18 FOREST INFLUENCES. 



The latter currents, being warmer and raoister, can be of influence on 

 the temperature and moisture conditions of a neighboring flekl by mod- 

 erating 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 circu- 

 lation becomes sensible is not ascertained. In winter time, when the 

 temperature differences become small, no such circulation is noticeable. 



(3) The general air currents in their lower portions are cut off en- 

 tirely by the forest, which acts as a windbreak. This influence can 

 of course be experienced only on the leeward side. How far this pro- 

 tection reaches it is difticult to estimate, but it certainly reaches far- 

 ther than that of a mere windbreak, 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 "windbreak 

 effect. Deforestation on a large scale would permit uninterrupted 

 sweep of the winds, a change more detrimental where the configura- 

 tion of the ground does not fulfil a similar function — in large plains 

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

 than in the interior. 



The upper air strata can' be modified only by the conditions existing 

 near and above the crowns. At the same time they jnust 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 cnrrents, 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 

 coudensation 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 Avarm winds, whde the cooling winter effect upon compar- 

 atively warmer moist winds is not noticeable. Theoretical considera- 

 tions lead to the conclusion that in monntain regions only the forest 

 on the leeward slope can possil^ly add moisture to a Avind coming over 

 the mountain, but this does not necessarily increase the i)recipitation 

 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 set- 

 tled than that of precipitation upon forest areas themselves. Wherever 

 moisture-laden winds ])ass over extensive forest areas the cooler and 

 moister condition of the atmosphere may at least not reduce the i)ossi- 

 bility of condensation, which a heated plain Avould do; but observa- 

 tions so far give no conclusive evidence that neighboring fields receive 

 more rain than they otherwise would. 



(4) With regard to comparative temperatures in forest stations and 

 open stations that are situated not far apart from each other, it would 



