Snow Crystals

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A snow crystal is any of several types of ice crystal found in snow. A snow crystal is a single crystal, in contrast to a snowflake, which is usually an aggregate of many single snow crystals.
A snow crystal is any of several types of ice crystal found in snow. A snow crystal is a single crystal, in contrast to a snowflake, which is usually an aggregate of many single snow crystals.
The basic structure of snow formed in the atmosphere is a hexagonal crystal. A-axes growth produces a stellar crystal or “snowflake”
The basic structure of snow formed in the atmosphere is a hexagonal crystal. A-axes growth produces a stellar crystal or “snowflake

A snow crystal is any of several types of ice crystal found in snow. A snow crystal is a single crystal, in contrast to a snowflake, which is usually an aggregate of many single snow crystals.

Snow crystals, also called snowflakes, are single crystals of ice that grow from water vapour.

They form in copious numbers in the atmosphere and are well known for their elaborate, symmetrical patterns. The physics of snow crystal formation is a specific example of the more general problem of how crystals grow and develop, creating complex structures on many length scales in the process.

Because crystallization is a basic phase transition and crystals make up the foundation of several major industries, much effort has been expended toward developing a detailed understanding of the physics of crystal nucleation and growth.

There are two features of snow crystal growth—

  1. the formation of simple, faceted prism crystals as a function of temperature and supersaturation.
  2. the transition to, and growth of, dendritic structures.

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).

In warmer clouds an aerosol particle or "ice nucleus" must be present in (or in contact with) the droplet to act as a nucleus. Ice nuclei are very rare compared to that cloud condensation nuclei on which liquid droplets form. Clays, desert dust and biological particles may be effective, 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.

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 by a process known as the Wegner-Bergeron-Findeison process. The corresponding depletion of water vapor causes 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.

Guinness World Records list the world’s largest snowflakes as those of January 1887 at Fort Keogh, Montana; allegedly one measured 38 cm (15 in) wide.

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.

The shape of the snowflake is determined broadly by the temperature and humidity at which it is formed. The most common snow particles are visibly irregular. See Snowflake Gallery

  • 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. 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.
  • 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."

A snowflake consists of roughly 1018 water molecules, which are added to its core 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.

[edit] Also See

[edit] Reference

  1. INSTITUTE OF PHYSICS PUBLISHING REPORTS ON PROGRESS IN PHYSICS The physics of snow crystals, Kenneth G Libbrecht [1]
  2. Glossary of Meteorology Snow Crystal [2][3]
  3. S. Yanagi Snow crystals observed in Hokkaido, Japan [4]
  4. Wikipedia Snow http://en.wikipedia.org/wiki/Snow

[edit] External links



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