RESULTS OF MEASUREMENTS. 



71 



upward will have little effect upon the humidity of the air in the zone 

 which it protects, except as the stagnation of the air in that zone 

 may permit it to accumulate more than the normal amount of 

 moisture. An inefficient windbreak, however, which permits the 

 wind to pass under the trees, may have a deleterious effect upon the 

 crops in the protected zone by bringing down from the higher strata 

 air which is not saturated to the normal degree. This effect of wind- 

 breaks of both classes in deflecting air currents is shown in diagram 33. 

 The results of an experiment conducted September 7, 1908, as ob- 

 tained from a large number of psychrometer, anemometer, and 

 evaporometer readings, are given in table 15. A careful study of these 

 figures shows that the wind near the trees on the leeward side was 

 almost as great as in an open situation on the windward side. The 

 lower absolute humidity of the air at this point indicates that the air 

 current measured must have comprised air from a higher stratum 

 mixed with that of the normal surface current. While greater 

 efficiency is obtained at a point some distance from the windbreak, 

 the figures furnish sufficient proof of the undesirability of a wind- 

 break of the open character of the one tested. 



TABLE 15. Showing the relation of an open windbreak to humidity and other atmospheric 



factors. 



On the whole, it must be admitted that the windbreak has no 

 important effect upon the humidity of the air. The reduction of 

 evaporation may be slightly affected by the increased or decreased 

 humidity due to the deflection or stagnation of the air, but will be 

 much more affected in all cases by the reduction of wind velocity. 



SUMMATION OP PHYSICAL AND PHYSIOLOGICAL EFFECTS. 



A summary of the influences of windbreaks upon the physical 

 factors which affect the growth of plants shows that 



(1) The zone of competition. In a narrow zone or belt adjacent to 

 the trees there is competition which results unfavorably to the annual 

 crops by reason of 



