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   » » Wiki: Frost
Tag Wiki 'Frost'.
Frost is the coating or deposit of ice that may form in air in cold conditions, usually overnight. In temperate climates it most commonly appears as fragile white crystals or frozen dew drops near the ground, but in cold climates it occurs in a greater variety of forms. ξ1 Frost is composed of delicate branched patterns of ice crystals formed as the result of process development.

Frost is known to damage crops or reduce future crop yields, therefore farmers in those regions where frost is a problem often invest substantial means to prevent its formation.

Frost forms when the temperature of a solid surface in the open cools to below the freezing point of water and for the most clearly crystalline forms of frost in particular, below the in still air. In most temperate countries such temperatures usually are the result of heat loss by radiation at night, so those types of frost sometimes are called radiation frost.

of frost include crystalline from of from air of low humidity, in humid conditions, on glass surfaces, from cold wind over cold surfaces, without visible ice at low temperatures and very low humidity, and under supercooled wet conditions.

The size of frost crystals varies depending on the time they have been building up and the amount of water vapor available. Frost crystals may be clear or , but, like snow, a mass of frost crystals will scatter light in all directions, so that a coating of frost appears white.

If a solid surface is chilled below the of the surrounding humid air and the surface itself is colder than freezing, ice will form on it. If the water deposits as a liquid that then freezes, it forms a coating that may look glassy, opaque, or crystalline, depending on its . Depending on context, that process also may be called . The ice it produces differs in some ways from crystalline frost, which consists of spicules of that typically project from the solid surface on which they grow.

The main difference between the ice coatings and frost spicules arises from the fact that the crystalline spicules grow directly from of water vapour from air, and desublimation is not a factor in icing of freezing surfaces. For desublimation to proceed the surface must be below the of the air, meaning that it is sufficiently cold for ice to form without passing through the . The air must be humid, but not sufficiently humid to permit the condensation of liquid water, or icing will result instead of desublimation. The size of the depends largely on the temperature, the amount of available, and how long they have been growing undisturbed.

As a rule, except in conditions where droplets are present in the air, frost will form only if the deposition surface is colder than the surrounding air. For instance frost may be observed around cracks in cold wooden sidewalks when humid air escapes from the warmer ground beneath. Other objects on which frost commonly forms are those with low or high , such as blackened metals; hence the accumulation of frost on the heads of rusty nails.

The apparently erratic occurrence of frost in adjacent localities is due partly to differences of elevation, the lower areas becoming colder on calm nights. Where static air settles above an area of ground in the absence of wind, the and specific heat of the ground strongly influence the temperature that the trapped air attains.


Hoar frost
Hoar frost (also hoarfrost, radiation frost, or pruina) refers to white , deposited on the ground or loosely attached to exposed objects such as wires or leaves. They form on cold, clear nights when conditions are such that heat radiates out to the open sky faster than it can be replaced from nearby sources such as wind or warm objects. Under suitable circumstances, objects cool to below the ξ2 of the surrounding air, well below the freezing point of water. Such freezing may be promoted by effects such as flood frost or frost pocket. These occur when ground-level radiation losses cool air till it flows downhill and accumulates in pockets of very cold air in valleys and hollows. Hoar frost may freeze in such low-lying cold air even when the air temperature a few feet above ground is well above freezing.

The name hoar comes from an adjective that means "showing signs of old age"; in this context it refers to the frost that makes trees and bushes look like white hair.

Hoar frost may have different names depending on where it forms:

  • air hoar is a deposit of hoar frost on objects above the surface, such as tree branches, plant stems, and wires;
  • surface hoar refers to fern-like ice crystals directly deposited on snow, ice or already frozen surfaces;
  • crevasse hoar consists of crystals that form in glacial crevasses where water vapour can accumulate under calm weather conditions;
  • depth hoar refers to faceted crystals that have slowly grown large within cavities beneath the surface of banks of dry snow. Depth hoar crystals grow continuously at the expense of neighbouring smaller crystals, so typically are visibly stepped and have faceted hollows.

When surface hoar covers sloping snowbanks, the layer of frost crystals may create an risk; when heavy layers of new snow cover the frosty surface, furry crystals standing out from the old snow hold off the falling flakes, forming a layer of voids that prevent the new snow layers from bonding strongly to the old snow beneath. Ideal conditions for hoarfrost to form on snow are cold clear nights, with very light, cold air currents conveying humidity at the right rate for growth of frost crystals. Wind that is too strong or warm destroys the furry crystals, and thereby may permit a stronger bond between the old and new snow layers. However, if the winds are strong enough and cold enough to lay the crystals flat and dry, carpeting the snow with cold, loose crystals without removing or destroying them or letting them warm up and become sticky, the frost interface between the snow layers still may present an avalanche danger, because the texture of the frost crystals differs from the snow texture and the dry crystals will not stick to fresh snow. Such conditions still prevent a strong bond between the snow layers. ξ3

In very low temperatures where fluffy surface hoar crystals form without subsequently being covered with snow, strong winds may break them off, forming a dust of ice particles and blowing them over the surface. The ice dust then may form , as has been observed in parts of Antarctica, in a process similar to the formation of and similar structures.

Hoar frost and also occurs in man-made environments such as in freezers or industrial facilities. If such cold spaces or the pipes serving them are not well insulated and are exposed to ambient , the moisture will freeze instantly depending on the . The frost may coat pipes thickly, partly insulating them, but such inefficient insulation still is a source of heat loss.

Advection frost
Advection frost (also called wind frost) refers to tiny ice spikes that form when there is a very cold wind blowing over branches of trees, poles and other surfaces. It looks like rimming the edge of flowers and leaves and usually it forms against the direction of the wind. It can occur at any hour, day or night.

Window frost
Window frost (also called fern frost or ice flowers) forms when a glass pane is exposed to very cold air on the outside and warmer, moderately moist air on the inside. If the pane is not a good (for example, if it is a single pane window), water vapour condenses on the glass forming frost patterns. With very low temperatures outside, frost can appear on the bottom of the window even with double pane energy efficient windows because the air convection between two panes of glass ensures that the bottom part of the glazing unit is colder than the top part. On unheated motor vehicles the frost will usually form on the outside surface of the glass first. The glass surface influences the shape of crystals, so imperfections, scratches, or dust can modify the way ice . The patterns in window frost form a with a greater than one but less than two. This is a consequence of the nucleation process being constrained to unfold in two dimensions, unlike a snowflake which is shaped by a similar process but forms in three dimensions and has a fractal dimension greater than two. ξ4

If the indoor air is very , rather than moderately so, water will first in small droplets and then freeze into .

Similar patterns of freezing may occur on other smooth vertical surfaces, but they seldom are as obvious or spectacular as on clear glass.

White frost
White frost is a solid of ice which forms directly from contained in .

White frost forms when there is a above 90% and a temperature below −8 °C (18 °F) and it grows against the direction, since air arriving from has a higher humidity than leeward air, but the wind must not be strong or it damages the delicate icy structures as they begin to form. White frost resembles a heavy coating of hoar frost with big, interlocking crystals, usually needle-shaped.

is a type of ice deposition that occurs quickly, often under conditions of heavily saturated air and windy conditions. Technically speaking, it is not a type of frost, since usually water drops are involved, in contrast to the formation of hoar frost, in which water vapour desublimates slowly and directly. Ships travelling through Arctic seas may accumulate large quantities of rime on the rigging. Unlike hoar frost, which has a feathery appearance, rime generally has an icy solid appearance.

Black frost
Black frost (or "killing frost") is not strictly speaking frost at all, because it is the condition seen in crops when the humidity is too low for frost to form, but the temperature falls so low that plant tissues freeze and die, becoming blackened, hence the term "black frost". Black frost often is called "killing frost" because white frost tends to be less cold, partly because the of freezing of the water reduces the temperature drop.

Effect on plants

Many plants can be damaged or killed by freezing temperatures or frost. This varies with the type of plant, the tissue exposed, and how low temperatures get: a "light frost" of will damage fewer types of plants than a "hard frost" below .

Plants likely to be damaged even by a light frost include vines—such as beans, grapes, squashes, melons—along with such as tomatoes, eggplants and peppers. Plants that may tolerate (or even benefit) from frosts include:

  • root vegetables (e.g. beets, carrots, parsnips, onions)
  • leafy greens (e.g. lettuces, spinach, chard, cucumber)
  • (e.g. cabbages, cauliflower, bok choy, broccoli, Brussels sprouts, radishes, kale, collard, mustard, turnips, rutabagas)
Even those plants that tolerate frost may be damaged once temperatures drop even lower (below ). Hardy perennials, such as , become dormant after the first frosts and regrow when spring arrives. The entire visible plant may turn completely brown until the spring warmth, or may drop all of its leaves and flowers, leaving the stem and stalk only. Evergreen plants, such as pine trees, withstand frost although all or most growth stops. is a bark defect caused by a combination of low temperatures and heat from the winter sun.

Vegetation is not necessarily damaged when leaf temperatures drop below the freezing point of their cell contents. In the absence of a site nucleating the formation of ice crystals, the leaves remain in a liquid state, safely reaching temperatures of . However, once frost forms, the leaf may be damaged by sharp ice crystals. is the process by which a plant becomes tolerant to low temperatures. See also .

Certain , notably , are particularly effective at triggering frost formation, raising the nucleation temperature to about . Bacteria lacking ice -active proteins () result in greatly reduced frost damage.

Protection methods
Typical measures to prevent frost or reduce its severity include one or more of:
  • deploying powerful blowers to simulate wind, thereby preventing the formation of accumulations of cold air. There are variations on this theme. One of them is the Selective Inverted Sink Rolex Awards site (won award in Technology and Innovation category) 1998. prevents frost by drawing cold air from the ground and blowing it up through a chimney. It was originally developed to prevent frost damage to fruits in . In , are used in a similar function, especially in the regions like . By dragging down warmer air from the , and preventing the ponding of colder air on the ground, the low-flying helicopters prevent damage to the fruit buds. As the operations are conducted at night, and have in the past involved up to 130 aircraft per night in one region, safety rules are strict. Helicopters Fight FrostVector, , September / October 2008, Page 8-9 Although not a dedicated method, have similar (small) effect of vertically mixing air layers of different temperature. Turbines and turbulence, , 468, 1001, 23 December 2010, DOI:10.1038/4681001a, published online 22 December 2010.Somnath Baidya Roy and Justin J. Traiteur. Impacts of wind farms on surface air temperatures, Proceedings of the National Academy of Sciences, Vol. 107, No. 42, October 19, 2010, p. 17,899. Wind farms impacting weather, Science Daily.
  • for high-value crops, farmers may wrap trees and cover crops.
  • heating to slow the drop in temperature. This is not practical except for high value crops grown over small areas.
  • production of smoke to reduce cooling by radiation
  • spraying crops with a layer of water that releases latent heat, preventing harmful freezing of the tissues of the plants that it coats.
Such measures need to be applied with discretion, because they may do more harm than good; for example, spraying crops with water can cause damage if the plants become overburdened with ice. An effective low cost method for small crop farms and plant nurseries, exploits the . A pulsed irrigation timer delivers water through existing overhead sprinklers at a low volumes to combat frosts down to . If the water freezes it giving off its latent heat, preventing the temperature of the foliage from falling much below zero.

Frost is personified in Russian culture as . such as the have their own traditions of frost deities.

English folklore tradition holds that , an elfish creature, is responsible for feathery patterns of frost found on windows on cold mornings.

File:Air Hoar.jpg|Air Hoar on pine branches File:Frost on a nettle, Netherlands.jpg|Frost on a nettle File:Frost.jpg|Rime frost File:Saskatoon-Frost.jpg|Large feathery crystals File:Saint-Amant 16 Gelée blanche 2008.jpg|Hoar frost melting on grass in File:Winter forest in Łużna.jpg|Frost on branches (Image made using techniques) File:Fern Frost.JPG|Fern Frost on a Window File:Frost on leaves.jpg|Frost on plant leaves in the File:Hoar Frost.JPG|Surface Hoar in File:Hoar frost crystals on fence in Central Oregon USA.jpg|Hoar frost crystals on fence in central , USA File:Bow River Ice Fog-003.JPG|Bow River ice fog and hoar frost on north shore of , , Alberta at −20 °C 10:00AM

See also

External links

    ^ (2005). 9781402032646, Springer Science & Business Media. .
    ^ (2013). 9781284054279, Jones & Bartlett Publishers. .
    ^ (2022). 9780898868098, The Mountaineers Books. .
    ^ (2022). 9780387955544, Springer. .

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