3 i8 THE POPULAR SCIENCE MONTHLY 



of the slow heating of the water in the leaves. To a certain slight ex- 

 tent, then, a forest cover ought to behave as does a water surface; it 

 ought to warm a little less rapidly and therefore it ought to cool less 

 rapidly. 



(h) The process of growth of the trees, and the chemical changes 

 which are going on during their life, must require an expenditure of 

 energy whose effect might possibly be observable in a difference of tem- 

 perature between the forest and the open. The rise and return of the 

 sap may also be expected to be accompanied by certain slight tempera- 

 ture effects resulting from the transfer of root temperatures upwards 

 and of crown temperatures downwards. 



In these, and perhaps in other ways, we may seek for the causes of 

 forest influences upon climate. But, whatever may be the theoretical 

 reasons for believing in such influences, we are here concerned only 

 with the facts as they are at present known. One further word of cau- 

 tion is necessary. It is one thing for a forest to have a climate of its 

 own within its own limits, under or above the trees. It is quite another 

 thing for a forest to affect the climate of the surrounding country, or of 

 distant regions. The latter effect is naturally the one in which the real 

 interest centers. But it is also the one which is by far the most diffi- 

 cult to study. It is clear that nothing more than reasonably local modi- 

 fications of climate ought to be expected. The special climate of the 

 forest itself — so far as it may appear to have one — can only affect the 

 surroundings by modifying the air currents which pass through or over 

 it, by producing an ascending movement of the forest air to take part in 

 the prevailing wind movement, or by causing, as may happen under 

 especially favorable conditions, local air currents of its own. Most, if 

 not all, of the above-mentioned theoretical effects of forests upon climate 

 have been overestimated. 



Forests as Wind-breaks 

 The most obvious effect of forests is that of the barrier, or wind- 

 break. First, there is far less wind movement within the forest than 

 there is outside. Second, friction on the tree-tops reduces the ve- 

 locity of the wind blowing over the forest. Third, to leeward of the 

 forest there is a belt of relative cairn which is roughly ten to fifteen 

 times as wide as the forest is high, as has been determined by measure- 

 ments in Iowa and in the Rhone Valley. More recently, in Roumania, 

 Murat has shown that within 165 feet to leeward the decrease in veloc- 

 ity may be from four to eight miles an hour, and that the effect of the 

 forest in decreasing velocity extends as far as 1,500 feet to leeward. 

 Some years ago, comparative observations in the harbor, city and sub- 

 urbs of New York and Boston showed a remarkable reduction in wind 

 velocities with increasing distance inland, the velocities in the city 

 being a little over three fifths, and those in the suburbs about one third, 

 of those in the harbor. 



