FROST

is the name we give to the state of the weather when the temperature is below the freezing-point of water (32° F.). The intensity of the cold in frost is conveniently specified in degrees of frost, or the number of degrees by which the temperature of the atmosphere is below the point at which the freezing of water commences. Frost is often very destructive to vegetation, owing to the fact that water, which is generally the chief constituent of the juices of plants, expands when freezing, and bursts and thus destroys the vesicles of the plant. In the same way rain-water, freezing in the crevices of rocks, breaks up their surfaces, and often detaches large fragments. When the dew-point, or temperature at which the air would just be able to hold the quantity of aqueous vapour it actually contains, is below 32° F., and the air is cooled to that dewpoint, the vapour is deposited directly in the solid form, as hoar frost. On the other hand, if the vapour has first been deposited at a higher temperature, and the ground is subsequently cooled below 32°, there is the phenomenon of frozen dew. Rime is distinct from hoar frost. Itis produced during fogs by accumulation of frozen moisture on trees or other objects. Glazed frost is a transparent coating of ice, due to the freezing of rain after falling, or the contact of moist and comparavals, warm, air with very cold sur‘aces.