36 Eighteenth Report State Entomologist of Minnesota — 1920 



The difference in temperature due to change in solar altitude is 

 not very noticeable during June and July, as the total noon varia- 

 tion during this period is only about 3 degrees. During August, how- 

 ever, the difference becomes much more marked, with a reduction of 

 noon altitude of nearly 12 degrees. This change in solar altitudes 

 affects the subcortical temperature in two ways : ( 1 ) By an actual re- 

 duction of the possible amount of radiation due to the greater absorption 

 of heat by the atmosphere as the altitude of the sun is lowered ; and 

 (2) by changing the angle of incidence of the sun's rays upon the 

 log. Thus on a warm sunny day in September we can not expect as 

 high a subcortical temperature as would obtain on a similar day in 

 June or July. 



The amount of heat absorbed by the surface of the bark depends 

 considerably upon the angle of incidence of the sun's rays. The more 

 vertical the rays the greater the absorption. When the angle of inci- 

 dence is very great, a large proportion of the rays will be reflected from 

 the surface of the bark and very little heat will be absorbed. This is 

 shown very clearly by a comparison of the late afternoon or early 

 morning temperatures of logs standing on end with those of logs 

 lying east and west. 



Effect of Character of Bark 



The color of the bark is one of the most important factors deter- 

 mining the height of subcortical temperatures. The lighter colored 

 the bark the cooler the log. This effect is evident when white pine 

 is compared with thin-barked norway or jack pine. The white pine 

 was the darkest colored log of the series of species used and its tem- 

 perature was higher in every case than any of the others. With a 

 white bark, such as that of the paper birch, the opposite extreme is 

 illustrated. While no birch logs were included in this series of experi- 

 ments, it has been noticed incidentally that there is a very marked 

 dift'erence in temperature between the normal white bark and the black 

 areas resulting from peeling the bark. This difference is often so 

 great in the bright sunlight that it can easily be observed without the 

 aid of a thermometer by touching a dark area and a white area with 

 the hand. 



The impf)rtance of color was well shown by an experiment in 

 which normal tem])eratures for a set of logs were compared with tem- 

 peratures in the same logs when painted first white and then black. In 

 this way only one factor was changed appreciably, and its effect could 



