SO9095 : Frosted field and trees near Colton Hills, Staffordshire
taken 13 years ago, near to Sedgley, Dudley, England
Hoar frost forms during clear frosty nights when soft ice crystals can form on vegetation or any object that has been chilled below freezing point by radiation cooling. This deposit of ice crystals may sometimes be so thick that it looks almost like snow. The interlocking ice crystals become attached to branches of trees, leaves, hedgerows and grass blades. However, the beautiful pure white fine 'feathers', 'needles' and 'spines' might also be found on any other object that is exposed to supersaturated air with a temperature below 0degC.
The relative humidity in supersaturated air is greater than 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.
Another way in which hoar frosts might form is as liquid dew that has subsequently frozen with a drop in temperature. This 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.
The presence of freezing fog (or mist) also needs to be considered. This is composed of supercooled water droplets (i.e. ones which remain liquid even though the temperature is below freezing-point). One of the characteristics of freezing fog is that rime - composed of feathery ice crystals - is deposited on the windward side of vertical surfaces such as lamp-posts, fence-posts, overhead wires, pylons and transmitting masts. Scenes such as that in this image may well have been affected by this.
In most cases, what is often described as hoar frost will have formed by a combination of the processes above.
(Information reduced from the explanations on the Weatheronline website Link and the BBC Weather Centre Link )