40 Eighteenth Report State Entomologist of Minnesota — 1920 



Effect of Air Temperature and Air Movement 



As previously stated, the term air temperatvire as used in this 

 article refers to the temperature in the open, as recorded by a white 

 bulb thermometer. The influence of air temperature is almost entirely 

 over-shadowed and obscured by the influence of light. At the same 

 time it is important and can not be disregarded. On cloudy days and 

 at night the subcortical temperatures followed very closely the tem- 

 perature of the air, so it is not unjustifiable to assume that in prac- 

 tically all cases in the absence of sunlight the temperature within the 

 log would follow air temperature in a general way. There would al- 

 ways, of course, be a lag on the part of log temperature depending upon 

 the insulating properties of the bark. 



The amount of this lag would also be influenced l)y the etTect 

 of air movement upon the rate of radiation. The graph illustrating the 

 daily course of temperature in a standing log ( Plate I ) shows how 

 air currents increase radiation and cause rapid fluctuation in the sub- 

 cortical temperature. 



This fluctuation is also influenced to a considerable extent by the 

 fact that the surface of the standing log is removed from close prox- 

 imity to other absorbing and radiating surfaces which tend to stabilize 

 the temperatures in the log near the ground. 



Effect of Evaporation From the Surface Layer of the Bark 



Frequently on clear mornings following a night when there was 

 a heavy dew the temperature of logs under the lath shades where there 

 was no dew was nearly equal to or occasionally slightly higher than that 

 of logs lying in the open with the sun striking directly upon them. 

 This condition lasted but a short time and as ^soon as the moisture had 

 evaporated from the surface of the bark the logs in the sun rapidly 

 increased in temperature. The most logical explanation of this phe- 

 nomenon appears to be based upon the cooling efl:'ect of evaporation. 



The cooling eft'ect of evaporation was also apparent after rains. 

 If two similar days are selected, one following a rain and the other 

 following a period of dry weather, the temperature of the log is usually 

 lower on the day following the rain than on the day following dry 

 weather. This condition is illustrated bv Table III. 



