FORESTS AND CLIMATE 319 



Clearly, then, wind-breaks such as those whiqh have been recom- 

 mended for, and are found in, much of our western treeless area furnish 

 considerable protection, over a narrow strip to leeward of the trees, 

 against the sweep of strong hot or cold winds. Such a reduction in 

 wind-velocity may have beneficial effects in reducing somewhat the ex- 

 tremes of heat or cold, and in diminishing evaporation from soil and 

 from plants, and perhaps also in checking the blowing away of the soil. 

 On the other hand, frost is more likely to occur where there is less air 

 movement. Deforestation, on a large scale, especially on extended level 

 areas, will therefore favor a freer sweep of the wind, which may be hos- 

 tile to the growth of crops. Over any extended treeless area, exposed to 

 high winds and with a severe climate, the best protection will be found 

 in the planting of narrow belts of trees, alternating with agricultural 

 strips. It should be noted, however, that this very wind-break, by de- 

 creasing wind velocity, keeps the air of the forest interior from affect- 

 ing the atmospheric conditions round about. In other words, the forest 

 diminishes its own climatic influence. 



Influence of Forests upon Temperature 



There is comparatively little popular interest in any possible influ- 

 ence of forests upon temperature, attention being almost altogether 

 focused on the rainfall factor. Upon their soil temperatures, forests 

 have a slight cooling effect (up to about 5°) attributable to the shade 

 and to the greater moisture of the forest floor; the extremes are re- 

 tarded and reduced; frost penetrates less deeply. Between evergreen 

 and deciduous forests there is this difference, that in the former sun- 

 shine has freer access to the ground, and warms and dries it better than 

 in the latter. In general, a forest climate bears a faint resemblance to 

 a marine climate in having a slightly smaller range of temperature tharj 

 the open, the extremes being most moderated in summer. In central 

 Europe the mean annual minima are about 2° higher in the forest, and 

 the mean annual maxima are about 4° lower. Individual summer 

 maxima may be 6° to 8° lower in the forest, and individual winter min- 

 ima 3° higher (Prussia). Conditions in the United States are probably 

 not very different, although our greater extremes of heat and cold here 

 would perhaps lead us to expect a slightly greater forest effect in mod- 

 erating these extremes. The sum-total effect is, therefore, a slightly 

 cooling one, chiefly because the forest is a little cooler than the open in 

 summer, and about the same, or very slightly warmer, in winter. But 

 these temperature differences in the average of the year are very small, 

 and even in individual cases are certainly usually inappreciable without 

 the use of thermometers. The considerable difference in our feelings of 

 heat and cold ("sensible temperature") within and outside of a forest 

 is probably chiefly due to the combination of the other factors, such as 

 wind movement, moisture, exposure to sunshine, etc. Indeed, a good 

 many of the reported differences between field and forest are probably 



