The phenomena included under the title frost, as commonly accepted, are understood to include both the processes of freezing and the mechanical effects produced thereby. Con sidered under this broad definition, frost plays an important part in the economy of nature, both beneficent and otherwise. It enters the crevices and minute cracks in the rocks and rends the rocks apart ; and is thus an important agent in aiding and hastening their disintegra tion, and in converting them and the solid materials of the earth into soil. Its beneficent action in loosen;ng and pulverizing the soil, by entering it and forcing the particles of com pacted soil and clods apart through its expan sive action upon the particles of moisture dis _ seminated therein, is well known, and is of inestimable value to agriculture and to human ity. The damage sometimes done to vegetation, trees, etc., through the frosts entering them, and rending their fibres, cells, etc., apart, is often very great, and partial failures of crops such as corn, vegetables, fruits, etc., are due to this cause. As any considerable motion of air, the presence of clouds covering the sky, or the placing of a light covering, as of cloth or similar material, over the objects to be protected, greatly reduces or prevents the formation of frost upon them and of injury thereby, artificial preventives are often resorted to. Sometimes smoke-producing fires are built around or within enclosures or fields containing plants, fruits, or vegetables, and light, tent-like coverings are placed over small fruit trees, shrubs, etc., and other tender vegetable or plant growths, and thus the damage by frosts is prevented, or minimized. In France an instrument has been devised for the predic tion of frost. It consists of a wet bulb and dry bulb thermometer, mounted on a board on which is also a scale of lines corresponding to the degrees of the dry bulb, and a pointer travers ing a scale graduated according to degrees of the wet bulb. Observations are taken shortly
before sunset. By means of the pointer and scale, the point may be found at which the line of the dry bulb reading meets the pointer set to the reading of the wet bulb. The scale is colored so that the point may fall in one of three zones, indicating certain frost, probable frost or no probability of frost. See also SNow.
Air Drainage is a term generally applied to a type of air circulition whichplays an import ant part in the distribution of frosts, more par ticularly in hill and valley districts. The cold heavy air of the higher slopes flows down and fills the valley, forcing upwards the warm and lighter air of the latter, and forming in the valley a lake of cold air. Thus there are heavy frost in the valleys while the higher slopes escape. The condition is understood by gar deners and others who use upper slopes for their gardens, orchards, etc.
Bibliography.—Andrews, 'Famous Frosts and Frost-Fairs in Great Britain' (London 1887) ; Beals, 'Forecasting Frost in the North Pacific States> (Weather Bulletin 41, Wash ington 1912) ; Cox, 'Frost and Temperature Conditions in the Cranberry Marshes of Wis consin' (ib. 1910) ; Day, 'Frost Data of the United States and Length of the Crop-Growing Season> (ib. 1911) ; Garriott, E. B., 'Cold Waves and Frost in the United States> (Weather Bulletin P, Washington 1906) ; 'Canada's Fertile Northland' (Department of the Interior, Ot tawa 1907) ; Hann, 'Zin Klima Manitoba' in Meteorologische Zeitschrift (Vienna 1894), and Monthly Weather Review.
or a stilt sandpiper (q.v.).