EFFECT OF FORESTS ON TEMPERATURE. 



11 



The temperature in forests is higher during the night and lower 

 during the day than on open ground, the differences being most 

 pronounced during summer. Hence, forests tend to moderate the 

 extremes of heat and cold. The effect depends, however, much 

 on the geographical position, the extent to which the localities are 

 exposed to, or protected against, air currents, and on the species. 

 Fully stocked beech woods have, during summer, twice as much 

 effect as spruce or Scots pine woods ; on the other hand, during 

 January, spruce woods have twice the effect of beech woods, due 

 to the dense foliage of beech during summer and its leafless 

 condition during winter. 



The reduction of temperature in the interior of forests is 

 communicated to the surrounding country, owing to the con- 

 tinuous interchange of air between the two. To ascertain the 

 extent to which this takes place, a system of so-called radial 

 stations has been established, that is to say, a series of points of 

 observation commencing in the centre of a considerable block of 

 forest and extending at regular intervals into the open country 

 surrounding the forest. In this way it has been ascertained that 

 the effect of forests on the temperature of the air in the open 

 country diminishes rapidly with the distance. If, however, the 

 percentage of forest in a district is very high, the effect may be 

 greater. On this point Dr. Woeikof , late Director of the Meteoro- 

 logical Institute of Petrograd, pubhshed some years ago data 

 giving the mean temperature for several series of places, each 

 series being situated as nearly as possible on the same degree of 

 latitude, reduced to the same latitude and altitude above sea level. 

 In this way he obtained the following figures for July and the 

 50th degree northern latitude : — 



