FROST. In a general sense, frost means the lowering of atmospheric temperature at the surface of the earth to or below the freezing point, 32" F. Iu a special sense, the term is used to signify the deposition of atmospheric moisture upon plants and other objects. It is usually stated that frost is formed by the deposition of frozen dew, which indicates that the dew is frozen before it is deposited. This is not strictly correct. It 'could not so be deposited in the perfect form that frost assumes. The freezing takes place at the moment of deposition of the moisture, before dew is formed, the surface of the object having been cooled slightly lower than the atmosphere by radiation into clear space; for a clear atmosphere, as a rule, is one of the conditions of the appearance of hoar-frost, as deposited "frost," or frozen deposited moisture, is called. It will be seen that this phenomenon requires a temperature not far below the freezing point, else the moisture will be deposited in the form of snow, or of a very imperfect hoar-frost. The perfection of the crystallization requires that it take place at the point of deposit.
Frost may appear suddenly without a crystalline deposit when the air is not very moist. From its effect upon vegetation, which it withers and turns dark, it is then called black frost. It may take place when the sky is cloudy; but clouds, because they
retard the radiation of heat from the surface of the earth, generally retard or prevent the appearance of frost. A sheet placed horizontally over a bed of plants, even at some height, will often protect them from frost. A considerable amount of atmospheric moisture will often retard radiation sufficiently to avoid freezing. Seine places are peculiarly favorable to the deposition of hoar-frost, and sometimes it forms in very large crystals. This is the case at Mt. Washington, when there is in some favorable locations so nice a balance between the moisture and the cold that crystals sometimes form a foot in length. A certain degree of cold will destroy the life, or at least the activity, of many species of disease germs; and it is a popular belief that the appear ance of frost causes the disappearance of yellow and other fevers. This is the common belief in regard to the Mississippi valley; but in 1873, the appearance of frost did not Check the fever there, probably because the cold was not sufficient. A certain elevation of temperature will undoubtedly kill all disease germs, and a certain depression will either kill them or render them innocuous. See RADIATION, SNOW, RAIN, and VAron, IZAT/ON.