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Rime ice forms whensupercooled water droplets freeze onto surfaces.[1] In the atmosphere, there are three basic types of rime ice:
Soft and hard rime are less dense than clear ice and less adhesive, thus generally cause less damage.Glaze ice is similar in appearance to clear ice, however it is the result of a completely different process, occurring duringfreezing rain ordrizzle.
Rime ice also forms whenice forms on the surface of an aircraft, particularly on the leading edges and control surfaces when it flies through a cloud made of supercooled water liquid droplets.Rime ice is the least dense,milky ice is intermediately dense andclear ice is the most dense. All forms of ice can spoil lift and may have a catastrophic effect on an airborne aircraft. These hazardous effects are due to the ice's ability to disrupt airflow, increase weight, and add drag. Ice forming on propellers or engine inlets are especially dangerous as it can cause severe vibration and/or damage if ingested.[5]
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Hard rime is a white ice that forms when the water droplets infog freeze to the outer surfaces of objects. It is often seen on trees atop mountains andridges in winter, when low-hangingclouds causefreezing fog. This fog freezes to thewindward (wind-facing) side of tree branches, buildings, or any other solid objects, usually with high wind velocities and air temperatures between −2 and −10 °C (28 and 14 °F).[6]
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Hard rime formations are more difficult to remove. They have a comb-like appearance, with the streaks of material parallel to that of the direction of the wind. This is unlikesoft rime, which looks feathery or spiky, orclear ice, which looks homogeneous and transparent.
Scientists at meteorologically extreme places, such asMount Washington inNew Hampshire, often have to break huge chunks of hard rime off weather equipment in order to keepanemometers and other measuring instruments operating.
Under some specific atmospheric conditions, forming and descendingsnow crystals may encounter and pass via atmospheric supercooled cloud droplets. These droplets, which have a diameter of about 10 μm, can exist in an unfrozen state down to temperatures near −40 °C (−40 °F). Contact between the snow crystal and the supercooled droplets results in the freezing of the liquid droplets onto the surface of the crystals. This process of crystal growth is known asaccretion. Crystals that exhibit frozen droplets on their surfaces are referred to as rimed. When this process continues to the point that the shape of the original snow crystal is no longer identifiable the resulting crystal gets referred to asgraupel.[7]
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 visible-wavelengthmicroscope because of the limited resolution and depth of field in the instrument. However, observations of snow crystals with a low-temperaturescanning 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, giving rise to agraupel particle.[7]
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Soft rime is a white ice deposition that forms when the water droplets in light freezing fog or mist freeze to the outer surfaces of objects during calm or light wind. The fog usually freezes to thewindward side of solid objects, particularly those with a likeness to that of tree branches and wires.
Soft rime is similar in appearance tohoar frost; but while rime is formed by vapour firstcondensing to liquid droplets (of fog, mist or cloud) and then attaching to a surface, hoar frost is formed by directdeposition from water vapour to solid ice. A heavy coating of hoar frost, calledwhite frost, is very similar in appearance to soft rime, but the formation process is different; it happens when there is no fog, but very high levels of airrelative humidity (above 90%) andtemperatures below −8 °C (18 °F).
Soft rime formations appear as narrow white icy needles and scales. These needles are fragile and can be easily shaken off objects and removed. Factors that favour soft rime include: small drop size, the slow accretion of liquid water, a high degree of supercooling, and fast dissipation of latent heat of fusion. The opposite of these conditions favour ice with higher densities, such as the aforementionedhard rime orclear ice.
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