Frost.

Frost. The term frost is used to describe the condition of bodies containing moisture when their temperature is below $32^\{\backslash circ\}$ F., the freezing-point of water. When the substance in question is the air, everything exposed to its influence and not otherwise heated passes also below the freezing-point. In no part of the British Isles, within 1000 feet of sea-level, is the average temperature at any time of the year below $32^\{\backslash circ\}$; and therefore the frosts experienced in Britain, though often lasting several days or even weeks, are essentially sporadic and of the nature of interruptions in the general character of the weather. It may be noted in passing that when severe frosts do occur, covering the rivers and lakes with ice, the weather is usually settled, there being a high barometer and little wind; so that the air over the British Isles or those parts of them where the frost prevails is not liable to be mixed with air from the warmer regions above the seas around. Loch Ness is one of the few lakes in Britain never known to freeze: its great depth prevents the cold having time to cool the whole mass of the water even in the longest and severest frosts that have occurred within the memory of man. Other large but shallower lakes, such as Loch Lomond, on the contrary get sufficiently frozen over to bear skaters and curlers during every exceptionally cold winter. A frequent and disagreeable effect of frost is the bursting of water-pipes, due to the expansion of water in the act of freezing. The breakage is not usually noticed till a thaw sets in and the water again circulates in the pipe, hence it is sometimes erroneously supposed that the thaw has burst the pipe.

Local low temperatures are often found in valleys when the air at a little height up is considerably warmer, producing what is known as an 'up-bank thaw.' This is caused by the air chilled by radiation from the sides of the hills settling down from its greater weight, and occurs on every night when there is not enough wind to mix the different layers together. In fact, on calm mornings a stream of cold air flows down valleys like their rivers, and often indicates its presence by the fog caused by its coming in contact with the damp air above the watercourses. In choosing sites for houses or gardens a less liability to great cold and damp fogs will be secured by placing them on knolls or a little up the sides of the hill than if they are planted in the bottom of the valley, and thus in the influence of this cold current. A position directly opposite the mouth of a valley is also to be avoided.

Frost may be present on the ground or on plants when the air is several degrees above the freezing-point. This hoar-frost is due to cooling by radiation (see HEAT, p. 609)—i.e. to the ground, leaves, &c. radiating their heat away faster than it can be replenished from the air around. Hoar-frost is most liable to occur on clear nights, clouds acting as a screen to check radiation, and is more common in country districts than in towns, where the smoke serves a similar purpose. It is the frost most dangerous to vegetation—coming as it does in clear weather when the air is otherwise warm, the days often hot from strong sunshine, and the tissues of the plants full of sap. It may sometimes be foretold by observing the hygrometer; if the dew-point (see DEW) is below $32^\{\backslash circ\}$ in the afternoon, hoar-frost may be expected at night. At the same time it is frequently a sign of warm days, as the low dew-point indicates that little moisture is present in the air to check the sun's rays. Hoar-frost being wholly due to radiation, it is a common custom to protect plants by spreading some light covering over them, or even by burning leaves, brushwood, &c. to make a smoke of sufficient density to act as a screen. This is usually effectual, but may fail either from the air cooling below $32^\{\backslash circ\}$, in which case the covering is almost useless; or by injuriously checking the circulation of air and confining a small quantity immediately over the plants, which, getting cooled by contact with the ground below the temperature of the free moving air around, may pass below $32^\{\backslash circ\}$ and allow the vegetation to be frost-bitten.

A well-known form of frost, closely allied to hoar-frost, is the crystalline deposit seen when the moisture in the air of a warm room condenses on the glass of the window. It takes most beautiful and varied forms, owing to the tendency of ice deposited in this manner to form hexagonal crystals.

Another form of deposition is fog-crystals, which appear whenever a frosty fog is accompanied by wind, the fog drifting along and depositing spicules of ice on all surfaces exposed to it. As frosty fogs in low-lying districts occur usually in calm weather fog crystals are not often observed there, but are of frequent occurrence on hills, where the driving mists cover all projecting stones, trees, &c., with great masses of loose feathery crystals, often reaching a thickness of several feet. Great damage is sometimes caused to trees and shrubs by rain falling immediately after frost, before the ground and the air near it has time to thaw. The rain freezes as soon as it touches any objects, and gradually encrusts them with solid ice, until even large branches of trees break down under the weight. For other matters connected with freezing and its effects, see ICE, TEMPERATURE, THERMOMETER, GLACIER, HAIL, SNOW, FREEZING MIXTURES, &c.

Lists of the most memorable frosts on record will be found in W. Andrews's Famous Frosts and Frost-fairs in Great Britain (1887), and in C. Walford's paper on 'Famines' in Journal of the Statistical Society (1878). Fairs were held on the ice on the Thames in 1564, 1607-8, 1620, 1683-84 (especially celebrated), 1688-89, 1715-16, 1739-40, 1788-89, 1813-14. The western parts of the Baltic were frozen, and in most years passable for men and horses, in 1294, 1296, 1306, 1323, 1349, 1402, 1459-60, 1548, 1658, 1767. Flanders and Holland were visited by unusually severe frosts in 1468, 1544, 1565, 1594, 1622, 1734, and 1785. Besides these, other memorable frosts occurred in the years and countries mentioned in the subjoined table: