Page:Popular Science Monthly Volume 82.djvu/324

320 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 temperature effect within the forest, but we have as yet very little reliable information 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 temperate 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:

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 expectableexpected [sic] 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 forests 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