THIRD LECTURE. 



INFLUENCE OF INVERSIONS OF TEMPERATURE, 



ASCENDING AND DESCENDING CURRENTS 



OF AIR, UPON DISTRIBUTION. 1 



PROF. D. T. MACDOUGAL. 



THE soil and the air receive the same amount of heat from 

 the sun during the course of the day, but the former, on account 

 of its greater conductivity, becomes warmer than the air resting 

 upon it. Both the soil and the air begin to lose heat shortly 

 before sunset, but the former undergoes cooling much more 

 rapidly than the latter, for the same reason that it becomes 

 warm more rapidly. If the air is poor in moisture, showing 

 a low relative humidity, it will conduct heat very slowly. As a 

 consequence of this fact, the layer of air, a few meters in thick- 

 ness, nearest the ground will become cooled by conduction and 

 radiation to the cold surface of the soil, and soon falls to a 

 temperature many degrees below that of the air a few yards 

 above. This is termed inversion of temperature by the mete- 

 orologist, and the effects of inversions of temperatures are 

 well known to those engaged in horticultural and agricultural 

 operations. 



The late spring frosts and the earlier frosts of the autumn 

 are generally due to such inversions of temperature. It is a 

 matter of common observation in these occurrences that the 

 lower branches of trees and shrubs will be killed, while the top- 

 most buds will be unharmed ; showing that the layer of cold 

 air was not deep enough to submerge the entire plant. As a 



1 Delivered August, 1899. Based upon investigations reported to the U. S. De- 

 partment of Agriculture in 1898. Printed by permission from the chief botanist. 



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