196 REPORT OI? THE FORESTRY COMMITTEE 



goes several changes. In ascending it becomes cooler, the temperature of air not 

 fully saturated decreasing one degree F. for every 182 feet of ascension. At the 

 same time the water-holding capacity of the air decreases until the saturation point 

 is reached and fogs, clouds, and precipitation begin to form. Further cooling of 

 the air in its iipward course is counteracted to some extent by the heat that is sep- 

 arated in the process of condensing vapor and from then on proceeds only at the 

 rate of about 0.5 degree F. for every 182 feet of ascension. After the air current 

 has passed the crest of the mountain and lost an amount of moisture in ratio to 

 the degree to which it has been cooled, it descends on the leeward side and becomes 

 heated. In its descent it absorbs the fogs and clouds and in this process takes on 

 some heat. Further heating goes on at the rate of 1 degree F. for every 182 feet 

 of descent. 



The more moisture the air loses in ascending a mountain the greater is the 

 amount of heat it can absorb in descending. If, for instance, a current of satu- 

 rated air, before ascending, had a temperature of 50 degrees F., and the crest 

 over which it passed was 900 feet high, then, on the leeward side at the same 

 altitude at which it began to ascend, it would have a temperature of 77 degrees 

 F., and, provided no moisture is absorbed in the descent, a relative humidity of 

 21 per cent.^ At other obstructions met by the same current of air the same 

 changes would take place, though on the next chain of mountains new precipita- 

 tion begins, as a rule, only at an altitude equal to that of the crest of the previous 

 mountain chain over which the current of air has passed. 



Prof. Mayr^ has shown that wherever, as on the Pacific coast, in the Rocky 

 Mountains, and in Caucasus and Turkestan, there are several parallel chains of 

 mountains at right angles to the moist air current, each chain higher than the 

 previous one, the forest on each consecutive mountain chain does not extend be- 

 low an altitude equal to that of the preceding chain. Between the mountain 

 chains are treeless, dry valleys. 



As a rule the moist air currents passing over wooded slopes, being chilled, 

 deposit most of their precipitation on the windward side. It is only in excep- 

 tional cases, such as when the air is not fully saturated, or when warm currents 

 rise from below, that the air current, instead of depositing moisture, becomes 

 enriched with moisture and carries it over the crest to the regions lying beyond. 

 This may occur on southern slopes, which are likely to be warm. The influence 

 of wooded windward slopes upon the humidity of the region to the leeward side 

 of the mountains, therefore, varies. It is apparent, however, that while the 

 forests in the mountains have a marked influence upon local precipitation, their 

 influence upon the humidity of regions lying to the leeward can not, on the whole, 

 be very great. 



If the effect of mountainous forests upon the precipitation of regions lying 

 in the lee of them is not entirely clear to us, the effect of forests in wide plains 

 of continents, especially in the path of moist winds, can not be doubted. By 

 increasing the evaporation from the land at the expense of surface run-off they 



1 Klossovsky, A. V. Osnovi meteorologii. Odessa, 1910, p. 48. 



2 Mayr, H. Waldungen von Nord Aerika, Munich, 1890. 



