frost

This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (December 2007)

A fine coating of hoar-frost on grass.

Frost is the solid deposition of water vapor from saturated air. It is formed when solid surfaces are cooled to below the dew point of the adjacent air.^[1] Frost crystals' size differ depending on time and water vapor available. Frost is also usually translucent in appearance. There are many types of frost, such as radiation and window frost. Frost causes economic damage when it destroys plants or hanging fruits. Road surfaces can also be damaged through a process known as frost heaving.

Formation

Frost on black pipes.

If a solid surface is chilled below the dew point of the surrounding air and the surface itself is colder than freezing, frost will form on the surface. Frost consists of spicules of ice which grow out from the solid surface. The size of the crystals depends on time, temperature, and the amount of water vapor available. Based on wind direction, "Frost arrows" might form.

A thin layer of frost will melt quickly, when exposure to sunlight raises an underlying object's temperature above the melting-point of water.

In general, for frost to form the deposition surface must be colder than the surrounding air. For instance frost may be observed around cracks in cold wooden sidewalks when moist air escapes from the ground below. Other objects on which frost tends to form 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. It is also affected by differences in absorptivity and specific heat of the ground which in the absence of wind greatly influences the temperature attained by the superincumbent air.

Because cold air is denser than warm air, in calm weather cold air pools at ground level. This is known as surface temperature inversion. It explains why frost is more common and extensive in low-lying areas. Areas where frost forms due to cold air trapped against the ground or against a solid barrier such as a wall are known as "frost pockets".

The formation of frost is an example of meteorological deposition.

Types

Hoar frost

Radiation frost

Hoar frost or soft rime on a cold winter day in Lower Saxony

Hoar frost visible on a blade of grass in England

Interior Alaska Ice Iris

Window frost on a skylight (contrast enhanced)

Rime frost

Frost on a grave, Netherlands

Radiation frost (also called hoar frost or hoarfrost) refers to the white ice crystals, loosely deposited on the ground or exposed objects, that form on cold clear nights when heat losses into the open skies cause objects to become colder than the surrounding air. A related effect is flood frost which occurs when air cooled by ground-level radiation losses travels downhill to form pockets of very cold air in depressions, valleys, and hollows. Hoar frost can form in these areas even when the air temperature a few feet above ground is well above freezing. Nonetheless the frost itself will be at or below the freezing temperature of water.

Hoar frost may have different names depending on where it forms. For example, air hoar is a deposit of hoar frost on objects above the surface, such as tree branches, plant stems, wires; surface hoar is formed by fernlike ice crystals directly deposited on snow, ice or already frozen surfaces; crevasse hoar consists in crystals that form in glacial crevasses where water vapour can accumulate under calm weather conditions; depth hoar refers to cup shaped, faceted crystals formed within dry snow, beneath the surface.

Surface hoar is a cause of avalanches when it forms on top of snow. Conditions that are ideal are cold clear nights, with a very light wind that is able to circulate more humidified air around the snow surface. Wind that is too abrupt will destroy the crystals. When buried by subsequent snows they may remain standing for easy identification, or become laid down, but still dangerous because of the weakness of the crystals

Hoar frost also occurs around man-made environments such as freezers or industrial cold storage facilities. It occurs in adjacent rooms that are not well insulated against the cold or around entry locations where humidity and moisture will enter and freeze instantly depending on the freezer temperature.

Advection frost

Advection frost (also called wind frost) refers to tiny ice spikes forming 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 of day and night.

Frost flowers

Frost flowers occur when there is a freezing weather condition but the ground is not already frozen. The water contained in the plant stem expands and causes long cracks along the stem. Water, via capillary action, goes out from the cracks and freezes on contact with the air. Also the frost can literally look like a flower, even a dead flower from the previous summer. These are rare and wonderful to see as they are very delicate and last usually less than a day. Due to their fleeting nature, they are difficult to find to photograph and the locations of these Frost Flowers are elusive as terrain plays a big part in their formation as well.

Window frost

Window frost (also called fern frost) forms when a glass pane is exposed to very cold air on the outside and moderately moist air on the inside. If the pane is not a good insulator (such as a single pane window), water vapour condenses on the glass forming patterns. With very cold temperatures outside frost can appear on the bottom of the window even with double pane energy efficient windows, due to air convection between two panes of glass. The bottom part of the glazing unit is always colder than the top part. The glass surface influences the shape of crystals, so imperfections, scratches or dust can modify the way ice nucleates. If the indoor air is very humid, rather than moderately so, water would first condense in small droplets and then freeze into clear ice.

Rime

Rime is a type of frost that occurs quickly, often under conditions of heavily saturated air and windy conditions. Ships traveling through Arctic seas may accumulate rime on the rigging. Unlike hoar frost, which has a feathery appearance, rime generally has an icy solid appearance. In contrast to the formation of hoar frost, in which the water vapor condenses slowly and directly into icy feathers, Rime typically goes through a liquid phase where the surface is wet by condensation before freezing.

Effect on plants

Overview

Frost on a nettle.

Frost on lingonberry leaves.

Many plants can be damaged or killed by freezing temperatures or frost. This will vary with the type of plant and tissue exposed to low temperatures.

Tender plants, like tomatoes, die when they are exposed to frost. Hardy plants, like radish, tolerate lower temperatures. Perennials, such as the hosta plant, become dormant after first frosts and regrow when spring arrives. The entire visible plant may completely turn brown until the spring warmth, or will drop all of its leaves and flowers, leaving the stem and stalk only. Evergreen plants, such as pine trees, will withstand frost although all or most growth stops. Frost crack is a bark defect caused by a combination of low temperatures and heat from the winter sun.

Vegetation will not necessarily be 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°C to −12°C. 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 cold temperatures. See also cryobiology.

Certain bacteria, notably Pseudomonas syringae, are particularly effective at triggering frost formation, raising the nucleation temperature to about −2°C^[2]. Bacteria lacking ice nucleation-active proteins (ice-minus bacteria) result in greatly reduced frost damage^[3].

Protection methods

The Selective Inverted Sink ^[4] 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 fruits in Uruguay.

In New Zealand, helicopters are used in a similar function, especially in the vineyard regions like Marlborough. By dragging down warmer air from the inversion layers, 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.^[5]

Personifications

Frost is personified in Russian culture as Ded Moroz. Indigenous peoples of Russia such as the Mordvins have their own traditions of frost deities.

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

See also

Wikimedia Commons has media related to: Frost

Look up frost in Wiktionary, the free dictionary.

• Air frost
• Frostbite
• Frost flower
• Frost line
• Hard frost
• Icing (nautical)
• Needle ice
• Rime: soft/hard
• White frost

References

External links

• Clouds R Us - Frost
• Guide to Frost
• BBC Understanding Weather - Frost
• American Meteorological Society, Glossary of Meteorology - Hoarfrost
• The Weather Doctor - Weather Whys - Frost
• Free Frost Alert system