INFLUENCE OF WINDJiREAKS. 



119 



tbe ratio of the width of the inotected strip to the height of the obstruc- 

 tion. M. Becquerel says that in the Rhone Valley a hedge -J meters 

 (() feet) high will protect delicate garden ])lants to a distance of 22 

 meters (70 feet). This is a ratio of 1 to 11, and this ratio is about that 

 given in the following cases gleaned from Dr. Hough's report on fores- 

 try for 1887. Judge Whiting, of Iowa, is quoted as saying that this 

 protection, " with almost mathematical precision, amounts to 1 rod on 

 the ground to each foot of height'' of the protecting trees. This is a 

 ratio of 10 to 165. Mr. Barnard, of Pawnee County, Nebraska, stated 

 that a wiiulbreak will protect an orchard a rod for every foot in height, 

 but Prof. Thomson wanted trees 25 feet high for every 10 rods of pro- 

 tection. The two latter cases relate to orchards which rise 10 or 20 

 feet above the surface. A probable estimate is that the forest creates a 

 calm area on its leeward side, which is, at the ground, ten to fifteen times 

 as wide as the forest is high. The protected area has the same relations 



Fig. 57. — Snowliaiik lief'ore and behind a fence. 



to temperature that a glade has. The air is relatively stagnant, and 

 temperatures rise higher in the direct sun's rays and fall lower on clear 

 nights. Evaporation is also somewhat decreased in this space, but its 

 chief advantage is found in the i^rotection from injurious cold or dry 

 winds. The rough surface of the forest must also decrease the velocity 

 of the wind, with the result of affecting the character of a storm which 

 passes over it, especially if the storm be small or local. Dr. Waldo 

 has given two interesting cases where the results are due undoubtedly 

 to several causes, one l)cing the retarding effect on the wind of a sur- 

 face of city buildings. At both New York and Boston there are three 

 meteorological stations, one out in the harbor, the second in the city, 

 the third fairly beyond the city (Central Park and Cambridge). The 

 mean velocities at these tiiree stations in order are, in ])ercentages: 



The reduction of the velocities as one goes inland through the city 

 is remarkable, and something similiar may be expected for a wind pass- 



ing over a forest. 



