FROST is the coating or deposit of ice that may form in humid 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
* 1 Introduction * 2 Formation
* 3 Types
* 3.1 Hoar frost * 3.2 Advection frost * 3.3 Window frost * 3.4 White frost * 3.5 Rime * 3.6 Black frost
* 4 Effect on plants
* 4.1 Overview * 4.2 Protection methods
* 5 Frost-free areas * 6 Personifications * 7 Gallery * 8 See also * 9 References * 10 External links
Types of frost include crystalline HOAR FROST from deposition of water vapor from air of low humidity, WHITE FROST in humid conditions, WINDOW FROST on glass surfaces, ADVECTION FROST from cold wind over cold surfaces, BLACK FROST without visible ice at low temperatures and very low humidity, and RIME 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 translucent , 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 dew point 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 type . Depending on context, that process also may be called atmospheric icing . The ice it produces differs in some ways from crystalline frost, which consists of spicules of ice 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 desublimation 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 frost point of the air, meaning that it is sufficiently cold for ice to form without passing through the liquid phase . 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 crystals depends largely on the temperature, the amount of water vapor available, and how long they have been growing undisturbed.
As a rule, except in conditions where supercooled 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 specific heat or high thermal emissivity , 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 absorptivity and specific heat of the ground strongly influence the temperature that the trapped air attains.
A spider web covered in air hoar frost Depth hoar, imaged with light and with scanning electron microscopy.
HOAR FROST (also HOARFROST, RADIATION FROST, or PRUINA) refers to white ice crystals , 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 frost point 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 until 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 Old English 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 avalanche 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.
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 yukimarimo , as has been observed in parts of Antarctica, in a process similar to the formation of dust bunnies and similar structures.
Hoar frost and white frost also occurs in man-made environments such as in freezers or industrial cold storage facilities. If such cold spaces or the pipes serving them are not well insulated and are exposed to ambient humidity , the moisture will freeze instantly depending on the freezer temperature . The frost may coat pipes thickly, partly insulating them, but such inefficient insulation still is a source of heat loss.
A flower with advection frost on the tips of its petals.
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 on 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 (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 insulator (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 nucleates . The patterns in window frost form a fractal with a fractal dimension 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.
If the indoor air is very humid , rather than moderately so, water will first condense in small droplets and then freeze into clear ice .
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 is a solid deposition of ice which forms directly from water vapour contained in air .
White frost forms when there is a relative humidity above 90% and a temperature below −8 °C (18 °F) and it grows against the wind direction, since air arriving from windward 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.
RIME 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 supercooled 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 (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 latent heat 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 −2 to 0 °C (28 to 32 °F) will damage fewer types of plants than a "hard frost" below −2 °C (28 °F).
Plants likely to be damaged even by a light frost include vines—such as beans, grapes, squashes, melons—along with nightshades 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 ) * cruciferous vegetables (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 −4 °C or 25 °F). Hardy
perennials, such as
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 supercooled liquid state, safely reaching temperatures of −4 to −12 °C (25 to 10 °F). However, once frost forms, the leaf cells may be damaged by sharp ice crystals. Hardening is the process by which a plant becomes tolerant to low temperatures. See also cryobiology .
Certain bacteria , notably Pseudomonas syringae , are particularly effective at triggering frost formation, raising the nucleation temperature to about −2 °C (28 °F). Bacteria lacking ice nucleation -active proteins (ice-minus bacteria ) result in greatly reduced frost damage.
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 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 citrus
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 latent heat of freezing . A pulsed irrigation timer delivers water through existing overhead sprinklers at a low volumes to combat frosts down to −5 °C (23 °F). If the water freezes it giving off its latent heat, preventing the temperature of the foliage from falling much below zero.
Frost-free areas are found mainly in the tropics, where they cover almost all land except at altitudes above about 3,000 metres or 9,800 feet near the equator and around 2,000 metres or 6,600 feet in the semi-arid middle tropics, but also in areas with subtropical climates that have winters tempered by strong oceanic influences. The most poleward frost-free areas are the lower altitudes of the Azores , Île Amsterdam , Île Saint-Paul , and Tristan da Cunha .
The only reliably frost-free areas in the contiguous United States are the Florida Keys and the coastal areas of the Channel Islands of California .
English folklore tradition holds that
Large feathery crystals *
Fern frost on a window *
Window frost *
Surface hoar in
* Clyde\'s Vegetable Garden Planner
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