SPRING FROSTS 127 



containing it gradually gets deeper and deeper as the lower 

 layers cool those immediately above them. 



Cold air is denser than warm air, and the layer of air which 

 has been cooled by contact with the cooling earth will remain 

 stagnant on the surface, if the ground is quite level; but if 

 the ground slopes, the heavy, cold air will gradually flow away, 

 just as water does, though much more sluggishly, and will 

 accumulate in any valley or hollow, forming a pond or lake of 

 cold air. 1 If this air be laden with mist, the mist will, of course, 

 also accumulate in the hollow, and will give the impression that 

 it has been formed there, though in reality it has been formed 

 on the higher ground, and has only slid down into the hollow. 

 The higher ground, in fact, is the seat of manufacture of the 

 cold air, for there the air is clearer, and radiation is more active ; 

 in the hollow radiation is less active, and, as soon as the mist 

 accumulates there, will cease altogether, for the mist covers the 

 ground with a cloud, and prevents radiation, just as a cloud in 

 the sky would* do. 



From these facts it is clear that one of the chief considera- 

 tions in selecting a site for a fruit plantation is to avoid the 

 presence of any hollows where cold air can accumulate : good 

 air-drainage is of just as much importance as good water- 

 drainage. 



There is no reason for supposing that the mist which accom- 

 panies the cold air in a hollow is itself the direct cause of damage 

 to the fruit blossoms, by increasing the fall of temperature, and, 

 inasmuch as it hinders radiation, it must tend to retard the cooling 

 of the trees by radiation. Erroneous conclusions must not be 

 drawn as to the relative effect of damp and dry air on trees, from 

 relative effect of them on the human body. Cold, damp air, of 

 the same temperature as dry air, feels much colder to us, simply 

 because we are warm-blooded animals, and because moisture, 

 being a good conductor of heat, causes a rapid loss of heat from 

 our bodies ; this would be so to only a very small extent, if at all, 

 with trees ; they would be practically unaffected by the dryness or 

 moistness of air at any given temperature, just as would a ther- 

 mometer. Yet the view commonly held 2 that dampness in the 

 atmosphere increases injury by frost, is, apparently, not without 

 foundation, for West and Edlefsen (loc. cit.) found that the 

 spraying of trees, even with warm water, before a frost, whilst 



1 See L. D. Batchelor and F. L. West, " Variation in Minimum Tem- 

 peratures due to the Topography of a Mountain Valley in its Relation to 

 Fruit Growing," Utah Agric. Coll., Bulletin 141, 1915. 



2 G. F. Hooper, Board of Agric. Journ., 1907, XIV, 25. 



