40 



WINDBREAKS. 



IV. MECHANICAL POWER OF WINDS. 



A Russian experimenter 1 has shown that the air which strikes a 

 body of trees is to some extent deflected upward; some of it leaks 

 through between the leaves and branches and some of it may pass 

 under the trees. The amount deflected upward is greatest in the 

 case of trees which are not too rigid. Willows are typical of the 

 class of trees which have this effect. These are very unimportant 

 considerations. 



In general, there is a mass of more or less calm air in the lee of 

 the windbreak which in time is set in motion by contact with the 

 rapid-moving strata above. Where a strong current of air passes 

 under the trees (see diagram 33) it gradually loses its velocity through 

 friction with the ground and with a calmer stratum of air above, so 



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jL 



* 

 s i 



d 





$ S /O /S *0 



MM y&oc/rr /M r# ofie/v (#/ts rs/r /tots/?) 

 DIAGRAM 15. Reduction of mechanical force of wind. 



tnat the wind velocity at the ground may be greater near the trees 

 than somewhat farther away on the leeward side. 



Distances at which the calming effect of the windbreak may be 

 felt will depend upon the depth of the mass of the air so calmed 

 (which will, of course, be equal to the height of the trees), and also 

 upon the mean velocity of the air in the area of calm (determined 

 by the leakage through the windbreak). Actually the average dis- 

 tance was found to be not more than twenty times the height of 

 the windbreak, and at that distance almost the same velocities were 

 experienced as were found on the windward side of the windbreak. 



When planting windbreaks for the protection of buildings or fields 

 from violent winds the farmer is most concerned with the effect 

 upon wind velocity in the immediate vicinity of the trees. A com- 



1 Nesterov, N. S., "The Influence of the Forest on the Force and Direction of the Wind.' 

 Trade Journal, Moscow, 1908, Vol. X, Nos. 8 and 9. 



Lumber 



