Frost

The process is similar to the formation of dew, except it occurs below the freezing point of water typically without crossing through a liquid state.

For example, water vapor adsorbs to glass very well, so automobile windows will often frost before the paint, and large hoar-frost crystals can grow very rapidly when the air is very cold, calm, and heavily saturated, such as during an ice fog.

The depth of frost crystals varies depending on the amount of time they have been accumulating, and the concentration of the water vapor (humidity).

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.

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 size of the crystals depends largely on the temperature, the amount of water vapor available, and how long they have been growing undisturbed.

For instance, frost may be observed around cracks in cold wooden sidewalks when humid air escapes from the warmer ground beneath.

The apparently erratic occurrence of frost in adjacent localities is due partly to differences of elevation, the lower areas becoming colder on calm nights.

[3] They form on cold, clear nights when conditions are such that heat radiates into outer space faster than it can be replaced from nearby warm objects or brought in by the wind.

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.

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.

If the pane is a bad insulator (for example, if it is a single-pane window), water vapour condenses on the glass, forming frost patterns.

The glass surface influences the shape of crystals, so imperfections, scratches, or dust can modify the way ice nucleates.

[7] If the indoor air is very humid, rather than moderately so, water first condenses in small droplets, and then freezes 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 that forms directly from water vapour contained in air.

[9][10] 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.

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).

Certain bacteria, notably Pseudomonas syringae, are particularly effective at triggering frost formation, raising the nucleation temperature to about −2 °C (28 °F).

An effective, low cost method for small crop farms and plant nurseries, exploits the latent heat of freezing.

A pulsed irrigation timer[22] delivers water through existing overhead sprinklers at a low volumes to combat frosts down to −5 °C (23 °F).

Although not frost in the atmospheric sense, it consists of dirt, soil, sand, rocks, clay, or organic matter (peat) bound firmly together by ice crystals, making the material very hard and difficult to penetrate.