July 18, 1913] 



SCIENCE 



73 



feet of ascension. In ascending the moun- 

 tain slope the water-holding capacity of 

 the air decreases until the saturation point 

 is reached, and fogs, clouds and precipita- 

 tion begin to form. The further cooling 

 of the air is counteracted to some extent by 

 the heat that is given off in the process of 

 the condensation of vapor. This further 

 cooling, therefore, proceeds only at the rate 

 of about 0.5° F. for every 182 feet of ascen- 

 sion, or only half as much as when the air 

 is dry. After the air current has passed 

 the crest of the mountain and lost an 

 amount of moisture corresponding to the 

 temperature which it had at the time of 

 passage, it descends on the leeward side 

 and becomes heated. 



In its descent it absorbs the fogs and 

 clouds. In this process it consumes some 

 heat. The further heating goes on at the 

 rate of 1° F. for every 182 feet of descent. 

 The more moisture there is extracted on the 

 windward side of the slope, the greater is 

 the temperature of the air on the leeward 

 side. 



If, for instance, an air current before as- 

 cending had a temperature of 50° F. at a 

 barometric pressure of 30 inches, and the 

 crest over which it passed was 9,900 feet 

 high, then, on the leeward side at the same 

 altitude at which it began to ascend, it 

 would not have a temperature of 50° F., 

 but of 77° F. at a relative humidity of 21 

 per cent. At other ascensions by the same 

 current of air, the same changes would take 

 place. But new precipitation, as a rule, 

 begins on the next chain of mountains only 

 at an altitude equal to that of the crest of 

 the previous mountain chain over which 

 the current of air has passed. 



Professor Mayr'^ has shown that wherever 

 there are several parallel chains of moun- 

 tains perpendicular to the moist-air cur- 

 rent, such as are found on the Pacific coast, 



" " Waldungen von Nord Amerika. " 



of which each one is higher than the pre- 

 vious one, the forest appears in each con- 

 secutive mountain chain only from an alti- 

 tude equal to the altitude of the top of the 

 preceding chain over which the air current 

 has passed. Between the mountain chains 

 there remain treeless, dry valleys. This is 

 strikingly observed in the Pacific coast and 

 Rocky Mountains, as well as in Caucasus 

 and Turkestan. 



As a rule, the moist air currents, in pass- 

 ing over wooded slopes, being chilled, de- 

 posit most of their precipitation on the 

 windward side. It is only in exceptional 

 eases, such as when the air that passes over 

 the wooded slopes is not fully saturated, or 

 when warm currents rise from below, that 

 the air current, instead of depositing mois- 

 ture, becomes enriched with moisture and 

 carries it over the crest to the regions lying 

 farther on its way. 



This may occur on southern slopes, which 

 are apt to become warm. The influence of 

 wooded windward slopes upon the humidity 

 of the regions lying to the leeward side of 

 the mountain chains, therefore, varies. It 

 is apparent, however, that, while the for- 

 ests in the mountains at right angles to pre- 

 vailing moist winds have a marked influ- 

 ence upon local precipitation, their influ- 

 ence upon the humidity of regions lying to 

 the leeward of them can not, on the whole, 

 be very great. 



CONCLUSIONS 



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 conti- 

 nents, 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 enrich with mois- 

 ture the passing air currents, and in this 

 way help to carry it in larger quantities 



