Cause: Water vapor freezing without first forming a liquid.
Frost, like fog, tends to occur on clear nights when the absence of cloud allows heat to rapidly radiate from the ground, resulting in a significant drop in temperature. For frost to form, the temperature must fall to below freezing (i.e. below 32° F or 0° C).
True frost, known as hoar frost, occurs when a thin layer of moist air near the ground cools to below freezing and immediately forms ice crystals, without first condensing as liquid (dew). These crystals will coat any cold surface including stone, grass, leaves, berries, and even spiders’ webs. Sometimes, hoar frost is so thick and white that it is mistaken for snow.
The ice crystals that result from hoar frost have exquisite, jewel-like patterns that branch outward from the edges of leaves and grass stems. These intricate structures are easy to see when hoar frost forms on window panes. This normally happens on the windows of an unheated house, when the exterior temperature falls to below freezing. Because moisture levels inside the house are higher than those outside, hoar frost crystals readily form on the inside of the cold window pane, coating the glass with delightful columns, plates, and feathers of frost.
If condensation takes place and dew forms before the air temperature falls below 32° F (or 0° C), the water or dew simply freezes, forming solid droplets rather than delicate ice crystals. These droplets are a form of ice rather than frost, and they occur in the same way as the ice on puddles, ponds, and lakes.
When temperatures fall below freezing, the water within the leaves and stems of plants will freeze. This can cause cell damage in the plants and produce a blackening of the leaves. Although this phenomenon is known in some parts of the world as black frost, it is not always accompanied by a frost. Air with a low dew point may cool to below 32° F (0° C) without reaching saturation point, which means that no water vapor is released by the air and no real frost formation can occur.
Exam Question Tips:
Hoar frost forms on an aircraft as a result of water vapour turning directly into ice crystals on the aircraft surface.
After a prolonged VMC descent in very cold air, you penetrate a humid air mass. Type of icing you will encounter is “Hoar Frost”.
Intensity and type of airframe icing most likely to occur when aircraft descends rapidly from FL 320 (temp -45oC) to FL 60 in warm, moist clear air is “Light or Moderate Hoar Frost”.
Airframe icing can occur in clear air.
What are the conditions required for the formation of hoar frost?
(a) The temperature of the surface is lower than the dew point of the air and the dew point is lower than 0 deg C.
Correct: As the surface is below the dew point air in contact with the surface will be saturated. Since the dew point is below 0 deg C the water vapour will skip the liquid stage and form ice crystals on the cold surface which is called hoar frost – the sublimation of water vapour to form ice crystals on the surface.
(b) Falling droplets on a surface while the air temperature is below freezing.
Wrong: The droplets will be supercooled (below 0 deg C). We don’t know what the surface temperature is but if it is above 0 deg C the water droplets will become dew and if it is below 0 deg C the water droplets will form clear ice or rime ice.
(c) The temperature of the surface is higher than the dew point of the air and the dew point is lower than 0 deg C.
Wrong: If the temperature of the surface is higher than the dew point the air wiil remain unsaturated and the water vapour will remain as water vapour.
(d) Falling droplets on a surface with a temperature below freezing.
Wrong: Liquid water doplets hitting a surface below 0 deg C will change to ice and, depending on the exact conditions, this will be clear ice or, more likely, rime ice.
Hoar frost is most likely to form when taking off from an airfield with a significant ground inversion.
Hoar frost will form in clear air when an aircraft has been parked overnight in sub-zero temperatures and will form when an aircraft flies from sub-zero temperature air into warm moist air, such as in descent or climbing through a temperature inversion layer. (http://www.skybrary.aero/index.php/Hoar_Frost)
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. (http://en.wikipedia.org/wiki/Frost)