Ice

A natural block of (water) ice
Snowflakes (ice crystals) by Wilson Bentley, 1902

Ice, technically, may be any one of the 15 known crystalline phases of water. In non-scientific contexts, the term usually means the phase known as ice Ih, which is the most abundant of these solid phases on the Earth's surface. It can appear transparent or opaque bluish-white colour, depending on the presence of impurities or air inclusions. The addition of other materials such as soil may further alter the appearance.

The most common phase transition to ice Ih occurs when liquid water is cooled below 0°C (273.15K, 32°F) at standard atmospheric pressure. It can also deposit from vapour with no intervening liquid phase, such as in the formation of frost.

Ice appears in nature in forms of precipitation snowflakes, hail, icicles, glaciers, pack ice, and entire polar ice caps. It is an important component of the global climate, and plays an important role in the water cycle. Furthermore, ice has numerous cultural applications, from ice cooling of drinks to winter sports and the art of ice sculpting.

The word is derived from Old English īs, which in turn stems from Proto-Germanic *isaz.

Contents

Characteristics

Crystal structure of hexagonal ice. Grey dashed lines indicate hydrogen bonds.

As a naturally occurring crystalline inorganic solid with an ordered structure, ice is considered a mineral.[1] It possesses a regular crystalline structure based on the molecule of water, which consists of a single oxygen atom covalently bonded to two hydrogen atoms, or H-O-H. However, many of the physical properties of water and ice are controlled by the formation of hydrogen bonds between adjacent oxygen and hydrogen atoms. It is a weak bond, but is critical in controlling the structure of both water and ice.

An unusual property of ice frozen at atmospheric pressure is that the solid is approximately 9% less dense than liquid water. Ice is the only known non-metallic substance to expand when it freezes. The density of ice is 0.9167 g/cm³ at 0°C, whereas water has a density of 0.9998 g/cm³ at the same temperature. Liquid water is densest, essentially 1.00 g/cm³, at 4°C and becomes less dense as the water molecules begin to form the hexagonal crystals[2] of ice as the freezing point is reached. This is due to hydrogen bonding dominating the intermolecular forces, which results in a packing of molecules less compact in the solid. Density of ice increases slightly with decreasing temperature and has a value of 0.9340 g/cm³ at −180 °C (93 K).[3]

The effect of expansion during freezing can be dramatic, and is a basic cause of freeze-thaw weathering of rock in nature. It is also a common cause of the flooding of houses when water pipes burst due to the pressure of expanding water when it freezes, then leak water after thawing.

The result of these properties is that ice floats on liquid water, which is an important factor in Earth's climate. It has been argued that natural bodies of water would freeze from the bottom up without this property,[4] resulting in the annual loss of wild life and vegetation.

When ice melts, it absorbs as much energy as it would take to heat an equivalent mass of water by 80 °C. During the melting process, the temperature remains constant at 0 °C. While melting, any energy added breaks the hydrogen bonds between ice (water) molecules. Energy becomes available to increase the thermal energy (temperature) only after enough hydrogen bonds are broken that the ice can be considered liquid water. The amount of energy consumed in breaking hydrogen bonds in the transition from ice to water is known as the heat of fusion.

It has been theoretically shown that ice may be superheated beyond its equilibrium melting point. Simulations of ultrafast laser pulses acting on ice suggest it may be heated to room temperature for an extremely short period (250 ps) without melting.[5]

Light reflecting from ice often appears blue due to the properties of Rayleigh scattering, the same reason the sky is blue. Blue light is scattered much more strongly than red in matter which does not absorb visible light, such as ice. Other colours can appear in the presence of light absorbing impurities, where the impurity is dictating the colour rather than the ice itself. For instance, icebergs containing impurities (e.g., sediments, algae, air bubbles) can appear brown, grey or green.[6]

Slipperiness

Frozen waterfall in southeast New York

Until recently, it was widely believed that ice was slippery because the pressure of an object in contact with it caused a thin layer to melt. For example, the blade of an ice skate, exerting pressure on the ice, melted a thin layer, providing lubrication between the ice and the blade.

This explanation is no longer accepted. There is still debate about why ice is slippery. The explanation gaining acceptance is that ice molecules in contact with air cannot properly bond with the molecules of the mass of ice beneath (and thus are free to move like molecules of liquid water). These molecules remain in a semiliquid state, providing lubrication regardless of pressure against the ice exerted by any object.[7]

Formation

Feather ice on the plateau near Alta, Norway. The crystals form at temperatures below −30 °C (i.e. −22 °F).

Ice that is found at sea may be in the form of sea ice, pack ice, or icebergs. The term that collectively describes all of the parts of the Earth's surface where water is in frozen form is the cryosphere. Ice is an important component of the global climate, particularly in regard to the water cycle. Glaciers and snowpacks are an important storage mechanism for fresh water; over time, they may sublimate or melt. Snowmelt is often an important source of seasonal fresh water.

Rime is a type of ice formed on cold objects when drops of water crystallize on them. This can be observed in foggy weather, when the temperature drops during the night. Soft rime contains a high proportion of trapped air, making it appear white rather than transparent, and giving it a density about one quarter of that of pure ice. Hard rime is comparatively denser.

Aufeis is layered ice that forms in Arctic and subarctic stream valleys. Ice, frozen in the stream bed, blocks normal groundwater discharge, and causes the local water table to rise, resulting in water discharge on top of the frozen layer. This water then freezes, causing the water table to rise further and repeat the cycle. The result is a stratified ice deposit, often several meters thick.

Ice can also form icicles, similar to stalactites in appearance, or stalagmite-like forms as water drips and re-freezes.

Clathrate hydrates are forms of ice that contain gas molecules trapped within its crystal lattice.

Pancake ice is a formation of ice generally created in areas with less calm conditions.

Candle Ice is a form of Rotten Ice that develops in columns perpendicular to the surface of a lake.

Ice discs are circular formations of ice surrounded by water in a river.

Ice pellets

An accumulation of ice pellets

Ice pellets are a form of precipitation consisting of small, translucent balls of ice. This form of precipitation is also known as sleet.[8] Ice pellets are usually (but not always) smaller than hailstones.[9] They often bounce when they hit the ground, and generally do not freeze into a solid mass unless mixed with freezing rain. The METAR code for ice pellets is PL.[10]

Ice pellets form when a layer of above-freezing air is located between 1,500 metres (4,900 ft) and 3,000 metres (9,800 ft) above the ground, with sub-freezing air both above and below it. This causes the partial or complete melting of any snowflakes falling through the warm layer. As they fall back into the sub-freezing layer closer to the surface, they re-freeze into ice pellets. However, if the sub-freezing layer beneath the warm layer is too small, the precipitation will not have time to re-freeze, and freezing rain will be the result at the surface. A temperature profile showing a warm layer above the ground is most likely to be found in advance of a warm front during the cold season,[11] but can occasionally be found behind a passing cold front.

Hail

See also: Hail
A large hailstone, about 6 cm (2.36 in) in diameter

Like other precipitation, hail forms in storm clouds when supercooled water droplets freeze on contact with condensation nuclei, such as dust or dirt. The storm's updraft blows the hailstones to the upper part of the cloud. The updraft dissipates and the hailstones fall down, back into the updraft, and are lifted up again. Hail has a diameter of 5 millimetres (0.20 in) or more.[12] Within METAR code, GR is used to indicate larger hail, of a diameter of at least 6.4 millimetres (0.25 in). GR is derived from the French word grêle. Smaller-sized hail, as well as snow pellets, use the coding of GS, which is short for the French word grésil.[10] Stones just larger than golf ball-sized are one of the most frequently reported hail sizes.[13] Hailstones can grow to 15 centimetres (6 in) and weigh more than .5 kilograms (1.1 lb).[14] In large hailstones, latent heat released by further freezing may melt the outer shell of the hailstone. The hailstone then may undergo 'wet growth', where the liquid outer shell collects other smaller hailstones.[15] The hailstone gains an ice layer and grows increasingly larger with each ascent. Once a hailstone becomes too heavy to be supported by the storm's updraft, it falls from the cloud.[16]

Hail forms in strong thunderstorm clouds, particularly those with intense updrafts, high liquid water content, great vertical extent, large water droplets, and where a good portion of the cloud layer is below freezing 0 °C (32 °F).[12] Hail-producing clouds are often identifiable by their green coloration.[17][18] The growth rate is maximized at about −13 °C (9 °F), and becomes vanishingly small much below −30 °C (−22 °F) as supercooled water droplets become rare. For this reason, hail is most common within continental interiors of the mid-latitudes, as hail formation is considerably more likely when the freezing level is below the altitude of 11,000 feet (3,400 m).[19] Entrainment of dry air into strong thunderstorms over continents can increase the frequency of hail by promoting evaporational cooling which lowers the freezing level of thunderstorm clouds giving hail a larger volume to grow in. Accordingly, hail is actually less common in the tropics despite a much higher frequency of thunderstorms than in the mid-latitudes because the atmosphere over the tropics tends to be warmer over a much greater depth. Hail in the tropics occurs mainly at higher elevations.[20]

Snowflakes

Snowflake viewed in an optical microscope

Snow crystals form when tiny supercooled cloud droplets (about 10 μm in diameter) freeze. These droplets are able to remain liquid at temperatures lower than −18 °C (0 °F), because to freeze, a few molecules in the droplet need to get together by chance to form an arrangement similar to that in an ice lattice; then the droplet freezes around this "nucleus." Experiments show that this "homogeneous" nucleation of cloud droplets only occurs at temperatures lower than −35 °C (−31 °F).[21] In warmer clouds an aerosol particle or "ice nucleus" must be present in (or in contact with) the droplet to act as a nucleus. Our understanding of what particles make efficient ice nuclei is poor — what we do know is they are very rare compared to that cloud condensation nuclei on which liquid droplets form. Clays, desert dust and biological particles may be effective,[22] although to what extent is unclear. Artificial nuclei include particles of silver iodide and dry ice, and these are used to stimulate precipitation in cloud seeding.[23]

Once a droplet has frozen, it grows in the water-supersaturated air, when the temperature remains below the freezing point. The droplet then grows by condensation of water vapor onto the ice surfaces. Air saturation with water is maintained by continuous simultaneous evaporation of water droplets. Thus ice crystals grow at the expense of water droplets in a process called the Wegner-Bergeron-Findeison process. These large crystals are an efficient source of precipitation, since they fall through the atmosphere due to their weight, and may collide and aggregate in clusters. These aggregates are snowflakes, and are usually the type of ice particle that falls to the ground.[24] Guinness World Records list the world’s largest snowflakes as those of January 1887 at Fort Keogh, Montana; allegedly one measured 38 cm (15 inches) wide.[25]

The exact details of the sticking mechanism remain controversial. Possibilities include mechanical interlocking, sintering, electrostatic attraction as well as the existence of a "sticky" liquid-like layer on the crystal surface. The individual ice crystals often have hexagonal symmetry. Although the ice is clear, scattering of light by the crystal facets and hollows/imperfections mean that the crystals often appear white in colour due to diffuse reflection of the whole spectrum of light by the small ice particles.[26] The shape of the snowflake is determined broadly by the temperature and humidity at which it is formed.[24] Rarely, at a temperature of around −2 °C (28 °F), snowflakes can form in threefold symmetry — triangular snowflakes.[27] The most common snow particles are visibly irregular, although near-perfect snowflakes may be more common in pictures because they are more visually appealing. It is unlikely that any two snowflakes are alike due to the estimated 10,000,000,000,000,000,000 water molecules which make up a typical snowflake,[28] which grow at different rates and in different patterns depending on the changing temperature and humidity within the atmosphere that the snowflake falls through on its way to the ground.[29] The METAR code for snow is SN, while snow showers are coded SHSN.[10]

Diamond dust

Diamond dust, also known as ice needles or ice crystals, forms at temperatures approaching −40 °F (−40 °C) due to air with slightly higher moisture from aloft mixing with colder, surface based air.[30] The METAR identifier for diamond dust within international hourly weather reports is IC.[10]

Production

Harvesting ice on Lake Saint Clair in Michigan, circa 1905
Ice being transported by cart in Mumbai, India

Ice is now mechanically produced on an large scale, but before appropriate coolants were developed ice was harvested from natural sources for human use.

Ice harvesting

Ice has long been valued as a means of cooling. Until recently, the Hungarian Parliament building used ice harvested in the winter from Lake Balaton for air conditioning. Icehouses were used to store ice formed in the winter, to make ice available all year long, and early refrigerators were known as iceboxes, because they had a block of ice in them. In many cities, it was not unusual to have a regular ice delivery service during the summer. For the first half of the 19th century, ice harvesting had become big business in America. Frederic Tudor, who became known as the “Ice King,” worked on developing better insulation products for the long distance shipment of ice, especially to the tropics. The advent of artificial refrigeration technology has since made delivery of ice obsolete.

In 400 BC Iran, Persian engineers had already mastered the technique of storing ice in the middle of summer in the desert. The ice was brought in during the winters from nearby mountains in bulk amounts, and stored in specially designed, naturally cooled refrigerators, called yakhchal (meaning ice storage). This was a large underground space (up to 5000 m³) that had thick walls (at least two meters at the base) made out of a special mortar called sārooj, composed of sand, clay, egg whites, lime, goat hair, and ash in specific proportions, and which was known to be resistant to heat transfer. This mixture was thought to be completely water impenetrable. The space often had access to a Qanat, and often contained a system of windcatchers which could easily bring temperatures inside the space down to frigid levels on summer days. The ice was then used to chill treats for royalty on such occasions.

Commercial production

Ice is now produced on an industrial scale, for uses including food storage and processing, chemical manufacturing, concrete mixing and curing, and consumer or packaged ice.[31] Most commercial ice makers produce three basic types of fragmentary ice: flake, tubular and plate, using a variety of techniques.[31] Large batch ice makers can produce up to 75 tons of ice per day.[32]

Ice production is a large business; in 2002, there were 426 commercial ice-making companies in the United States, with a combined value of shipments of $595,487,000.[33]

For small-scale ice production, many modern home refrigerators can also make ice with a built in icemaker, which will typically make ice cubes or crushed ice. Stand-alone icemaker units that make ice cubes are often called ice machines.

Uses

Sports

Ice sailing on the Żnin Small Lake

Ice also plays a central role in winter recreation and in many sports such as ice skating, tour skating, ice hockey, ice fishing, ice climbing, curling, broomball and sled racing on bobsled, luge and skeleton. Many of the different sports played on ice get international attention every four years during the Winter Olympic Games.

A sort of sailboat on blades gives rise to ice yachting. The human quest for excitement has even led to ice racing, where drivers must speed on lake ice, while also controlling the skid of their vehicle (similar in some ways to dirt track racing). The sport has even been modified for ice rinks.

Other uses

Ice pier during 1983 cargo operations. McMurdo Station, Antarctica

Ice and transportation

U.S. Coast Guard icebreakers near McMurdo Station, February 2002

Ice can also be an obstacle; for harbours near the poles, being ice-free is an important advantage; ideally, all year long. Examples are Murmansk (Russia), Petsamo (Russia, formerly Finland) and Vardø (Norway). Harbours which aren't ice-free are opened up using icebreakers.

Ice forming on roads is a dangerous winter hazard. Black ice is very difficult to see, because it lacks the expected frosty surface. Whenever there is freezing rain or snow which occurs at a temperature near the melting point, it is common for ice to build up on the windows of vehicles. Driving safely requires the removal of the ice build-up. Ice scrapers are tools designed to break the ice free and clear the windows, though removing the ice can be a long and laborious process.

Far enough below the freezing point, a thin layer of ice crystals can form on the inside surface of windows. This usually happens when a vehicle has been left alone after being driven for a while, but can happen while driving, if the outside temperature is low enough. Moisture from the driver's breath is the source of water for the crystals. It is troublesome to remove this form of ice, so people often open their windows slightly when the vehicle is parked in order to let the moisture dissipate, and it is now common for cars to have rear-window defrosters to solve the problem. A similar problem can happen in homes, which is one reason why many colder regions require double-pane windows for insulation.

When the outdoor temperature stays below freezing for extended periods, very thick layers of ice can form on lakes and other bodies of water, although places with flowing water require much colder temperatures. The ice can become thick enough to drive onto with automobiles and trucks. Doing this safely requires a thickness of at least 30 centimetres (one foot).

For ships, ice presents two distinct hazards. Spray and freezing rain can produce an ice build-up on the superstructure of a vessel sufficient to make it unstable, and to require it to be hacked off or melted with steam hoses. And icebergs — large masses of ice floating in water (typically created when glaciers reach the sea) — can be dangerous if struck by a ship when underway. Icebergs have been responsible for the sinking of many ships, the most famous probably being the Titanic.

Ice formation on window glass of high altitude flying airplane

For aircraft, ice can cause a number of dangers. As an aircraft climbs, it passes through air layers of different temperature and humidity, some of which may be conducive to ice formation. If ice forms on the wings or control surfaces, this may adversely affect the flying qualities of the aircraft. During the first non-stop flight of the Atlantic, the British aviators Captain John Alcock and Lieutenant Arthur Whitten Brown encountered such icing conditions - Brown left the cockpit and climbed onto the wing several times to remove ice which was covering the engine air intakes of the Vickers Vimy aircraft they were flying.

A particular icing vulnerability associated with reciprocating internal combustion engines is the carburetor. As air is sucked through the carburettor into the engine, the local air pressure is lowered, which causes adiabatic cooling. So, in humid near-freezing conditions, the carburettor will be colder, and tend to ice up. This will block the supply of air to the engine, and cause it to fail. For this reason, aircraft reciprocating engines with carburettors are provided with carburettor air intake heaters. The increasing use of fuel injection—which does not require carburettors—has made "carb icing" less of an issue for reciprocating engines.

Jet engines do not experience carb icing, but recent evidence indicates that they can be slowed, stopped, or damaged by internal icing in certain types of atmospheric conditions much more easily than previously believed. In most cases, the engines can be quickly restarted and flights are not endangered, but research continues to determine the exact conditions which produce this type of icing, and find the best methods to prevent, or reverse it, in flight.

Phases

Most liquids freeze at a higher temperature under pressure, because the pressure helps to hold the molecules together. However, the strong hydrogen bonds in water make it different: water freezes at a temperature below 0 °C under a pressure higher than 1 atm (0.10 MPa). Consequently, water also remains frozen at a temperature above 0 °C under a pressure lower than 1 atm. The melting of ice under high pressures is thought to contribute to the movement of glaciers.

Ice formed at high pressure has a different crystal structure and density from ordinary ice. Ice, water, and water vapour can coexist at the triple point, which is exactly 273.16 K (by definition) at a pressure of 611.73 Pa.

Subjected to higher pressures and varying temperatures, ice can form in fifteen separate known phases. With care all these types can be recovered at ambient pressure. The types are differentiated by their crystalline structure, ordering and density. There are also two metastable phases of ice under pressure, both fully hydrogen-disordered; these are IV and XII. Ice XII was discovered in 1996. In 2006, XIII and XIV were discovered.[36] Ices XI, XIII, and XIV are hydrogen-ordered forms of ices Ih, V, and XII respectively. In 2009, ice XV was found at extremely high pressures and −143 °C.[37]

As well as crystalline forms, solid water can exist in amorphous states as amorphous solid water (ASW), low-density amorphous ice (LDA), high-density amorphous ice (HDA), very high-density amorphous ice (VHDA) and hyperquenched glassy water (HGW).

In outer space, hexagonal crystalline ice (the predominant form found on Earth) is extremely rare. Amorphous ice is more common; however, hexagonal crystalline ice can be formed via volcanic action.[38]

Pressure-temperature phase diagram of water showing several ice phases listed in the table below
Phase Characteristics
Amorphous ice Amorphous ice is an ice lacking crystal structure. Amorphous ice exists in three forms: low-density (LDA) formed at atmospheric pressure, or below, high density (HDA) and very high density amorphous ice (VHDA), forming at higher pressures. LDA forms by extremely quick cooling of liquid water ("hyperquenched glassy water", HGW), by depositing water vapour on very cold substrates ("amorphous solid water", ASW) or by heating high density forms of ice at ambient pressure ("LDA").
Ice Ih Normal hexagonal crystalline ice. Virtually all ice in the biosphere is ice Ih, with the exception only of a small amount of ice Ic.
Ice Ic A metastable cubic crystalline variant of ice. The oxygen atoms are arranged in a diamond structure. It is produced at temperatures between 130 and 220 K, and can exist up to 240 K,[39][40] when it transforms into ice Ih. It may occasionally be present in the upper atmosphere.[41]
Ice II A rhombohedral crystalline form with highly ordered structure. Formed from ice Ih by compressing it at temperature of 190–210 K. When heated, it undergoes transformation to ice III.
Ice III A tetragonal crystalline ice, formed by cooling water down to 250 K at 300 MPa. Least dense of the high-pressure phases. Denser than water.
Ice IV A metastable rhombohedral phase. It can be formed by heating high-density amorphous ice slowly at a pressure of 810 MPa. It doesn't form easily without a nucleating agent.[42]
Ice V A monoclinic crystalline phase. Formed by cooling water to 253 K at 500 MPa. Most complicated structure of all the phases.[43]
Ice VI A tetragonal crystalline phase. Formed by cooling water to 270 K at 1.1 GPa. Exhibits Debye relaxation.[44]
Ice VII A cubic phase. The hydrogen atoms' positions are disordered. Exhibits Debye relaxation. The hydrogen bonds form two interpenetrating lattices.
Ice VIII A more ordered version of ice VII, where the hydrogen atoms assume fixed positions. Formed from ice VII, by cooling it below 5 °C (278 K).
Ice IX A tetragonal phase. Formed gradually from ice III by cooling it from 208 K to 165 K, stable below 140 K and pressures between 200 MPa and 400 MPa. It has density of 1.16 g/cm3, slightly higher than ordinary ice.
Ice X Proton-ordered symmetric ice. Forms at about 70 GPa.[45]
Ice XI An orthorhombic, low-temperature equilibrium form of hexagonal ice. It is ferroelectric. Ice XI is considered the most stable configuration of ice Ih. The natural transformation process is very slow and ice XI has been found in Antarctic ice 100 to 10,000 years old. That study indicated that the temperature below which ice XI forms is −36 °C (240 K).[46]
Ice XII A tetragonal, metastable, dense crystalline phase. It is observed in the phase space of ice V and ice VI. It can be prepared by heating high-density amorphous ice from 77 K to about 183 K at 810 MPa.
Ice XIII A monoclinic crystalline phase. Formed by cooling water to below 130 K at 500 MPa. The proton-ordered form of ice V.[47]
Ice XIV An orthorhombic crystalline phase. Formed below 118 K at 1.2 GPa. The proton-ordered form of ice XII.[47]
Ice XV The proton-ordered form of ice VI formed by cooling water to around 80–108 K at 1.1 GPa.

Non-water ice

The solid phases of some other substances are also referred to by the term ice: dry ice is a popular term for solid carbon dioxide.

A "magnetic analogue" of ice is also realized in some insulating magnetic materials in which the magnetic moments mimic the position of protons in water ice and obey energetic constraints similar to the Bernal-Fowler ice rules arising from the geometrical frustration of the proton configuration in water ice. These materials are called spin ice.

See also

Detail of an ice cube
Ice formations
  • Amorphous solid water
  • Anchor ice
  • Black ice
  • Diamond dust
  • Firn
  • Frazil ice
  • Frost flowers
  • Glacier
  • Iceberg
  • Icicle
  • Ice circle
  • Ice crystals
  • Ice disc
  • Ice nucleus
  • Ice spike
  • Névé
  • Needle ice
  • Sea ice
  • Slurry ice
Climate
Ice sports
Man-made ice
Density

References

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  2. ↑ The word crystal derives from Greek word for frost.
  3. ↑ Lide, D. R., ed. (2005), CRC Handbook of Chemistry and Physics (86th ed.), Boca Raton (FL): CRC Press, ISBN 0-8493-0486-5 
  4. ↑ Neil deGrasse Tyson. "Water, Water". http://www.haydenplanetarium.org/tyson/read/essays/nathist/waterwater. 
  5. ↑ Iglev, H.; et. al. (12 January 2006). "Ultrafast superheating and melting of bulk ice". Nature 439 (7073): 183–186. doi:10.1038/nature04415. PMID 16407948. 
  6. ↑ Why some ice looks blue
  7. ↑ Chang, Kenneth (February 21, 2006). "Explaining Ice: The Answers Are Slippery". New York Times. http://www.nytimes.com/2006/02/21/science/21ice.htm. Retrieved 8 April 2009. 
  8. ↑ "Sleet (glossary entry)". National Oceanic and Atmospheric Administration's National Weather Service. http://www.weather.gov/glossary/index.php?word=sleet. Retrieved 2007-03-20. 
  9. ↑ "Hail (glossary entry)". National Oceanic and Atmospheric Administration's National Weather Service. http://www.weather.gov/glossary/index.php?word=hail. Retrieved 2007-03-20. 
  10. ↑ 10.0 10.1 10.2 10.3 Alaska Air Flight Service Station (2007-04-10). "SA-METAR". Federal Aviation Administration via the Internet Wayback Machine. http://web.archive.org/web/20080501074014/http://www.alaska.faa.gov/fai/afss/metar+taf/sametara.htm. Retrieved 2009-08-29. 
  11. ↑ Weatherquestions.com. "What causes ice pellets (sleet)?". http://www.weatherquestions.com/What_causes_ice_pellets.htm. Retrieved 2007-12-08. 
  12. ↑ 12.0 12.1 Glossary of Meteorology (2009). "Hail". American Meteorological Society. http://amsglossary.allenpress.com/glossary/search?id=hail1. Retrieved 2009-07-15. 
  13. ↑ Ryan Jewell and Julian Brimelow (2004-08-17). "P9.5 Evaluation of an Alberta Hail Growth Model Using Severe Hail Proximity Soundings in the United States". http://www.spc.noaa.gov/publications/jewell/hailslsc.pdf. Retrieved 2009-07-15. 
  14. ↑ National Severe Storms Laboratory (2007-04-23). "Aggregate hailstone". National Oceanic and Atmospheric Administration. http://www.photolib.noaa.gov/htmls/nssl0001.htm. Retrieved 2009-07-15. 
  15. ↑ Julian C. Brimelow, Gerhard W. Reuter, and Eugene R. Poolman (October 2002). "Modeling Maximum Hail Size in Alberta Thunderstorms". Weather and Forecasting 17 (5): 1048–1062. doi:10.1175/1520-0434(2002)017<1048:MMHSIA>2.0.CO;2. 
  16. ↑ Jacque Marshall (2000-04-10). "Hail Fact Sheet". University Corporation for Atmospheric Research. http://www.ucar.edu/communications/factsheets/Hail.html. Retrieved 2009-07-15. 
  17. ↑ American Broadcasting Company News online (2004-10-19). "Hail storms rock southern Qld". http://www.abc.net.au/news/australia/qld/toowoomba/200410/s1222665.htm. Retrieved 2009-07-15. 
  18. ↑ Michael Bath and Jimmy Degaura (1997). "Severe Thunderstorm Images of the Month Archives". http://australiasevereweather.com/storm_news/arc1997.htm. Retrieved 2009-07-15. 
  19. ↑ Pete Wolf (2003-01-16). "Meso-Analyst Severe Weather Guide". University Corporation for Atmospheric Research. http://www.meted.ucar.edu/resource/soo/MesoAnalyst.htm. Retrieved 2009-07-16. 
  20. ↑ Thomas E. Downing, Alexander A. Olsthoorn, Richard S. J. Tol (1999). Climate, change and risk. Routledge. pp. 41–43. ISBN 9780415170314. http://books.google.com/?id=UbtG3vFfNtoC&pg=PA41&lpg=PA41&dq=average+height+freezing+level+tropics. Retrieved 2009-07-16. 
  21. ↑ Basil John Mason (1971). Physics of Clouds. Clarendon Press. ISBN 0198516037. 
  22. ↑ Brent Q. Christner; Cindy E. Morris, Christine M. Foreman, Rongman Cai, David C. Sands (2008). "Ubiquity of Biological Ice Nucleators in Snowfall". Science 319 (5867): 1214. doi:10.1126/science.1149757. PMID 18309078. 
  23. ↑ Glossary of Meteorology (2009). "Cloud seeding". American Meteorological Society. http://amsglossary.allenpress.com/glossary/search?p=1&query=cloud+seeding&submit=Search. Retrieved 2009-06-28. 
  24. ↑ 24.0 24.1 M. Klesius (2007). "The Mystery of Snowflakes". National Geographic 211 (1): 20. ISSN 0027-9358. 
  25. ↑ William J. Broad (2007-03-20). "Giant Snowflakes as Big as Frisbees? Could Be". New York Times. http://www.nytimes.com/2007/03/20/science/20snow.html. Retrieved 2009-07-12. 
  26. ↑ Jennifer E. Lawson (2001). Hands-on Science : Light, Physical Science (matter) - Chapter 5: The Colors of Light. Portage & Main Press. p. 39. ISBN 9781894110631. http://books.google.com/?id=4T-aXFsMhAgC&pg=PA39&lpg=PA39. Retrieved 2009-06-28. 
  27. ↑ Kenneth G. Libbrecht (2006-09-11). "Guide to Snowflakes". California Institute of Technology. http://www.its.caltech.edu/~atomic/snowcrystals/class/class.htm. Retrieved 2009-06-28. 
  28. ↑ John Roach (2007-02-13). ""No Two Snowflakes the Same" Likely True, Research Reveals". National Geographic News. http://news.nationalgeographic.com/news/2007/02/070213-snowflake.html. Retrieved 2009-07-14. 
  29. ↑ Kenneth Libbrecht (Winter 2004/2005). "Snowflake Science". American Educator. http://www.aft.org/pubs-reports/american_educator/issues/winter04-05/Snowflake.pdf. Retrieved 2009-07-14. 
  30. ↑ Glossary of Meteorology (June 2000). "Diamond Dust". American Meteorological Society. http://amsglossary.allenpress.com/glossary/search?p=1&query=diamond+dust&submit=Search. Retrieved 2010-01-21. 
  31. ↑ 31.0 31.1 ASHRAE. "Ice Manufacture". 2006 ASHRAE Handbook: Refrigeration. Inch-Pound Edition. p. 34-1. ISBN 1931862869.
  32. ↑ Rydzewski, A.J. "Mechanical Refrigeration: Ice Making." Marks' Standard Handbook for Mechanical Engineers. 11th ed. McGraw Hill: New York. p. 19-24. ISBN 9780071428675.
  33. ↑ U.S. Census Bureau. "Ice manufacturing: 2002." 2002 Economic Census.
  34. ↑ "Unique ice pier provides harbor for ships," Antarctic Sun. January 8, 2006; McMurdo Station, Antarctica.
  35. ↑ Wildwood Survival - Fire From Ice - Rob Bicevskis
  36. ↑ Salzmann, C.G.; et al. (2006). "The Preparation and Structures of Hydrogen Ordered Phases of Ice". Science 311 (5768): 1758–1761. doi:10.1126/science.1123896. PMID 16556840. 
  37. ↑ Laurua Sanders (September 11, 2009). "A Very Special Snowball". Science News. http://www.sciencenews.org/view/generic/id/47258/title/A_very_special_snowball. Retrieved 2009-09-11. 
  38. ↑ Kenneth Chang (December 9, 2004). "Astronomers Contemplate Icy Volcanoes in Far Places". New York Times. http://www.nytimes.com/2004/12/09/science/09ice.html?ex=1260334800&en=9326ecdbb6f20b0a&ei=5090&partner=rssuserland. Retrieved 2009-08-20. 
  39. ↑ Murray, B.J.; Bertram, A. K. (2006). "Formation and stability of cubic ice in water droplets". Phys. Chem. Chem. Phys. 8 (1): 186–192. doi:10.1039/b513480c. PMID 16482260. 
  40. ↑ Murray, B.J. (2008). "The Enhanced formation of cubic ice in aqueous organic acid droplets". Env. Res. Lett. 3: 025008. doi:10.1088/1748-9326/3/2/025008. 
  41. ↑ Murray, B.J.; et al. (2005). "The formation of cubic ice under conditions relevant to Earth’s atmosphere". Nature 434 (7030): 202–205. doi:10.1038/nature03403. PMID 15758996. 
  42. ↑ Chaplin, Martin (2007-07-01). "Ice-four structure". Water Structure and Science. http://www.lsbu.ac.uk/water/ice_iv.html. Retrieved 2008-01-02. 
  43. ↑ Chaplin, Martin (2007-11-11). "Ice-five and ice-thirteen structures". Water Structure and Science. http://www.lsbu.ac.uk/water/ice_v.html. Retrieved 2008-01-02. 
  44. ↑ Chaplin, Martin (2007-11-11). "Ice-six structure". Water Structure and Science. http://www.lsbu.ac.uk/water/ice_vi.html. Retrieved 2008-01-02. 
  45. ↑ Chaplin, Martin (2007-10-26). "Ice-seven and ice-ten structures". Water Structure and Science. http://www.lsbu.ac.uk/water/ice_vii.html. Retrieved 2008-01-02. 
  46. ↑ Chaplin, Martin (2007-11-11). "Hexagonal ice structure". Water Structure and Science. http://www.lsbu.ac.uk/water/ice1h.html#icexi. Retrieved 2008-01-02. 
  47. ↑ 47.0 47.1 Chaplin, Martin (2007-07-01). "Ice-twelve and ice-fourteen structures". Water Structure and Science. http://www.lsbu.ac.uk/water/ice_xii.html. Retrieved 2008-01-02. 

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