Windbreaks and Shelterbelts 



Board barrier, 33 percent solid 

 16 feet high 



>ISTANCE LEEWARD IN TREE-HEIGHT L 



Wind velocity at instrument stations 16 inches above the ground in 15-mile-per-hour 

 wind blowing at right angles to three types of windbreaks: (1) A 16-foot high board fence 

 of 33 percent density; (2) a dense belt of green ash, 290 feet wide; (3) a thin, rather open 

 cotton wood belt, 165 feet wide. The velocities are given in percentages of wind velocities 



in an open field nearby. 



more favorable conditions in orchards 

 for pollination by bees. 



Another benefit of windbreaks was 

 cited by F. L. Overly, superintendent 

 of the Tree Fruit Branch Experiment 

 Station near Wenatchee, Wash. He 

 pointed out that spraying for insect 

 control results in more even and com- 

 plete coverage in protected areas be- 

 cause of lower wind velocities. More- 

 over, protected orchard trees do not 

 develop as much lean or become as lop- 

 sided as those in exposed areas. 



ANYONE who has stood in the pro- 

 tection of a belt of trees on a windy 

 day has observed that the wind was 

 considerably reduced near the trees. 

 How much is this reduction in wind 

 velocity, and how far does it extend? 



The zone of influence is most easily 

 shown graphically. The chart shows 

 what this effect is for a 15-mile-an- 

 hour wind for several different types 

 of barriers. In this study, distances 

 were expressed in terms of windbreak 

 heights, in order to provide a con- 



venient comparison of zones of influ- 

 ence for the tree belts of different 

 heights; for instance, the term 3H 

 refers to a horizontal distance equal 

 to three times the height of a tree belt. 



It is seen that the wind velocity near 

 a dense wide belt of ash may be re- 

 duced to as low as 30 percent of that 

 in the open; for a thin cottonwood 

 belt, it is about 66 percent of normal 

 velocity; for a board barrier, it is about 

 58 percent. All three windbreaks show 

 some effect out to about 30 times their 

 height, but the effect beyond 20H is 

 rather minor. 



The results are substantiated by 

 studies made in other parts of the 

 United States. 



Pioneer tree planters, especially in 

 Nebraska, planted east-west shelter- 

 belts for protection of fields against 

 south winds. It has often been reported 

 that such protection may reduce the 

 drying power of winds, and may at 

 times prevent the firing of crops when 

 the temperature of southwesterly winds 

 is excessive. 



