FOREST INFLUENCES. 311 



hand in hand with decrease and increase of rainfall, the possible secular change in rainfall must 

 also be considered. Yet the experience of increased rainfall over the station at Lintzel, with 

 increase of forest area, points strongly toward a possible interdependence under given conditions. 

 (Pp. 111-118, Bill. 7.) 



By condensing dew, hoar frost, and ice on their branches, trees add thereby a little to the 

 precipitation which reaches the ground, and by preventing the rapid melting of snow more water 

 remains available under forest cover. (P. 121, Bui. 7.) 



The question as to the march of destructive hailstorms with reference to forest areas, which 

 seems settled for some regions in France, remains in doubt for other, especially mountain, regions. 



( JL })« X—J L—"J-jU%Jm JD 1 111. »•/ 



From these statements we would expect as a consequence of deforestation an effect on the 

 climate of the deforested area in three directions, namely: (a) extremes of temperature of air as 

 well as soil are aggravated; (b) the average humidity of the air is lessened; and possibly (o) the 

 distribution of precipitation throughout the year, if not its quantity, is changed. 



INFLUENCE OF FORESTS UPON THE CLIMATE OF THE SURROUNDING COUNTRY. 



(1) An influence of the forest upon the climate of its surroundings can only take place by 

 means of diffusion of the vapor which is transpired and evaporated by the crowns and by means 

 of air currents passing through and above the forests being modified in temperature and moisture 

 conditions; the mechanical effect upon such air currents by which they are retarded in their 

 progress may also be effective in changing their climatic value. 



(2) Local air currents are set up by the difference in temperature of the air within and without 

 the forest, analogously to those of a lake or pond, cooler currents coming from the forest during 

 the day in the lower strata and warmer currents during the night in the upper strata. The latter 

 currents, being warmer and moister, can be of influence on the temperature and moisture con- 

 ditions of a neighboring field by moderating temperature extremes and increasing the humidity 

 of the air. 



This local circulation is the one most important difference between forest and other vegetation. 

 How far away from the forest this circulation becomes sensible is not ascertained. In winter time, 

 when the temperature differences become small, no such circulation is noticeable. (P. 120, Bui. 7.) 



(3) The general air currents in their lower portions are cut off entirely by the forest, which 

 acts as a wind break. This influence can of course be experienced only on the leeward side. How 

 far this protection reaches it is difficult to estimate, but it certainly reaches farther than that of a 

 mere wind break, since by the friction of the air moving over the crowns a retardation must be 

 experienced that would be noticeable for a considerable distance beyond the mere wind-break 

 effect. Deforestation on a large scale would permit uninterrupted sweep of the winds, a change 

 more detrimental where the configuration of the ground does not fulfill a similar function — in 

 large plains more than in hilly and mountainous regions, and at the seashore more than in the 

 interior. (Pp. 118-120, 133, Bui. 7.) 



The upper air strata can be modified only by the conditions existing near and above the 

 crowns. At the same time they must carry away the cooler and moister air there and create an 

 upward movement of the forest air, and thereby in part the conditions of this become also active 

 in modifying air currents. The greater humidity immediately above the crowns is imparted to the 

 air currents, if warm and dry, and becomes visible at night in the form of mists resting above and 

 near forest areas. These strata protect the open at least against insolation and loss of water by 

 evaporation, and have also a greater tendency to condensation as dew or light rain if conditions 

 for such condensation exist. This influence can be felt only to the leeward in summer time, and 

 with dry, warm winds, while the cooling winter effect upon comparatively warmer moist winds is 

 not noticeable. Theoretical considerations lead to the conclusion that in mountain regions only 

 the forest on the leeward slope can possibly add moisture to a wind coming over the mountain, 

 but this does not necessarily increase the precipitation on the field beyond. Altogether, the 

 theoretical considerations are as yet neither proved nor disproved by actual observations, and as 

 to rainfall the question of influence on the neighborhood is still less settled than that of precipita- 

 tion upon forest areas themselves. Wherever moisture-laden winds pass over extensive forest 

 areas the cooler and moister condition of the atmosphere may at least not reduce the possibility 



