Phenomenon: Hoar Frost

S P E C I E S  L I S T


Hoar Frost

hoar frost rime cold winter

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 forms.[1] Frost is composed of delicate branched patterns of ice crystals formed as the result of fractal 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 it forming.


hoar frost rime cold winter

A willow covered with ice crystals due to fog and frost, which causes hoar frost

A frosted willow on a very cold and bright winter day against a  blue sky in a snow-covered landscape

A frosted willow on a very cold and bright winter day against a
blue sky in a snow-covered landscape


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 frost point in still air.[2] 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.

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.[1]

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

hoar frost rime cold winter

A cold winter day, everything covered with rime


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.

hoar frost rime cold winter

Hoar frost

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[3] 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.[4] 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 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. 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.[5]

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.

hoar frost rime cold winter

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.

hoar frost rime cold winter

A cold winter day, verything covered with rime

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 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. The glass surface influences the shape of crystals, so imperfections, scratches, or dust can modify the way ice nucleates. The patterns in window frost are formed by fractalprocess development and have 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.[6]

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.

hoar frost rime cold winter

A cold winter day, verything covered with rime

White frost

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.

hoar frost rime bike biker person cold winter

A biker cycles on a snow-covered road under a frosted hedge of trees on a cold morning.


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.

hoar frost rime cold winter

A lane of hoar frost covered trees and snow

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 latent heat of freezing of the water reduces the temperature drop.

hoar frost rime cold winter

A cold winter day, verything covered with rime

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).[7]

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:[8]

  • root vegetables (e.g. beets, carrots, parsnips, onions)
  • leafy greens (e.g. lettuces, spinach, chard, cucumber[9])
  • 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).[7] Hardy perennials, such as Hosta, 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. Frost crack 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 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).[10] Bacteria lacking ice nucleation-active proteins (ice-minus bacteria) result in greatly reduced frost damage.[11]


hoar frost rime cold winter

A cold winter day, verything covered with rime