Rime (frost)
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, and any other solid object.
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 surface, hoar frost is formed by direct deposition from water vapour to solid ice.
Scientists at meterologically-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.
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 though a cloud made of supercooled water liquid droplets.
Rime ice is formed also inside of
freezers, and on objects taken out of freezers on humid days, hoar frost is formed on their surfaces in a while.
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Rime frost on both ends of a "capped column" snowflake. |
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 know 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.
["[ftp://198.77.171.17/pub/High%20resolution%20TIFF%20Snow%20Images%20from%20webpage/RimeGraupel/RG.HTM Rime and Graupel]". Electron Microscopy Unit, Beltsville Agricultural Research Center, U.S. Department of Agriculture. Public domain. URL accessed 2006-07-23.]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 mm 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.
A natural scenery in
Jilin City of
China, called
Jilin ice-rimed trees, is one of the
Four Natural Wonders in China, along with
Guilin scenery,
Yunnan stone forest, and
Three Gorges on the Yangtze River.
During the coldest period in January or February in Jilin City, the weeping
willow trees along the long bank of
Songhua river get ice-rimed in the early morning, as if they're wearing beautiful snow clothes. It is a natural result when water vapor rises up from the warm Songhua River to meet the cold âˆ'20 °C night air, causing the crystalisation of water vapour on the willows' branches. Unfortunately, it has been more difficult to see this marvellous spectacle in recent years.
