32o THE POPULAR SCIENCE MONTHLY 



too large, owing to unfavorable exposure of the thermometers. It is, 

 however, significant that the presence of relatively cool air over forests 

 has been indicated by the fact that balloons, in passing over forested 

 sections, often have a distinct tendency to descend. This cooling effect 

 above the forest is pretty clearly of more importance than any tempera- 

 ture effect within the forest, but we have as yet very little reliable in- 

 formation on this phase of our problem. 



It is to be expected that equatorial forests should have more marked 

 effects in lowering the temperature than temperate forests. The high 

 maxima reached over the deserts of the lower latitudes, largely as a 

 result of the excessive heating of the sandy surface, do not occur where 

 the dense equatorial forests shade the ground; increase the radiating . 

 surface by means of their leaves; supply much water vapor through 

 transpiration and evaporation, and possibly also, by favoring fog and 

 cloud formation, cut off sunshine. Woeikof has done good service in 

 calling attention to this important function of tropical forests. We 

 must not, however, suppose that scattering forest patches in our tem- 

 perate latitudes can have any notable effects upon temperature. As 

 Supan has well stated the case, in speaking of the very " moderate " 

 effect of forests on temperature: 



No one will care to maintain that the system of isotherms would be radically 

 altered if Europe and Asia were one great forest from ocean to ocean. 



Influence of Forests upon Humidity and Evaporation 

 Within European forests the relative humidity exceeds that over 

 the neighboring glades or fields by a few per cent. (2-10 per cent.). 

 This is an expectable condition, and no doubt in part due to the slightly 

 lower average temperature in the forest. The local formation of dew 

 might be favored on this account. It appears, further, that evergreen 

 forests have more influence in increasing relative humidity than do 

 deciduous forests. Evaporation from free water surfaces within for- 

 ests is a little less than one half of that in the open, a fact which is to 

 be explained chiefly by the decreased air movement, and, to a much 

 less extent, by the slightly lower temperature and the slightly higher 

 relative humidity. In addition to the action of forests in decreasing 

 evaporation, there is the positive effect of supplying moisture to the 

 air through the process of transpiration. The amount of moisture 

 thus given off from the leaves of the forest has been estimated to vary 

 from three times that from a horizontal water surface of the same extent 

 to less than half that from the water. Evaporation is, of course, much 

 the most active under sunshine. In Central Europe the annual amount 

 of transpiration in forests consisting of well-grown beeches and oaks 

 has been estimated to be about one quarter of the total precipitation. 



It is apparent that, as rain-bearing winds progress inland from the 

 ocean, their tendency to continue rainy will be favored if they pass over 

 extended forest areas instead of over bare soil, or even over grass or 



