THE OCCURRENCE OF FROST 347 



surfaces cools it becomes more dense and tends to sink. It is then 

 replaced by air somewhat warmer, probably for the most part flowing in 

 from the same level, which air in turn cools and sinks. If the supply of 

 relatively warm air be extensive enough and warm enough, the radiating 

 surfaces may be kept from reaching the freezing point. This frequently 

 happens on hillsides where the coolest air is continuously being pushed 

 downward by air nearly as cool and warmer air is flowing in from the side. 

 So much cool air may accumulate, however, that it fills a depression com- 

 pletely and raises the level of warm air. The warm air may be raised 

 so high above a given object that, as radiation proceeds, the replacing 

 air has little heat to give up. It therefore fails to warm the surface 

 sufficiently to prevent freezing. ^^2 Little replacement can be expected 

 by warm au from above since it is lighter. 



However, other things being equal, the wider a valley the greater 

 its area in proportion to its circumference; consequently the reservoir 

 of free warmer air at any level is greater in proportion to the radiating 

 shorehne at that level. The higher levels, in a given valley, therefore, 

 in addition to having better "drainage facilities" for removal of cold 

 air have larger reservoirs of warm air on which they can draw. For 

 the same reasons a slight elevation above a wide valley may be con- 

 siderably freer from frost than a higher elevation above a more restricted 

 valley. 



The term "air drainage," used to signify the resemblance of the 

 flow of cold air to the flow of water, is more or less unscientific and 

 inexact. ^^^ Nevertheless it is a convenient term; it suffices for practical 

 purposes and doubtless will continue in use. In many cases there is 

 an actual flow of air, closely comparable to the flow of water. This 

 flow of air is frequently the salvation of orchards in narrow valleys 

 which otherwise would fill quickly with cold air. 



In the discussion of Sites the statement is made that air drainage 

 insuring as much freedom from spring frosts as possible is one of the 

 most important considerations in picking the site for an orchard. It 

 should be stated here conversely that the best method of insuring against 

 frost and against the continual tax of frost-fighting is the proper selec- 

 tion of a site. There are certain sections where to secure proper soil 

 or plentiful moisture it becomes necessary for the prospective fruit 

 grower to locate on low sites that are subject to frost. He should recog- 

 nize clearly that he is exchanging relative immunity from frost for other 

 advantages; the exchange may be profitable if the frosts are not too 

 numerous and too severe. Over a large part of the country, however, 

 a considerable latitude in choice is available and intelligent discrimi- 

 nation in the choice of site may very easily make the difference between 

 considerable profit and heavy loss. The grower who is forced to protect 

 his orchard may make a profit in spite of his heavy overhead expense 



