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[[ga:Calóg sneachta]]
[[ga:Calóg sneachta]]

Revision as of 19:33, 2 January 2010

This article is about snow. For other uses, see Snowflake (disambiguation).
Snowflake viewed in an optical microscope

Snowflakes are conglomerations of frozen ice crystals which fall through the Earth's atmosphere. They begin as snow crystals which develop when microscopic supercooled cloud droplets freeze. Snowflakes come in a variety of sizes and shapes. Complex shapes emerge as the flake moves through differing temperature and humidity regimes. Individual snowflakes are nearly unique in structure. Types which fall in the form of a ball due to melting and refreezing, rather than a flake, are known as graupel, with ice pellets and snow grains as examples of graupel.

Formation

Sketch of snow crystal by René Descartes

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 Template:C to 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 Template:C to F.[1] In warmer clouds an aerosol particle or "ice nucleus" must be present in (or in contact with) the droplet to act as a nucleus. The particles that make ice nuclei are very rare compared to nuclei upon which liquid cloud droplets form, however it is not understood what makes them efficient. Clays, desert dust and biological particles may be effective,[2] 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.[3]

Once a droplet has frozen, it grows in the supersaturated environment, which is one where air is saturated with respect to ice when the temperature is below the freezing point. The droplet then grows by diffusion of water molecules in the air (vapor) onto the ice crystal surface where they are collected. Because water droplets are so much more numerous than the ice crystals due to their sheer abundance, the crystals are able to grow to hundreds of micrometers or millimeters in size at the expense of the water droplets. This process is known as the Wegner-Bergeron-Findeison process. The corresponding depletion of water vapor causes the droplets to evaporate, meaning that the ice crystals grow at the droplets' expense. These large crystals are an efficient source of precipitation, since they fall through the atmosphere due to their mass, and may collide and stick together in clusters, or aggregates. These aggregates are snowflakes, and are usually the type of ice particle that falls to the ground.[4] 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.[5]

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 color due to diffuse reflection of the whole spectrum of light by the small ice particles.[6]

Geometry

Ice crystals formed in the well-controlled laboratory conditions are often thin and flat. These planar crystals may be in the shape of simple hexagons, or if the supersaturation is high enough, develop branches and dendritic (fern-like) features and have six nearly identical arms. The sixfold symmetry arises from the hexagonal crystal structure of ordinary ice, the branch formation is produced by unstable growth, with deposition occurring preferentially near the tips of branches.[1]

The shape of the snowflake is determined broadly by the temperature and humidity at which it is formed.[4] Rarely, at a temperature of around −2 °C (28 °F), snowflakes can form in threefold symmetry — triangular snowflakes.[7] The most common snow particles are visibly irregular, although near-perfect snowflakes may be more common in pictures because they are more visually appealing.

Planar crystals (thin and flat) grow in air between 0 °C (32 °F) and −3 °C (27 °F). Between −3 °C (27 °F) and −8 °C (18 °F), the crystals will form needles or hollow columns or prisms (long thin pencil-like shapes). From −8 °C (18 °F) to −22 °C (−8 °F) the shape reverts back to plate-like, often with branched or dendritic features. The maximum difference in vapor pressure between liquid and ice is at about −15 °C (5 °F) where crystals grow most rapidly at the expense of the liquid droplets. At temperatures below −22 °C (−8 °F), the crystal development becomes column-like, although many more complex growth patterns also form such as side-planes, bullet-rosettes and also planar types depending on the conditions and ice nuclei.[8][9][10] If a crystal has started forming in a column growth regime, at around −5 °C (23 °F), and then falls into the warmer plate-like regime, then plate or dendritic crystals sprout at the end of the column, producing so called "capped columns."[4]

In artistic representations, snowflakes are sometimes 5- or 8-sided. In nature, snowflakes sometimes form 3-sided and 12-sided figures (halfing and doubling the normally 6-sided crystal arrangement), but the other representations are scientifically inaccurate.[11]

Uniqueness

It is next to impossible that two snowflakes are exactly alike due to the roughly 1018 water molecules which make up a snowflake,[12] 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 united.[13] Initial attempts to find identical snowflakes by photographing thousands of them with a microscope from 1885 onward by Wilson Alwyn Bentley found the wide variety of snowflakes we know about today.[14] It is more likely that two snowflakes could become virtually identical if their environments were similar enough. Matching snow crystals were discovered in Wisconsin in 1988. The crystals were not flakes in the usual sense but rather hollow hexagonal prisms.[15]

Use as a symbol

The snowflake is often a traditional seasonal image/motif used around the Christmas period, especially in Europe, representing the traditional White Christmas.

Snowflakes are also often used as symbols representing winter or cold conditions. For example, tires which enhance traction during harsh winter driving conditions are labelled with a snowflake on the mountain symbol.[16] A snowflake was the symbol of the 2002 Winter Olympic Games in Salt Lake City, Utah.[17]

Multiple snowflake symbols are encoded in Unicode: "snowflake" at U+2744 (❄); "tight trifoliate snowflake" at U+2745 (❅); and "heavy chevron snowflake" at U+2746 (❆).

See also

References

  1. ^ a b Mason, Basil John. (1971). Physics of Clouds. Clarendon Press. ISBN 0198516037.
  2. ^ Christner, Brent Q. (2008). "Ubiquity of Biological Ice Nucleators in Snowfall". Science. 319 (5867): 1214. doi:10.1126/science.1149757. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  3. ^ Glossary of Meteorology (2009). "Cloud seeding". American Meteorological Society. Retrieved 2009-06-28.
  4. ^ a b c M. Klesius (2007). "The Mystery of Snowflakes". National Geographic. 211 (1): 20. ISSN 0027-9358.
  5. ^ William J. Broad (2007-03-20). "Giant Snowflakes as Big as Frisbees? Could Be". New York Times. Retrieved 2009-07-12.
  6. ^ Jennifer E. Lawson (2001). Hands-on Science : Light, Physical Science (matter) - Chapter 5: The Colors of Light. Portage & Main Press. p. 39. ISBN 9781894110631. Retrieved 2009-06-28.
  7. ^ Kenneth G. Libbrecht (2006-09-11). "Guide to Snowflakes". California Institute of Technology. Retrieved 2009-06-28.
  8. ^ Bailey, Matthew, united united united (2004). "Growth rates and habits of ice crystals between -20 and -70C". Journal of the Atmospheric Sciences. 61: 514. doi:10.1175/1520-0469(2004)061<0514:GRAHOI>2.0.CO;2. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: multiple names: authors list (link)
  9. ^ Kenneth G. Libbrecht (2006-10-23). "A Snowflake Primer". California Institute of Technology. Retrieved 2009-06-28.
  10. ^ Kenneth G. Libbrecht (January–February 2007). "The Formation of Snow Crystals". American Scientist. 95 (1): 52–59.{{cite journal}}: CS1 maint: date format (link)
  11. ^ What's Wrong With This Snowflake? by Jon Hamilton. All Things Considered, National Public Radio, 23 Dec 2009.
  12. ^ John Roach (2007-02-13). ""No Two Snowflakes the Same" Likely True, Research Reveals". National Geographic News. Retrieved 2009-07-14.
  13. ^ Kenneth Libbrecht (Winter 2004/2005). "Snowflake Science" (PDF). American Educator. Retrieved 2009-07-14. {{cite journal}}: Check date values in: |date= (help)
  14. ^ Chris V. Thangham (2008-12-07). "No two snowflakes are alike". Digital Journal. Retrieved 2009-07-14.
  15. ^ Randolph E. Schmid (15 June 1988). "Identical snowflakes cause flurry". The Boston Globe. Associated Press. Retrieved 27 November 2008. But there the two crystals were, side by side, on a glass slide exposed in a cloud on a research flight over Wausau, Wis.
  16. ^ Tim Gilles (2004). "Automotive chassis". Cengage Learning. p. 271. ISBN 9781401856304. Retrieved 2009-07-15.
  17. ^ International Olympic Committee (2009). "Olympic Games Salt Lake City 2002 - The emblem". Retrieved 2009-07-15.

Further reading

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