THE OCCURRENCE OF FROST 357 



effective at the height of trees and they are of little importance to the 

 orchardist. They are, however, of extreme importance to the grower of 

 small fruits. His is the most difficult problem in heating his fruit planta- 

 tion but, on the other hand, he can do more than any other fruit grower to 

 prevent frost. Generally he has the same freedom as the orchardist in 

 the selection of site; in addition he can take advantage of minor localized 

 variations. In aiming to profit by them he is following cultural prac- 

 tices that are beneficial to his fruit plants in other ways. 



Summary. — Spring frost is important in setting geographic limits to 

 the commercial culture of fruits of some kinds and in determining the 

 regularity of crops, yields and profits in practically all deciduous fruit 

 growing sections. Frost formation depends to a considerable extent on 

 the radiation of heat by exposed surfaces during the night. Because of 

 radiation on still clear nights, temperatures close to the earth are lower 

 than those at somewhat greater elevations, giving rise to the condition 

 known as temperature inversion. On account of radiation the real tem- 

 peratures of plants may be several degrees lower than those registered by 

 sheltered thermometers. When the dewpoint is very low, freezing will 

 occur without frost formation. Clouds and wind both protect against 

 frost, the former by reducing the total effect of radiation, the latter by 

 mixing warm air with that which has been cooled. In a general way 

 both the blossoming dates of fruits and the average dates of the last 

 killing frosts range later with each increase in latitude, though the prog- 

 ress of the two is not always parallel. Study of Weather Bureau records 

 showing average last dates of killing frosts, together with the standard 

 deviations therefrom, will make possible an accurate determination of 

 frost danger beyond any particular date for any given locality, though not 

 for any site. Air drainage secured by suitable elevation is of considerable 

 importance in determining danger from frost in particular sites. Minor 

 differences in temperature within narrow limits in space are occasioned by 

 minor differences in elevation, amount of soil moisture, character of the 

 soil covering, type of soil and system of cultivation. These are seldom 

 important in influencing frost injury to tree fruits; however, they may be 

 of considerable importance in small fruit culture. 



