56 FOREftT INFLUENCES. 



Fig-. 14 (Sclioo) shows a small reduction of mean temperature limited 

 to late summer and early autumn. Fig. 1.") ( Marienthal) shows a curious 

 aberrant case. The woods in March and April are warmer than the 

 fields. But Fig. IG (Hadersleben) is perhaps the most curious of all. 

 Here the uiaxima of temperature are higher in the woods from Novem- 

 ber to Ai)ril than they are in the fields, aiul during the same time the 

 minima are lower. The temperature range through these months is, 

 therefore, greater in woods than outside. But the increase in range is 

 small and the mean temj^eratures are about the same in woods and 

 outside. 



These figures, all of which, except the first, have been selected be- 

 cause they were peculiar, are instructive in showing how extremely 

 forest action varies while preserving consistently its characteristic 

 features. The variations are due to the causes mentioned under soil 

 temperatures, with the addition, in this case, of disturbances due to 

 prevailing winds. The average effects in which these disturbing ac- 

 tions are eliminated more or less completely are illustrated in Fig. 

 17. As this represents the result from sixteen stations, through about 

 two hundred years of observation, it may be considered fairly typical 

 for middle latitudes. From this diagram it appears that — 



(1) As to miuinui, the forest reduces them with fair uniformity dur- 

 ing the entire year. For about half of the year they are reduced about 

 a degree Fahrenheit. In April this reduction begins to increase; it 

 grows gradually until it reaches a maximum of about 2 degrees in 

 Angust. From this it lessens until November, when its average value 

 is 1 degree. 



(2) As to the maxinm, these are reduced about 1 degree in December. 

 This reduction grows slowly until it reaches about li degre(^s in April. 

 From this it grows very rapidly until it reaches abcmt 5 degrees. It 

 remains at about this value during the summer and decreases rapidly 

 during the autumn to its December value. 



(3) The reduction of range is about two degrees in the winter, six or 

 seven degrees in the summer, and an intermediate amount in spring and 

 autumn. ^ 



(4) As to mean temperature, it is one or two degrees cooler in the 

 woods in summer, about the same in woods and open fields during the 

 winter and intermediate in si)riug and autumn. 



(o) The moderating and cooling effects of woods already mentioned as 

 a phencmienon of mean annual temi)erature, extend, it appears, through 

 the entire year, but they are especially marked in summer. The cool- 

 ing effect tends to disappear in winter. The moderating effect is the 

 most important one and it is the most characteristic. 



It has often been claimed that forests warm the air of their interior 

 in winter, but the (German observations do not show this. On the other 

 hand, there is an occasional warming effect in early spring, while in 

 winter the woods are usually slightly cooler than the open fields. 



