352 



FUNDAMENTALS OF FRUIT PRODUCTION 



the probability of frost the following morning. Furthermore Cox states, 

 "It is practically impossible for frost to occur in the bogs on the first cool 

 night following a warm spell, but it is likely, if conditions are favorable, 

 on the second night after the soil has become cold." 



The difference in temperature over the two soils is due probably to 

 their difference in radiating and conducting powers. Peat absorbs and 

 radiates heat readily but of course the heat lost by radiation warms the 

 air exceedingly little; peat is a poor conductor and cannot warm the air 

 greatly by conduction. The sand, though not as good an absorber of 

 heat is a better conductor and warms the air above it at night. 



Influence of Soil Covering, — A thick mat of vegetation covering the 

 soil prevents much heating during sunshine. At night, though it pre- 

 vents conduction of heat from the soil, it radiates heat and thus tends to 

 lower the air temperature further. It is not strange therefore that lower 

 temperatures are found over vegetation than over bare ground. Table 



Table 60. — Temperatures Over Sod and Over Bare Ground 



(After Seeley^'"') 



(Degrees Fahrenheit) 



60, giving the means of observations on 18 morning at Peoria, III., shows 

 the increase in difference of surface temperatures between sod and bare 

 ground from afternoon to morning. The sod surface is 2° cooler in the 



