Rime (frost)
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Rime ice 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.
Rime ice is similar in appearance to hoar frost; but whereas rime ice is formed by vapour first condensing to liquid droplets (of fog, mist or cloud) and then attaching to a surface, hoar frost is formed by direct deposition from water vapour to solid ice.
Scientists at meteorologically-extreme places such as Mount Washington in New Hampshire often have to break huge chunks of rime ice off weather equipment, in order to keep anemometers and other measuring instruments operating. This type of ice can spoil lift and have catastrophic effects on airborne aircraft.
Sometimes the rime ice takes on a feathery look, and looks very much like "snow feathers".
Ice storms may consist of either glaze ice or rime ice. Meteorologists classify transparent and homogeneous ice forming on vertical and horizontal surfaces as glaze. Glaze ice resembles ice-cube ice in appearance. Its amorphous, dense structure helps it cling tenaciously to any surface on which it forms. In contrast, if the ice is milky and crystalline, like sugar, it is termed rime. Rime ice is less dense than glaze ice and clings less tenaciously, therefore damage due to rime is generally minor compared to glaze ice.
Rime ice and glaze ice are also the two types of ice that can form on the surfaces of an aircraft, if it flies through a cloud made of supercooled water liquid droplets.
Rime ice is also formed inside of freezers, and on humid days, objects taken out of freezers will form hoar frost on their surfaces.
[edit] Formation on snow crystals
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, can exist in the unfrozen state down to temperatures near -40 °C. 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.[1]
The frozen droplets on the surface of rimed crystals are hard to resolve and the topography of a graupel particle is not easy to record with a light microscope because of the limited resolution and depth of field in the instrument. However, observations of snow crystals with a low-temperature scanning electron microscope (LT-SEM) clearly show cloud droplets measuring up to 50 μm on the surface of the crystals. The rime has been observed on all four basic forms of snow crystals, including plates, dendrites, columns and needles. As the riming process continues, the mass of frozen, accumulated cloud droplets obscures the identity of the original snow crystal, thereby giving rise to a graupel particle.[1]
[edit] References
- ^ a b "Rime and Graupel". Electron Microscopy Unit, Beltsville Agricultural Research Center, U.S. Department of Agriculture. Public domain. URL accessed 2006-07-23.
