340 FUNDAMENTALS OF FRUIT PRODUCTION 



difference between the free radiation from the exposed fruit and the restricted 

 radiation from that which is covered may usually be sufficient, even when there 

 is a marked temperature inversion, to subject the former and not the latter to the 

 greatest danger from frost and freeze." 



Radiation and Thermometer Readings. — The full importance of radia- 

 tion to the horticulturist needs emphasis. Lack of recognition of this 

 factor has diminished the value of much investigational work. A ther- 

 mometer exposed to the open air is radiating and receiving heat. During 

 a clear night the outgoing exceeds the incoming heat and the thermometer 

 registers a lower temperature than that of the air. Inside a shelter prac- 

 tically all the outgoing radiation heat is reflected to the thermometer 

 which consequently registers very close to the actual air temperature. 

 During May in cranberry marshes in Wisconsin there were found differ- 

 ences between sheltered and exposed thermometers over bare soil averaging 

 2.3°F. for all nights of record, including nights not clear. Occasionally 

 the exposed thermometers recorded as much as 5.7 and 6.4° lower than 

 the thermometers in shelters.*^ Inasmuch as these temperatures were 

 taken near the ground it is possible that they represented extreme con- 

 ditions and would be of direct importance only to the cranberry and 

 strawberry grower. Sheltered and unsheltered thermometers at a 

 height of 5.5 feet from the ground at Williamstown, Mass., showed dif- 

 ferences at the time of the minimum temperature averaging 1.6° and 

 a maximum difference of 4°F.^^^ It is evident, then, that the exposed and 

 sheltered, thermometers do not check and that the differences are not 

 constant. 



Radiation and Plant Temperatures. — Plants as well as thermometers 

 lose heat by radiation, Seeley^^^ working with strawberries found con- 

 siderable difference between plant temperatures and air temperatures. 



He reports in part on his results as follows: "The plant thermometer readings 

 were usually lower than the air temperature in the early morning, the minimum 

 usually being about 3 or 4°[F]. lower than the air, the difference being greater, 

 of course, when the weather was clear with but httle wind velocity. The plant 

 cooled off more rapidly than the air in the early evening so that at 7 p. m. it was 

 usually 3 or 4°[F.] lower in temperature than the surrounding air." At times 

 the temperature of the plant may fall to 8°C. below that of the surrounding air 

 and plants may be frozen stiff though the thermometer indicates one or two 

 degrees above zero (C.),i" and there are records showing that occasionally plants 

 are cooled by radiation to a temperature 12 to 15°F. below that of the surrounding 

 air."2 Tomato vines under apple trees sometimes escape frost when those 

 exposed to radiation are killed and the temperature on a lawn under a tree may 

 be 5° higher than in the open.i^'^ 



Observations, predictions and conclusions, then, must be made with 

 three standards in mind: the air temperature, the exposed thermometer 

 temperature and the plant temperature. Though the exposed ther- 



