between 

 20 days before Apr. 15 and 20 days after, or between Mar. 27 and May 

 5; in approximately one-fourth of these years it will occur before Mar. 27 

 and in approximately one-fourth of the years it will occur after May 5. 

 The record shows that the latest date of last frost for this station is 

 May 10. Figure 34 shows the rather considerable range of standard 

 deviations in dates of last frosts at various points in the United States. 



Fig. 34. — Standard de%'iations of dates of last killing frosts in spring. {After Reed^^^) 



Of greater immediate value to the fruit grower is Fig. 35, showing 

 dates " when the chance of killing frost falls to 1 in 10. "^^^ If the average 

 date of blossoming at a given point is identical with the date of the 1:10 

 chance for that point the probability of damage is slight, being in fact 

 3^ X Ko = Ho, or one chance in 20. This may happen very frequently 

 in cane fruits and grapes, though in most cases the average date for 

 orchard fruits would precede that of the 1:10 chance. Comparison of 

 such average blossoming dates as are available and of real validity shows 



