Forest Influences. 17 



The absolute amount of water-vapor in forest air is ven' 

 slightly greater than that in the open, and on account of the lower 

 temperature of forest air, its relative humidity is generally higher 

 than that of air in the open, 



The Influence of Forests upon Precipitation. 



It has been generally stated by those anxious to emphasize 

 the deleterious effects of deforestation, that the wholesale de- 

 struction of the forests has resulted in decreased rainfall over 

 the deforested areas. This may be the case, but positive proof 

 is lacking. It seems probable that the rainfall over certain parts 

 of Ontario has decreased within the last 25 years. But whether 

 this is a consequence of the clearing of the land, or merely a re- 

 sult of those changes of climate that occur over large cycles of 

 time, who can say? There is no component of the climate that 

 is more variable and uncertain than the rainfall. An experi- 

 mental inquiry into the relation between the forest and rainfall 

 is, from the very nature of the problem, exceedingly difficult. To 

 conduct such an inquiry, all factors, other than the forest, that 

 can cause a difference in the observed rainfall, must be eliminat- 

 ed, and the experiment must cover a long series of years. Many 

 such attempts bave been made, with conflicting results. Ex- 

 perimentally the question is still undecided, but the concensus 

 of opinion is that the forest has little or no influence on the amount 

 of rainfall. 



The question may be examined theoretically with some in- 

 terest. Rainfall is the consequence of evaporation of moisture 

 from the earth's surface, with subsequent condensation of vapor 

 caused by cooling of the air containing it. Where there is no 

 evaporation there can be no rainfall, unless vapor is carried over 

 from some other area. Deserts are rainless because there is no 

 evaporation there, and because the air above desert regions is too 

 hot and dry to allow of cooling and condensation of vapor that 

 may be carried over them ; also these areas are usually so situated 

 that vapor-bearing winds do not reach them. 



Rainfall over any locality may be increased either by an in- 

 crease of evaporation from the area in question, and a subse- 

 quent condensation and fall over this area, or bv an attraction 

 exercised upon large air movements whereby clouds and vapor- 

 laden winds are diverted to the locaHty and precipitate rain there- 

 upon. 



Of the various causes of rainfall, the principal is the upward 

 movement of moisture-laden air and the cooling bv expansion 

 as the ascending air reaches levels of less and still less pressure. 

 This upward movement may be part of a great cyclonic move- 

 ment, covering hundreds of miles in horizontal extent, such as 



