70 WINDBREAKS. 



season of the year is very likely to be accomplished by precipitation, 

 and it will undoubtedly be found that the influence of the windbreak 

 at the tune is beneficial more often, and in a much greater degree, 

 than it is detrimental. 



SENSIBLE TEMPERATURES. 



It is a well-known fact that the same temperatures as registered 

 by thermometers may very differently affect human comfort owing 

 to differences in the quality of the air, its humidity, or, most of all, 

 its rate of circulation. Low temperatures which cause no discomfort 

 when the air is calm become unbearable when a wind springs up. 

 This is so widely appreciated tnat windbreaks are, perhaps, chiefly 

 valued for the protection they give against strong winter winds. 

 That there is considerable saving of fuel in heating a house so pro- 

 tected requires no proof, though exact determination of the amount 

 would be difficult. A good windbreak grove may reduce wind 

 velocity as much as 80 or 90 per cent immediately to the leeward, 

 and, at the same time, provide the required fuel without loss of 

 efficiency. If located with due regard for the direction of the pre- 

 vailing winds, such a grove need not be more than a few acres in 

 extent to serve the double purpose admirably. 



Conifers are obviously much superior to deciduous trees where only 

 winter protection is desired. A narrow belt of white pine, consisting 

 of only two rows planted close together and 20 feet high, reduces the 

 wind velocity at 100 feet to leeward by four-fifths. In other words, 

 if a 25-mile wind was blowing the velocity in the shelter of such a 

 windbreak would be only about 5 miles per hour.. This reduction in 

 wind velocity is equivalent to a reduction of 19 F. in the cooling 



effect of the wind upon the skin. 1 



\ 



VII. HUMIDITY. 



It has been thoroughly demonstrated that large areas of forest ap- 

 preciably increase the humidity of the atmosphere above them by rea- 

 son of the excessive transpiration from the trees as compared with 

 that from fields of grain or the evaporation from bare surfaces. This 

 fact, however, is of no economic importance in considering the influ- 

 ences of windbreaks consisting of a row or relatively small body of trees. 



A windbreak may more seriously affect the humidity of the air by 

 its disturbing influence upon air currents. The air obtains its mois- 

 ture by evaporation from the ground and from the plants on the 

 ground. There is more moisture, consequently, near the ground than 

 in the upper strata. A windbreak -which deflects the air currents 



i A French experimenter, Vincent, has calculated that the cooling effect of a wind upon the skin is about 

 1.2 C. for each increase in wind velocity of 1 meter per second. This, reduced to our terms, is 0.966 F. 

 for each mile per hour increase in wind. ( J. Vincent: La Determination de la Temperature Climatologique, 

 Bruzelles, 1890. Reprinted from Annuaire de 1'Observatoire Royal pour 1890.) 



