Frost (from the root of freeze), in a general sense, the act or process of freezing, but more commonly used to signify crystals of frozen dew; in the latter case called also hoar frost. When the atmosphere contains so little aqueous vapor or is itself already at so low a temperature that a reduction to a point below 32° F. is necessary before condensation can take place, the deposit will be frozen, and instead of being technically dew it will have the form of hoar frost. The process is precisely similar to the deposition of crystals of salts from their solution in water. In this latter case the operation must be conducted slowly and at a certain low temperature; if the water be evaporated by violent ebullition, we have an amorphous powder, but no true crystals. In a similar manner the atmosphere deposits its aqueous burden in crystals or liquid form according to the temperature. The most remarkable formations of frost are witnessed on the summit of Mount Washington during the autumn and winter, when crystals a foot or more in length attach themselves to every object. The conditions favorable to frost are but an exaggeration of those that facilitate the formation of dew.

The destructive effects of frosts on tender vegetation, and their beneficial influences in a sanitary point of view, have caused much attention to be given to this phenomenon. In the Mississippi valley it is commonly said that the spread of the yellow fever is completely checked by a heavy frost. This however was notably not the case in 1873; and it may be fairly questioned whether some other agency, especially the dryness of the air, be not the true antagonistic element. Whatever hinders the deposition of dew acts also to mitigate the severity of a frost; to this end a thin or loose covering of cloth, straw, etc, is sufficient. In low flat regions it is found practicable to produce clouds of smoke, which, lying quiescent above the regions to be protected, serves to completely protect the ground from the radiation and consequent frost.-The word frost is somewhat loosely applied also to the action of winter's cold in freezing the solid ground and the water it may contain, when the frost is said to be in the ground; again, in the spring, the frost is said to come out of the ground. These expressions allude of course to the simple phenomenon of freezing, and not to the frost deposited on the surface of grass, plants, etc.

The effect of cold in freezing the water within the earth and the crevices of wells is recognized as a powerful agent in the preparation of the earth, for cultivation and the growth of forests, and is sometimes called into requisition in the quarrying of rocks. The term frostwork is applied to the formation of ice crystals on the inside surface of the window panes of a warm room. During cold weather the glass panes are cooled to a temperature below the freezing point, and a coating of true dew is deposited upon them; this dew water is then cooled and frozen by the continued cold of the pane of glass. The phenomenon is that of the freezing of a thin film of water, not that of the direct deposition of ice crystals as in the true frost. A similar distinction is to be made in the case of the formation of snow and of hail. In the former the minute crystals are deposited at a temperature lower than the freezing point; but in the formation of hail the water is deposited first, and the freezing is a subsequent process.

Black frost is the effect produced when the moisture within a plant is frozen, but without any hoar frost being deposited on its exterior.