Canadian Forcslry Journal, December, 1918 



Do Forests Increase Rainfall? 



Bij Dr. B. E. Fernow, Dean, FacuUij of Forestry, 

 University of Toronto. 



A Speculative Problem on Which Con- 

 vincing Data is Yet to be Developed. 



1965 



This question, often asked, in- 

 cludes two questions, namely, whe- 

 ther such increase is found over the 

 forest itself, or whether the forest 

 increases the rainfall over the adjoin- 

 ing field; the latter being, of course, 

 the one practically most important. 

 To ask the question is to throw doubt 

 on what is axiomatic natural philoso- 

 phy with every popular writer, but 

 the candid professional student of 

 the influence of forests on their sur- 

 roundings will have to admit that we 

 cannot yet demonstrate positively 

 that, how, and to what degree the 

 rainfall of a locality is influenced by 

 forest cover. This ignorance is due 

 partly to the complicated character 

 of the problem, partly to the inef- 

 ficiency of meteorological instru- 

 ments for measuring rainfall, so that 

 an exact proof cannot, as yet, be 

 reached by measurement. Natural 

 philosophy, however, leads us to 

 assert that there is no condition on 

 earth that does not have an influence 

 on all other conditions more or less, 

 and hence we can infer such an in- 

 fluence of forest on rainfall. Whether 

 this influence in one or the other 

 direction is of practical import is 

 still another question. 



A human rain maker. 



Some years ago the Congress of 

 the United States appropriated sev- 

 eral thousand dollars for the writer — 

 to his dismay — to be devoted to the 

 production of rain by artificial means. 

 This reference of the appropriation 

 was made, because the writer had 

 published a bulletin on the influences 

 of forest cover on its surroundings, 

 including rainfall. The experiments, 

 however, were not to be made by 

 forest planting and waiting a half 

 century or so for an answer, but by 

 bombarding the atmosphere, in the 

 belief of the assertion that large bat- 



tles were usually followed by rainfall 

 — an assertion which was found then 

 not supported by the existant evid- 

 ence, although a book under the 

 title, "War and the Weather," had 

 brought together the statistics in 

 this respect, leaving out, of course, 

 the battles which were not followed 

 by rain. In looking up the history 

 of rainfall, it was amusing to find 

 that in France suffering from floods 

 the opposite effect, namely of driving 

 away thunder clouds by bombarding 

 the air, was expected to prevent these 

 floods. The efficacy of prayer in 

 breaking a drouth, and in some vil- 

 lages in the East Indies an official 

 rainmaker is employed to perform 

 the miracle. The Snake Dance of 

 the Hopi Indians is such a prayer for 

 rain. 



The cause of rain. 



At any rate, it became necessary 

 for the writer to find out what was 

 known as to the conditions under 

 which and by what means Nature 

 herself produces rain. It was found 

 that even this fundamental knowledge 

 was not very fully developed beyond 

 the ])rimary physical law that air at 

 a given temperature and under a 

 given barometric pressure could con- 

 tain only a given amount of moisture 

 and thai by decrease of that tempera- 

 lure, as when a cold wind blew into a 

 moisture-laden, warm current or by 

 decrease of barometric pressure, as 

 when such a current had to ascend a 

 mountain, some of the moisture in 

 the current would have to be pre- 

 cipitated as rain or snow. The same 

 would take place if a warm, moisture- 

 laden air current added its quota of 

 humidity to bring to saturation a 

 passing current. 



In the first place, then, the sun, 

 the oceans, the distribution of land 

 and water areas, the air currents due 



