38 Farmers' Bulletin 1096. 



fall several degrees below that of the surrounding air. The tem- 

 perature of mature citrus fruits falls more slowly than that of the sur- 

 rounding air but the rate of fall follows more closely that of the 

 outside air when radiation is rapid than when it is S!OW T . Growers 

 of citrus fruits are familiar "with the fact that the first fruit to be 

 damaged is that which is exposed to the sky ; fruit on the interior of 

 the tree and screened from the sky by leaves or branches often will 

 show no injury with air temperatures several degrees lower. Radia- 

 tion goes on more rapidly when the air is relatively dry than when 

 it is moist and the temperature of the fruit is likely to follow more 

 nearly that of the surrounding air when the humidity is low. 



On nights when the humidity is high, considerable ice is de- 

 posited on the fruit or blossoms. When this moisture condenses and 

 freezes, some of the latent heat liberated tends to retard the rate of 

 fall in temperature of the fruit. After sunrise the thawing of the 

 ice and evaporation of the resulting water retard the thawing to a 

 slight extent and this has a tendency to lessen the damage. 



It is possible that under actual orchard conditions, these influences 

 which tend to lessen the amount of damage on a night with relatively 

 high humidity more than counteract the influence of the increased 

 conductivity of the moist air. 



DECIDUOUS FRUITS 



Damage by frost to deciduous fruits usually takes place in the 

 spring when the trees are in bucl or blossom or shortly after the fruit 

 has set. The stage of advancement is of the greatest importance in 

 estimating resistance to low temperature; the same degree of frost 

 that causes little or no damage to fruit in bud, may injure the greater 

 portion of the crop two or three days later. 



In the case of most deciduous fruits, the same temperature will 

 cause far more permanent damage after the fruit has set than during 

 the period when the trees are in full bloom, and the later the frost 

 after the fruit has set, the greater is the actual loss. This is due to 

 the fact that there is nearly always a great overproduction of bloom 

 and usually from 50 to 90 per cent of the blossoms can be killed 

 without materially reducing the final crop of fruit. This fact often 

 causes orchardists to overestimate the amount of damage to their 

 crops early in the season. One or two uninjured blossoms in each 

 cluster are usually enough for a good crop. With some small fruits 

 and nuts a larger percentage of the blossoms must mature in order to 

 obtain a full crop and damage during full bloom is more serious. 



Another point to be considered is the fact that the blossoms do 

 not all open at once ; there are often unopened buds and small fruits 

 on the trees at the same time. Even though a heavy frost at this 



