Hoarfrost

Hoar frost, originally uploaded by tina_manthorpe.

This is a fantastic image of hoarfrost (yes, it's one word) on a tree in Worcestershire. As explained below, hoarfrost is similar to, but not the same as, the more common rime.

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 hoarfrost 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.

Hoarfrost 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. Hoarfrost deposits might also derive by sublimation, when water vapour is forming ice directly on the surfaces concerned. In most cases hoarfrost will have formed by a combination of the processes above. (Weather Online)

rime—A white or milky and opaque granular deposit of ice formed by the rapid freezing of supercooled water drops as they impinge upon an exposed object. It is denser and harder than hoarfrost, but lighter, softer, and less transparent than glaze. Rime is composed essentially of discrete ice granules and has densities as low as 0.2–0.3 g cm−3. (American Meteorological Society glossary)