58 FOREST INFLUENCES. 



M. Becquerel foiiiu] in liis observations that the woods were decidedly 

 cooler during persistent cold weutlier. For instance, at La Salvionniere 

 during nnusually cold weatlier, from the 24th to the 30th of December, 

 18G7, the temperature in the woods Avas 21.4° while that outside was 

 22.0°. From the Lst to the 12th of January, 180S, the temi)erature in 

 the woods was 14.5° while that outside was 15.8°. For the stations 

 near Chatillon-sur-Loing, at which Becquerel took his observations, the 

 forest temperature for the latter dates average 13.3o while the temper- 

 ature in open fields was 14.4°. He noted this in otlier cases and thought 

 that the lower the temperature in extreme cold weather the lower rel- 

 atively it was in the woods. This is, however, not always the case. In 

 Dr. Eberniayer's published series of observatioiis there is only one case 

 out of seven where the extreme teniperaturo was lowest in the woods 

 and this was only by a little over one degree. On the other hand, at 

 Seeshaupt, on January 25, 1808, the temperature in the fields was —20.4° 

 while in the woods it was (mly -f^.l'^, or a diftereuce of 22.5° in favor 

 of the woods. These are extreme and not mean temperatures, but the 

 publislied data do not permit the study of the daily mean temperatures. 

 Eivoli made a special study of this subject and <'amo to the conclusion 

 that the relative winter cold of forests was quite dependent on the direc- 

 tion of the wind. In Posen, 140 single observations from I^Tovember to 

 March gave a warmer temperature in the woods except lor south and 

 southwest winds, when the forest temperature was lower by a fraction 

 of a degree. 



The ])ublished observations are not complete enough to give a good 

 idea of the diurnal imigress of temperatures under trees, but several 

 stndeuts of the subject (Becquerel, Kentzsch, Bergen) find that it is of 

 the same character as the annual progress. They generally represent 

 the daily minimum temperatures of woods as being higher tlian those 

 outside. 



DECIDUOrS AND EVERGREEN FORESTS. 



The German observations permit a study of the eifects of difi'erent 

 kinds of trees. The stations were classified for decidnous and ever- 

 green tbrests as in the section on soil temperatures, and the means of 

 tlie values of ir— were calculated separately for each. Tlie results 

 are graphically represented in Figs. 18 and 19. The difference between 

 them is much more striking than was the case with soil temi)eratures. 

 The evergreen trees show a symmetrical in<'rease in the reductions of 

 maxima and minima until late summer, when they rapidly and sym- 

 metrically decrease. The greatest reduction of the maxima is 4.21° 

 (August), the least 1.20° (Decend)er). Tlie average reduction of max- 

 ima is 2.920. The gieatest reduction of minima is 2.20° for Septeml)er, 

 the least 1.29° in November, but it runs (capriciously from 1.29° to 1.08° 

 from November to May. 



