Ice crystals are solid

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 opaq ...

exhibiting atomic ordering on various length scales and include

hexagonal In geometry, a hexagon (from Greek , , meaning "six", and , , meaning "corner, angle") is a six-sided polygon. The total of the internal angles of any simple (non-self-intersecting) hexagon is 720°. Regular hexagon A '' regular hexagon'' has ...

columns, hexagonal plates, dendritic crystals, and

diamond dust Diamond dust is a ground-level cloud composed of tiny ice crystals. This meteorological phenomenon is also referred to simply as ''ice crystals'' and is reported in the METAR code as IC. Diamond dust generally forms under otherwise clear or near ...

Formation

The hugely symmetric shapes are due to depositional

growth Growth may refer to: Biology * Auxology, the study of all aspects of human physical growth * Bacterial growth * Cell growth * Growth hormone, a peptide hormone that stimulates growth * Human development (biology) * Plant growth * Secondary growth ...

, namely, direct deposition of water vapor onto the ice crystal. Depending on environmental temperature and humidity, ice crystals can develop from the initial hexagonal prism into numerous symmetric shapes. Possible shapes for ice crystals are columns, needles, plates and dendrites. If the crystal migrates into regions with different environmental conditions, the growth pattern may change, and the final crystal may show mixed patterns. Ice crystals tend to fall with their major axis aligned along the horizontal, and are thus visible in polarimetric weather radar signatures with enhanced (positive) differential reflectivity values. Electrification of ice crystals can induce alignments different from the horizontal. Electrified ice crystals are also well detectable by polarimetric weather radars.

Temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

and

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 depe ...

determine many different crystalline forms. Ice crystals are responsible for various atmospheric optics displays.

Ice cloud An ice cloud is a colloid of ice particles dispersed in air. The term has been used to refer to clouds of both water ice and carbon dioxide ice on Mars. Such clouds can be sufficiently large and dense to cast shadows on the Martian surface. Cirru ...

s are composed of ice crystals, the most notable being

cirrus cloud Cirrus ( cloud classification symbol: Ci) is a genus of high cloud made of ice crystals. Cirrus clouds typically appear delicate and wispy with white strands. Cirrus are usually formed when warm, dry air rises, causing water vapor deposition on ...

s and

ice fog Ice fog is a type of fog consisting of fine ice crystals suspended in the air. It occurs only in cold areas of the world, as water droplets suspended in the air can remain liquid down to . It should be distinguished from diamond dust, a pre ...

. The slight whitening of a clear blue sky caused by ice crystals high in the

troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...

can be a sign that a

weather front A weather front is a boundary separating air masses for which several characteristics differ, such as air density, wind, temperature, and humidity. Disturbed and unstable weather due to these differences often arises along the boundary. For in ...

(and

rain Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water f ...

) is approaching, as moist air is carried to high levels and freezes to ice crystals.

Geometry

At ambient temperature and pressure, water molecules have a V shape. The two hydrogen atoms bond to the oxygen atom at a 105° angle. Common ice crystals are symmetrical and have a hexagonal pattern.

Square crystals

Square ice crystals form at room temperature when squeezed between two layers of

graphene Graphene () is an allotrope of carbon consisting of a single layer of atoms arranged in a hexagonal lattice nanostructure.

. The material is a new crystalline phase of ice, joining 17 others. The research derived from the earlier discovery that

water vapor (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, water vapour or aqueous vapor is the gaseous pha ...

and liquid water could pass through laminated sheets of

graphene oxide An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the ...

, unlike smaller molecules such as

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...

. The effect is thought to be driven by the

van der Waals force In molecular physics, the van der Waals force is a distance-dependent interaction between atoms or molecules. Unlike ionic or covalent bonds, these attractions do not result from a chemical electronic bond; they are comparatively weak and th ...

, which may involve more than 10,000 atmospheres of pressure.

References

External links

SnowCrystals.com!
at

Caltech The California Institute of Technology (branded as Caltech or CIT)The university itself only spells its short form as "Caltech"; the institution considers other spellings such a"Cal Tech" and "CalTech" incorrect. The institute is also occasional ...

American Meteorological Society Glossary
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Crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

Clouds, fog and precipitation Atmospheric optical phenomena

Formation

Geometry

Square crystals

See also

References

External links