AIR TEMrERATURES IN FORESTS. 51 



TEMPERATURES OF AIR IN THE INTERIOR OF FORESTS. 



We pass uow from the soil, Avliicli is fixed and tlie teiiipeiatiue changes 

 of which are due to the changes of the siirroimdiug. medium, to the con- 

 stantly moving air, the temperature changes of which are to some 

 extent due to the motions of the medium itself. The results for the 

 air from this cause are less definite, more variable, and more cai»ricions 

 than those derived from observations on the soil. At the surface of 

 the soil there is only an imperfect relation between the temperatures of 

 the soil and air. Yet it is undoubtedly true that the air temperatures 

 uear the surface are due in the first place to heat derived from the soil. 

 It is to the motion of the air, carrying with it its temperature, that 

 this disparity between soil and air temperatures is due. 



The observations used here are those of maximum and minimum 

 temperatures.* These afford satisfactory results, because what we are 

 concerned with is not so much the temperatures in woods as tlie differ- 

 ences between temperatures in woods and those outside. Besides, in 

 using the extreme teniperatures, all danger is avoided of being misled 

 by a difference in the diurnal progress of temperatures at the two sta- 

 tions. The extreme temperatures, also, are those which best show the 

 influence of the forest, for this is exercised chiefiy here, as in soil tem- 

 peratures, in reducing the extremes. The data used arc always, unless 

 otherwise stated, the values of W— (woods minus open fields reading), 

 and it is to be remembered that a minus sign (or position below zero 

 line) indicates colder temperatures in woods, a i)lus sign (or position 

 above zero line) warmer. The observations are taken at the same 

 height from the ground in the woods and outside. 



ANNUAL RANGE. 



Fig. 9 shows at a glance the mean annual values of the temperature 

 difference of W—0 in the case of maxima, minima, and the mean de- 

 rived from them. The heavy line across the figure represents the zero 

 line (for W—0=0). The annual values for 11^— O for the maxima are 

 always negative and they are laid off to scale below the zero line. The 

 annual values for W— for the minima are always positive aiul they are 

 laid off above the zero line to tlie same scale. 



It appears at once that the action of the forest varies greatly in the 

 different stations. It always cuts down the annual mean of the monthly 

 maxima, but to a very variable extent. In the case of St. Johann (five 

 years of observation) it cuts oft" the maxima by an average of 4o.89, 

 but in tlie case of Schoo it is only 1°.44, or less than one-fourth, and in 

 the case of the young forest of Lintzel, it cuts down the nnixima by 

 only 0°.35. The average amount of reduction for all stations is 2o.85. 



"Dr. Miittrich lias published ii eoinploto I'cduol ion dl" tin- air tciiiperatures for the 

 German service \\\\ to lSi)0. It m ly he fomid in Danckelniaun's ZritschriJ'ffiir I'orst 

 uiid Jaydwexeii, xxii, 1890, oWi aud 397. 



