Fri, 18/May/2012

Snow Flake

From Wikipedia, the free encyclopedia

Jump to: navigation, search

This article is about snow. For other uses, see Snowflake (disambiguation).

Article

Snowflakes begin as snow crystals which develop when tiny supercooled cloud droplets (about 10 μm in diameter) freeze. Snowflakes come in a variety of sizes and shapes. Colder environments lead to column-like development of the flakes, while warmer temperatures lead to thin and flat crystal development. Complex shapes emerge as the flake moves through differing temperature and humidity regimes. 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. Snowflakes are used as a symbol for winter tires, and for the 2002 Winter Olympics.

Definition

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).[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. 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,[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]

... ...

A view...

Snowflake viewed in an optical microscope

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]

... ...

Uniqueness

No two snowflakes are alike due to the roughly 1018 water molecules which make up a snowflake,[11] 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.[12] Initial attempts to find identical snowflakes by photographing thousands their images under a microscope from 1885 onward by Wilson Alwyn Bentley found the wide variety of snowflakes we know about today.[13] 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.[14]

Login