THE OCCURRENCE OF FROST 339 



amount of radiation from the air. The rapid cooling of air after sunset is largely 

 a radiation effect, especially if the air at higher elevations is rather free from 

 water vapor as is usually the case with a high barometer. However, in compari- 

 son with radiation from the earth's surface that from the air is small. Coming 

 in contact with radiating and therefore cooler substances, air loses heat to them 

 by conduction and is thereby cooled. If the air is in motion, the cooled air and 

 the warmer air form a mixture which is constantly coming in contact with the 

 radiating substances bringing to them fresh supplies of heat. If, on the other 

 hand, the air is calm, the cooling of the radiating substances and therefore of the 

 adjacent air continues as long as conditions remain stable, frequently till the 

 sun rises. 



Temperature Inversion. — Evidently the nocturnal cooling of the 

 air is largely dependent on the cooling of the earth's surface by radia- 

 tion. The cooling effect is, therefore, most marked near the surface, 

 but since on even the stillest night the air becomes somewhat mixed its 

 temperature may be affected for from 200 to 600 feet above the surface, 

 the effect becoming less with increasing height. ^^^ During the day the 

 temperature decreases at the normal adiabatic rate with increasing 

 distance from the earth. This relation is unchanged at night except in 

 so far as it is disturbed, as shown above, by radiation up to a height of 

 200 to 600 feet. There is, then, at night, first an increase in temperature 

 with distance above the earth, followed by the normal adiabatic decrease. 

 This phenomenon is known to meteorologists as the temperature inversion. 



The extent of the temperature inversion is indicated by Table 53, showing the 

 averages of observations made throughout the year at varying heights above a 

 thermometer placed on grass and fully exposed to the sky, expressed in relation 

 to the readings of this thermometer. The steepness of the inversion varies from 

 night to night and it is more marked in some localities than in others but it 



Table 53. — Average Temperatures at Different Heights Compared to that 



IN Grass"! 



Distance above Grass Increase (Degrees Fahrenheit) 



1 inch 3 



6 inches 6 



1 foot 7 



12 feet 8 



50 feet 10 



150 feet 12 



necessarily exists whenever frost occurs. This temperature inversion causes 

 frost but it also makes possible the combatting of frosts by orchard heating as 

 shown later. Humphreys^^ points out another interesting relation of this in- 

 version to frost damage: " . . . it is obvious that the tops of open and sparsely 

 foliaged trees, especially if rather tall, often are less subject to frost and more 

 easily protected than are the lower limbs. On the other hand, when the tree is 

 low and its outer foliage sufficiently dense to produce a protecting canopy over 

 the under and inner branches, as is generally the case with orchard trees, the 



