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Rime ice forms when
supercooled water Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid or a gas below its melting point without it becoming a solid. It achieves this in the absence of a seed crystal or nucleus around which a crystal ...
liquid droplets freeze onto surfaces.
Meteorologist A meteorologist is a scientist who studies and works in the field of meteorology aiming to understand or predict Earth's atmospheric phenomena including the weather. Those who study meteorological phenomena are meteorologists in research, while t ...
s distinguish between three basic types of ice forming on vertical and horizontal surfaces by deposition of supercooled water droplets. 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 Frost is a thin layer of ice on a solid surface, which forms from water vapor in an above-freezing atmosphere coming in contact with a solid surface whose temperature is below freezing, and resulting in a phase change from water vapor (a ga ...
. *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
citation needed " itation needed''" is a tag added by Wikipedia editors to unsourced statements in articles requesting citations to be added. The phrase is reflective of the policies of verifiability and no original research on Wikipedia and has become a gen ...
]
, dense structure helps it cling tenaciously to any surface on which it forms. Both rime types are less dense than clear ice and cling less tenaciously, therefore damage due to rime is generally minor compared to clear ice. Glaze ice is similar in appearance to clear ice but it is the result of a completely different process, occurring during
freezing rain Freezing rain is rain maintained at temperatures below freezing by the ambient air mass that causes freezing on contact with surfaces. Unlike a mixture of rain and snow or ice pellets, freezing rain is made entirely of liquid droplets. The rain ...
or
drizzle Drizzle is a light precipitation consisting of liquid water drops smaller than those of rain – generally smaller than in diameter. Drizzle is normally produced by low stratiform clouds and stratocumulus clouds. Precipitation rates from dri ...
. These three types occur also when ice 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 intermediate and clear ice is the most dense. All forms of ice can spoil lift and may have a catastrophic effect on an airborne aircraft. Ice is hazardous to flight as it disrupts airflow, increases weight, and adds drag. Ice forming on propellers and/or engine inlets can cause severe vibration and/or damage if ingested. Rime is also a weather form.


Hard rime

Hard rime is a white
ice Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaqu ...
that forms when the
water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
droplet A drop or droplet is a small column of liquid, bounded completely or almost completely by free surfaces. A drop may form when liquid accumulates at the lower end of a tube or other surface boundary, producing a hanging drop called a pendant ...
s in fog freeze to the outer surfaces of objects. It is often seen on
tree In botany, a tree is a perennial plant with an elongated stem, or trunk, usually supporting branches and leaves. In some usages, the definition of a tree may be narrower, including only woody plants with secondary growth, plants that are ...
s atop
mountain A mountain is an elevated portion of the Earth's crust, generally with steep sides that show significant exposed bedrock. Although definitions vary, a mountain may differ from a plateau in having a limited summit area, and is usually highe ...
s and ridges in winter, when low-hanging
cloud In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may ...
s cause freezing fog. This fog freezes to the
windward Windward () and leeward () are terms used to describe the direction of the wind. Windward is ''upwind'' from the point of reference, i.e. towards the direction from which the wind is coming; leeward is ''downwind'' from the point of reference ...
(
wind Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few ho ...
-facing) side of tree branches, buildings, or any other solid objects, usually with high wind velocities and air temperatures between .


Characteristics

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 Clear ice refers to a solid precipitation which forms when air temperature is between and and there are supercooled, relatively large drops of water (from freezing fog). A rapid accretion and a slow dissipation of latent heat of fusion favor the ...
, which looks homogeneous and transparent. Scientists at meteorologically extreme places such as Mount Washington in
New Hampshire New Hampshire is a state in the New England region of the northeastern United States. It is bordered by Massachusetts to the south, Vermont to the west, Maine and the Gulf of Maine to the east, and the Canadian province of Quebec to the nor ...
often have to break huge chunks of hard rime off weather equipment, in order to keep
anemometer In meteorology, an anemometer () is a device that measures wind speed and direction. It is a common instrument used in weather stations. The earliest known description of an anemometer was by Italian architect and author Leon Battista Alberti ...
s and other measuring instruments operating.


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 . 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 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. 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-wavelength
microscope A microscope () is a laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic means being invisi ...
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.


Soft rime

Soft rime is a white
ice Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaqu ...
deposition that forms when the
water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
droplet A drop or droplet is a small column of liquid, bounded completely or almost completely by free surfaces. A drop may form when liquid accumulates at the lower end of a tube or other surface boundary, producing a hanging drop called a pendant ...
s in light freezing fog or mist freeze to the outer surfaces of objects, with calm or light wind. The fog freezes usually to the
windward Windward () and leeward () are terms used to describe the direction of the wind. Windward is ''upwind'' from the point of reference, i.e. towards the direction from which the wind is coming; leeward is ''downwind'' from the point of reference ...
side of tree branches, wires, or any other solid objects. Soft rime is similar in appearance to
hoar frost Frost is a thin layer of ice on a solid surface, which forms from water vapor in an above-freezing atmosphere coming in contact with a solid surface whose temperature is below freezing, and resulting in a phase change from water vapor (a ga ...
; but while rime is formed by vapor 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 vapor to solid ice. A heavy coating of hoar frost, called
white frost Frost is a thin layer of ice on a solid surface, which forms from water vapor in an above-freezing atmosphere coming in contact with a solid surface whose temperature is below freezing, and resulting in a phase change from water vapor (a ga ...
, 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 air
relative humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity dep ...
(above 90%) and
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...
s below . Soft rime formations have the appearance of white ice needles and scales; they are fragile and can be easily shaken off objects. Factors that favor soft rime are small drop size, the slow accretion of liquid water, high degree of supercooling, and fast dissipation of latent heat of fusion. The opposite conditions favor ice with higher densities, such as
hard rime Rime ice forms when supercooled water liquid droplets freeze onto surfaces. Meteorologists distinguish between three basic types of ice forming on vertical and horizontal surfaces by deposition of supercooled water droplets. There are also interm ...
or
clear ice Clear ice refers to a solid precipitation which forms when air temperature is between and and there are supercooled, relatively large drops of water (from freezing fog). A rapid accretion and a slow dissipation of latent heat of fusion favor the ...
.


See also

*
Atmospheric icing Atmospheric icing occurs in the atmosphere when water droplets suspended in air freeze on objects they come in contact with. It is not the same as freezing rain, which is caused directly by precipitation. Icing conditions can be particularly dang ...


References


External links


Search Hard Rime in Glossary, ''American Meteorological Society''Soft Rime in Glossary, ''American Meteorological Society''


{{Authority control Water ice Frost and rime