Under clear frosty nights in winter soft ice crystals might form on vegetation or any object that has been chilled below freezing point by radiation cooling. This deposit of ice crystals is known as hoar frost and may sometimes be so thick that it might look like snow. The interlocking ice crystals become attached to branches of trees, leafs, hedgerows and grass blades and are one of the most prominent features of a typical 'winter wonderland' day. However, the fine 'feathers', 'needles' and 'spines' might also be found on any other object that is exposed to supersaturated air below freezing temperature.
The relative humidity in supersaturated air is greater then 100% and the formation of hoar frost is similar to the formation of dew with the difference that the temperature of the object on which the hoar frost forms is well below 0°C, whereas this is not the case with dew. Hoar frost crystals often form intitially on the tips of plants or other objects.
Hoar frost might form as liquid dew that has subsequently frozen with a drop in temperature, which is then known as silver frost or white frost. Usually the dew drops do not freeze immediately, even if the air temperature is slightly below zero. Rather they become supercooled dew droplets at first. Supercooled dew will eventually freeze if the temperature falls below about -3°C to -5°C. Hoar frost deposits might also derive by sublimation, when water vapour is forming ice directly on the surfaces concerned. In most cases hoar frost will have formed by a combination of the processes above.
Hoar frost must not be confused with rime, which derives from freezing fog or glaze which forms as a continuos thick layer of ice, rather than individual frozen droplets.
