22 LOCALITY AND FOREST VROETATION. 



contiiK'iitiil {'ouiitries ; n;reater in forest countries than in l»are 

 tracts. 



Direct observations have shown that the rehxtive liuniidily 

 of forest air is greater than that of open air, the difference 

 amounting in Central Euroi:)e to as much as 10 per cent, 

 during summer, and a])Out lialf that amount in winter. The 

 practical vahie of this fact in sylviculture is, that radiation of 

 heat is much slower in moist than in dry air; hence tlie 

 danger of late and early frosts is smaller in the former than 

 in the latter. 



Snow and ice have a considerable effect upon trees. During 

 early youth snow may protect forest plants against excessive 

 cold. Afterwards a heavy fall of snow, or the formation of 

 ice or rime, may break the branches and tops of trees, or even 

 fell them to the ground. 



4. Air Currents. 



The atmosphere is, practically, in constant motion. The 

 principal cause of this is the uneven heating of the earth by 

 the sun. The heat, which becomes free on or near the surface 

 of the earth, w^arms the adjoining air and causes it to rise, its 

 place being taken by colder air from other parts of the earth. 

 The ascending air, after cooling, sinks again in its tuin. In 

 this manner a circular motion exists between the equator and 

 the jioies. Owing to a combination of these currents with the 

 motion of the earth, modified wind directions are produced. 

 The prevailing wind directions on the northern hemisphere 

 are, llicreforc, from the south-west and the north-east, 

 according as to whether the original current started from the 

 tropics or the polar region. 



A second cause of wind currents, especially of storms or 

 gales, is the sudden condensation of considerable quantities of 

 aqueous vapour, which forces air to rush from all sides into 

 the space of reduced pressure. 



Air currents arc of paramount importance to all organic 

 life on the earth, because they produce a thorough mixture of 



