46 



WINDBREAKS. 



(2) The area protected is proportional to the height and density of 

 the windbreak, and the distance to which protection is felt increases 

 with wind velocity. The protection is appreciable for a distance equal 

 to five times the height in the windward direction and fifteen or twenty 

 times the height leeward. 



DIAGRAM 16. Protective efficiency of underplanted cottonwood groves. For wind velocities of 5, 10, 

 15, and 20 miles per hour. Average height, 75 feet, 70 readings. 



(3) The distance from the windbreak of the area of greatest protec- 

 tion depends upon the position of the mass of foilage which affords the 

 protection. With a dense grove, it is immediately in the lee of the 

 trees ; with a narrow belt of trees that lack lower branches, it may be 

 as far from the trees as five times their height, and it moves outward 

 as the velocity of the wind increases. 



