The New International Encyclopædia/Frost
FROST (ME. frost, forst, AS. forst, from freosan, Eng. freeze). A formation of ice on the ground or on plants; also the temperature 32° Fahrenheit or 0° Centigrade that corresponds to the formation of ice and snow. When air whose dew-point is below 32° Fahrenheit comes in contact with a substance whose surface is colder than this, a portion of the aqueous vapor in the air is condensed upon that surface in the form of ice or frost; although this deposition is truly ice, yet the particles of ice are usually small, separate from each other, and reflect the rays of light in such a way as to make the deposit appear white, like crushed ice, instead of being transparent, as is the case with solid ice. In fact, the particles of ice usually have a crystalline structure, more or less perfect, as may easily be seen when moisture is deposited on the inside of a window-pane when the temperature outdoors is below freezing. In the latter case, when the air within the room has a dew-point far above the freezing temperature, the moisture first condenses in drops of dew upon the pane of glass, but is afterwards frozen into ice if the exterior cold is sufficiently intense.
When the air of the room has a dew-point below the freezing temperature, then the moisture is deposited upon the window-pane directly in the form of spiculæ or slender prisms of ice, and it is under these circumstances that the most delicate frost figures are formed. The latter is also the ordinary case in the formation of frost on vegetation and on the ground in the open air; in such a case every object is studded more or less thickly with small crystals of ice; the whole deposit is as white as snow, and is usually called ‘hoar frost.’ It frequently happens that rain (or sleet, which is frozen rain) falls on objects that are already colder than 32° Fahrenheit. In such cases the rain or sleet remains congealed as a layer of almost transparent ice on the upper surfaces of the limbs, the leaves, the fences, and other objects. This usually happens when rain falls at the close of a period of very cold weather. On the summits of high mountains, notably Mount Washington and the mountain stations of Southern Europe, it frequently happens that although the air is apparently clear, yet it is filled with the most minute drops of water, which are cooled far below the freezing-point, but retain their liquid condition. When these strike any object they lose their spherical shape, and are converted immediately into ice at the temperature of thirty-two degrees. They therefore build up an accumulation of ice on the windward side of every object, giving rise to remarkable displays of so-called ‘frostwork.’
Aëronauts have occasionally ascended into and through thin layers of air bearing similar aqueous globules that are cooled l)elow freezing, but still liquid water. These layers appear from a distance like thin stratus clouds, but are scarcely perceptible when viewed directly from below on account of their transparency. The globules instantly change to snowflakes or frostwork when they strike any object.
Tender vegetables in northern gardens and tropical plants in the southern portion of the United States and in California are severely injured or killed by freezing temperatures. The mere deposition of frost on the outside of such plants does not necessarily argue that the plant is frozen through and through; it may, therefore, produce only slight damage; on the other hand, when the air is too dry to deposit much moisture, and when it deposits frost only when cooled greatly below 32° Fahrenheit, it often happens that the plants are frozen under a clear sky or during a cold, dry wind without the deposition of much, if any, frost upon their exterior surfaces; in such cases the sap within the cells and especially within the medullary rays is frozen; the structure of the plant is destroyed, and when the sun's warmth has melted the frozen sap, the leaves and stalks sink to the ground, wilt away, and turn black, being in fact dead. This phenomenon is known as the black frost. It is, however, more properly a freeze than a frost.
The interval between the last frost of spring and the first frost of autumn is the so-called growing season of the agriculturists. Between these dates tender plants of all kinds must perfect their crops, while those that can withstand frosts continue their growth uninterruptedly. Especially must the great staple crops of the country—the Indian corn or maize, the cotton, tobacco, and a large range of tender fruits, as well as spring wheat, rye, and buckwheat—all complete their growth between these dates. The accompanying maps show by curved lines the regions in the United States over which the first and last frosts occurred on given dates on the average of the past thirty years. A comparison of these maps will therefore show the length of time that is available as a normal growing season in any part of the country. Although agriculturists always select seed that is likely to produce a quick-growing crop that may be harvested before the early frost of autumn, yet, owing to the irregularities of climate, the late spring frosts and the early autumn frosts frequently bring their crops into jeopardy. This has stimulated the invention of many methods of frost protection, which are fully described in the Monthly Weather Review for the years 1894-97, and especially in Bulletins on Frost Protection, Nos. 23 and 29, issued by the Weather Bureau.
The methods of frost protection are divided into several categories, as follows: (1) A light screen of any material, even a few slats or a gauzy veil, stretched above a plant prevents the radiation of the plant's heat into space, and by reflecting back the heat from the soil may keep the temperature of the plant so high as to prevent frosty temperatures. (2) Fires with clouds of smoke warm the air of a field while the smoke cloud prevents radiation; in perfectly still air such a cloud of smoke will spread evenly in all directions, and continue effective through the night. (3) Without reliance upon a cloud of smoke, one may warm the ground and the air either by fires, or by streams of water, or by flooding the field. All these methods and various combinations of them are in regular use for the protection of tropical fruits in California and Florida, and for the protection of tobacco, cranberries, and early vegetables in Northern States. Many patented devices for making smudges are on the market, but in general the smudge disfigures the fruit, and other protective devices are preferred.
Although the tender portions of plants are destroyed by frost, yet the ripened mature seed is much less susceptible. The kernels of both corn and wheat may be subjected to very low temperatures without being injured. Unfortunately, many of the bacterial germs and fungoid spores also are not injured by cold. It was formerly supposed that freezing weather destroyed the germs of malarial and yellow fever; but it is now probable that such germs are not affected by cold, but that, on the other hand, the cold checks the mosquitoes and other insects by which these germs are introduced into the human body.
The prediction of frost is a matter of great importance to a farmer, and can usually be made with great exactness by the help of the daily weather map, wherefore special attention is paid to this subject by the officials of the Weather Bureau. All persons whose interests depend largely upon the knowledge of frost keep in close touch with the Weather Bureau, and receive special telegrams when freezing temperatures are approaching. In general, when the dew-point is below 32° F. and the night is still and clear, the temperature will fall rapidly, and it may reach the freezing-point before sunrise.
Inasmuch as severe frosts have sometimes been very destructive to the staple crops, they have occasionally been the direct cause of severe famines; lists of memorable frosts for the last four hundred years will be found in Andrews, Famous Frosts and Frost-Fairs in Great Britain (London, 1887); also Walford, paper on “Famines” in Journal of the Statistical Society (London, 1878). The record for the United States will be found most conveniently in Pierce on The Weather (Philadelphia, 1860), and in the successive numbers of the Monthly Weather Review (Washington).
A special and elegant form of frostwork occurs as ‘ice needles’ or ‘ice columns,’ that rise up in masses from gravelly ground, raising up the top layer of gravel and small stones on their summits to a height of two, four, or six inches. These ice columns are hollow, and are apparently formed by the freezing of the films of moisture that rise up from the lower warm wet soil and freeze on the under side of the top layer of stones when the latter are chilled by radiation during clear nights. These columns do not form when the air is cold enough to freeze the soil below the top layers. A similar formation exudes from a thin crack in the bark of a tree when the warm body of the tree affords sap enough. The mechanics of this process is partially explained by Prof. Cleveland Abbe in the American Meteorological Journal for April (1893).
|COPYRIGHT, 1902, BY DODD, MEAD & COMPANY.|
|COPYRIGHT, 1902, BY DODD, MEAD & COMPANY.|