Atmospheric Drainage 51 



tion of a light frost. A rail fence, a stone wall, a row of 

 bushes, a slight elevation of land, the earth thrown out of 

 a ditch, all of these, when they extend across a slope, 

 are obstacles to drainage of cold air. In some cases, there 

 may be a difference of 10 in temperature in as many feet 

 of elevation. A dense row of trees standing diagonally 

 across a slope may convey away the cold air that settles 

 down against it, and thereby prevent injury to plants on 

 the lower levels. 



The range of elevation through which atmospheric 

 drainage acts beneficially to the fruit-grower is limited. 

 A fall of a few feet in a plantation is often sufficient for 

 the very best protection from light frosts; and a fall of 

 100 to 200 feet on a farm or large plantation may be 

 regarded as the general maximum throughout which the 

 benefit may be observed, for very high elevations are, as 

 we have seen, bleaker and colder in sum-temperature than 

 comparatively low ones. What may be gained by air- 

 drainage may be lost by coldness of elevation. The tem- 

 perature decreases by 1 F. for each 300 feet elevation, and, 

 according to Hann, is independent of latitude. 



"It is a common experience," writes W. M. Wilson, 

 "that vegetation at the surface is sometimes killed when 

 the temperature of the air 4 feet above the surface remains 

 above freezing; but rarely is there an absence of frost or 

 of injurious temperature when the air temperature 4 feet 

 above the surface falls to 32. This is due to the fact that 

 on clear, quiet nights when frost is likely to occur, the air 

 at the surface is nearly always colder than it is a few feet 

 above the surface. This difference may amount to as 

 much as 10, or even 15, in as many feet, but usually it 

 is much less. The difference is greater on clear nights 

 than on cloudy nights." 



