Hoarfrost (Or What I Learned Today)

Hoarfrost aka Rime
From Wikipedia:

Hard rime is a white ice that forms when the water droplets in fog freeze to the outer surfaces of objects. It is often seen on trees atop mountains and ridges in winter, when low-hanging clouds cause freezing fog. This fog freezes to the windward (wind-facing) side of tree branches, buildings, or any other solid objects, usually with high wind velocities and air temperatures between −2 °C (28 °F) and −8 °C (18 °F). Hard rime formations are difficult to shake off; they have a comb-like appearance, unlike soft rime, which looks feathery or spiky, or clear ice, which looks homogeneous and transparent. Scientists at meteorologically extreme places such as Mount Washington in New Hampshire often have to break huge chunks of hard rime off weather equipment, in order to keep anemometers and other measuring instruments operating.

Rime ice can accumulate on the leading edges and control surfaces of aircraft operating in certain meteorological conditions. Meteorologists distinguish between three basic types of ice forming on vertical and horizontal surfaces by deposition of supercooled water droplets. Of course there are also intermediate formations. Soft rime is less dense than hard rime and is milky and crystalline, like sugar. Soft rime appears similar to hoar frost. Hard rime is somewhat less milky, especially if it is not heavy.
Clear ice is transparent and homogeneous and resembles ice-cube ice in appearance. Its amorphous, dense structure helps it cling tenaciously to any surface on which it forms.

Under some atmospheric conditions, forming and descending snow crystals may encounter and pass through atmospheric supercooled cloud droplets. These droplets, which have a diameter of about 10 µm (0.00039 in), can exist in the unfrozen state down to temperatures near −40 °C (−40 °F). Contact between the snow crystal and the supercooled droplets results in freezing of the liquid droplets onto the surface of the crystals. This process of crystal growth is known as accretion. Crystals that exhibit frozen droplets on their surfaces are referred to as rimed. When this process continues so that the shape of the original snow crystal is no longer identifiable, the resulting crystal is referred to as graupel.

- Posted using BlogPress from my iPad