The Effects of Forests on Precipitation

Abstract

Debate has continued for many years as to the influence of the forest on the amount of precipitation falling in the vicinity of the forest itself as well as over wide regions. Evidence for such an influence is reviewed with reference to measurement of rainfall within the forest and moistening of the atmosphere by forest transpiration. Much of this evidence is circumstantial, some of which is based only on casual observation. Meteorological factors such as orographic effect, condensation and freezing nuclei, and vapor content of the atmosphere are discussed. This evidence, while based on analyses of the physical system, is not conclusive or completely satisfying either. The conclusion reached is that the maximum effect that may reasonably be attributed to the forest is a five per cent increase in precipitation, and this only in particular circumstances.