Rime ice forms when supercooled water droplets freeze onto surfaces. In the atmosphere, there are three basic types of rime ice: *Soft rime forms when supercooled water freezes under calm wind conditions. It is milky and crystalline, like sugar, and similar to

hoar frost Frost is a thin layer of ice on a solid surface, which forms from water vapor that deposits onto a freezing surface. Frost forms when the air contains more water vapor than it can normally hold at a specific temperature. The process is similar ...

. *Hard rime forms by rapid freezing of supercooled water under at least moderate wind conditions. The droplets freeze more or less individually, leaving air gaps. *Clear ice forms by slow freezing of supercooled water. Clear ice is typically transparent and homogeneous. Its amorphous and dense structure makes it adhesive. Soft and hard rime are less dense than clear ice and less adhesive, thus generally cause less damage.

Glaze ice Glaze or glaze ice, also called glazed frost or verglas, is a smooth, transparent and homogeneous ice coating occurring when freezing rain or drizzle hits a surface. It is similar in appearance to clear ice, which forms from supercooled water dro ...

is similar in appearance to clear ice, however it is the result of a completely different process, occurring during

freezing rain Freezing rain is rain maintained at temperatures below melting point, freezing by the ambient air mass that causes freezing on contact with surfaces. Unlike rain and snow mixed, a mixture of rain and snow or ice pellets, freezing rain is made en ...

drizzle Drizzle is a light precipitation which consists of liquid water drops that are smaller than those of rain – generally smaller than in diameter. Drizzle is normally produced by low stratiform clouds and stratocumulus clouds. Precipitation r ...

. Rime ice also forms 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 intermediately dense and clear 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.

Hard rime

Hard rime is a white ice that forms when the water droplets in

fog Fog is a visible aerosol consisting of tiny water droplets or ice crystals suspended in the air at or near the Earth's surface. Reprint from Fog can be considered a type of low-lying cloud usually resembling stratus and is heavily influenc ...

freeze to the outer surfaces of objects. It is often seen on trees atop mountains and

ridge A ridge is a long, narrow, elevated geomorphologic landform, structural feature, or a combination of both separated from the surrounding terrain by steep sides. The sides of a ridge slope away from a narrow top, the crest or ridgecrest, wi ...

s 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 Fog is a visible aerosol consisting of tiny water droplets or ice crystals suspended in the air at or near the Earth's surface. Reprint from Fog can be considered a type of low-lying cloud usually resembling stratus and is heavily influenc ...

. This fog freezes to the

windward In geography and seamanship, windward () and leeward () are directions relative to 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 ...

(wind-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 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 unlike soft rime, which looks feathery or spiky, or

clear ice Clear may refer to: * Transparency, the physical property of allowing light to pass through Arts and entertainment Music Groups * Clear (Christian band), an American CCM group from Cambridge, Minnesota * Clear (hardcore band), a vegan straight ...

, which looks homogeneous and transparent. Scientists at meteorologically extreme places, such as

Mount Washington Mount Washington is an ultra-prominent mountain in the state of New Hampshire. It is the highest peak in the Northeastern United States at and the most topographically prominent mountain east of the Mississippi River. The mountain is notorio ...

New Hampshire New Hampshire ( ) is a U.S. state, state in the New England region of the Northeastern United States. It borders Massachusetts to the south, Vermont to the west, Maine and the Gulf of Maine to the east, and the Canadian province of Quebec t ...

, 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 specific atmospheric conditions, forming and descending snow 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 as accretion. 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 as

graupel Graupel (; ), also called soft hail or hominy snow or granular snow or snow pellets, is precipitation that forms when supercooled water droplets in air are collected and freeze on falling snowflakes, forming balls of crisp, opaque rime. Gra ...

. 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 equipment, 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 ...

because of the limited resolution and depth of field in the instrument. However, observations of snow crystals with a low-temperature

scanning electron microscope A scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning the surface with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that ...

(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 a

particle.

Soft rime

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 the

side of solid objects, particularly those with a likeness to that of tree branches and wires. Soft rime is similar in appearance to

; but while rime 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. 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 that deposits onto a freezing surface. Frost forms when the air contains more water vapor than it can normally hold at a specific temperature. The process is similar ...

, 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 (meteorology), precipitation, dew, or fog t ...

(above 90%) and

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

s below . 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 aforementioned

hard rime Rime ice forms when supercooled water droplets freeze onto surfaces. In the atmosphere, there are three basic types of rime ice: *Soft rime forms when supercooled water freezes under calm wind conditions. It is milky and crystalline, like suga ...

References

External links