THE OCCURRENCE OF FROST 



343 



might change in another season the hnes shown in the figure. However, 

 the figure shows in a general way tlie northward progress of the blossom- 

 ing season. 



Unpublished figures compiled by Phillips give average data for several 

 kinds and varieties of fruits and show that the blossoming season moves 

 northward more rapidly in the Mississippi valley than along the Atlantic 

 seaboard (c/. Table 54). Philhps finds the rate for each degree of lati- 

 tude to be: along the Atlantic coast, 5.7 days; in the Mississippi valley, 

 4.8 days and for the Pacific region, 3.4 days. Somewhat similar relative 

 progress has been found for certain phases of insect life.^^ These differ- 

 ences between sections assume importance in connection with the dates 

 of the last killing frosts. 



Table 54. — Avekage Date of P'ull Bloom for Several Fruits at Different 



Latitudes 



{After Phillips^'-^) 



Avei^age Date of Last Spring Frost and Latitude. — The average dates 

 of the last killing spring frosts are shown in Fig. 33. Though there is 

 a general northward recession, local conditions evidently complicate 

 this process in the extreme so that latitude alone is not a safe guide in 

 determining the date of the last frost. The date lines of last frosts are 

 obviously not parallel. As an example, the last-frost date line for 

 June 1 is worth consideration. Barely dipping below the forty-fifth 

 parallel in New England it leaves the United States to reappear in 

 Minnesota where it remains well above the forty-fifth parallel until it 

 leaves the states at the Canadian boundary. Entering the United 

 States again in Montana it moves southward to New Mexico, almost 

 to the thirty-fifth parallel, embracing a wide range of territory until it 

 leaves Idaho, reappearing again in Washington. 



Average Dates and Frost Danger. — An average date, if the data on 

 which it is based be suflficient to give it validity, means that approximately 

 50 per cent, of the occurrences are prior to this date and 50 per cent. 



