Snow

For other uses of "Snow", see Snow (disambiguation).

Image:Utahsnow.jpeg Snow is precipitationin the form of crystallinewaterice, consisting of a multitude of snowflakes. Since it is composed of small rough particles it is a granular material. It has an open and therefore soft structure, unless packed by external pressure.

Snow is commonly formed when water vaporundergoes depositionhigh in the atmosphereat a temperature of less than 0°C (32°F), and then falls to the ground.

Occurrence

The probability of snowfall varies with season, location, and other geographic factors such as latitudeand elevation. In the latitudinal area closer to the equator, there is a lesser chance of snowfall, 35° N and 40°S are often quoted as a rough delimiter. The western coasts of the major continents remain devoid of snow to much higher latitudes.

As temperature decreases with altitude, high mountains, even near the Equator, have permanent snow cover on their upper portions. Examples include Mount Kilimanjaroin Tanzaniaand the TropicalAndesin South America; however, the only snow actually to appear on the Equator is at 4,690 m altitude of the southern slope of Volcán Cayambein Ecuador(Google Earthimages). Conversely, many regions of the Arcticand Antarcticreceive very little precipitation and therefore experience little snow despite the bitter cold (below a certain temperature, air essentially loses its ability to retain water vapor). Image:Falling snow Graz 2005 original.jpg

Substantial snowfall can, at times, even disrupt the infrastructure and services of a region that is accustomed to such weather. Automotive traffic may be greatly inhibited or may be stifled entirely. Basic infrastructures such as electricity, telephone lines, and gas supply can also be shut down. This can lead to a "snow day", which is a day on which the school or other services are cancelled due to unusually heavy snowfall. In areas that normally have very little snow, this may occur even with light accumulation, something often ridiculed by those people accustomed to colder climates, where streetswould remain passable given the same amount of snow.

The highest seasonally cumulative precipitation of snow ever measured was on Mount Baker, Washington, U.S.Aduring the 1998?1999season. Mount Bakerreceived a staggering 28.96 meters (1,140 in) of snow, thus surpassing the previous record holder, Mount Rainier, Washington, U.S.Awhich during the 1971?1972season received 28.5 meters (1,122 in) of snow.

See also: List of Countries receiving snowfall

Geometry

Image:SnowflakesWilsonBentley.jpg Image:Tsuzumi snow crystal.jpg A snowflake always has six lines of symmetry, which arises from the hexagonal crystal structureof ordinary ice(known as ice I_h) along its 'basal' plane.

There are, broadly, two possible explanations for the symmetry of snowflakes. Firstly, there could be communication (information transfer) between the arms, such that growth in each arm affects the growth in each other arm. Surface tensionor phononsare among the ways that such communication could occur. The other explanation, which appears to be the prevalent view, is that the arms of a snowflake grow independently in an environment that is believed to be rapidly varying in temperature, humidity and so on. This environment is believed to be relatively spatially homogeneous on the scale of a single flake, leading to the arms growing to a high level of visual similarity by responding in identical ways to identical conditions, much in the same way that unrelated trees respond to environmental changes by growing near-identical sets of tree rings. The difference in the environment in scales larger than a snowflake leads to the observed lack of correlation between the shapes of different snowflakes.

However, the concept that no two snowflakes are alike is not necessarily true. Strictly speaking, it is extremely unlikely for any two objects in the universe to contain an identical molecular structure; but, there are, nontheless, no known scientific laws which prevent it. In a more pragmatic sense, it's more likely, albeit not much more, that a pair of snowflakes are visually identical if their environments were similar enough, either because they grew very near one another, or simply by chance. The American Meteorological Societyhas reported that matching snow crystals were discovered by Nancy Knight of the National Center for Atmospheric Research. The crystals were not flakes in the usual sense but rather hollow hexagonalprisms.

Electron Microscope Gallery

Recreation

Image:Small snowman.jpg Forms of recreationdependent on snow:

• Many winter sports, such as skiing, snowmobiling, snowshoeingand snowboarding
• Playing with a sledor riding in a sleigh
• Building a snowman(or 'snowwoman') or snow fort
• Throwing snowballs mutually in a snowball fightor at others to teasethem. (Humansseem to be the only being that throw their snowballs. Pygmy chimpanzeeshave been known to carry snowballs around, but never to throw them.)
• Making a snow angel

Where snow is scarce but the temperature is low enough, snow cannonsmay be used to produce an adequate amount for such sports.

The world´s biggest snowcastle is built in Kemi, Finland, every winter.

Density

The water equivalent of a snow pack is the amount of water that it contains, regardless of its depth. For example, if the snow covering a given area has a water equivalent of 20 inches, then it will melt into a pool of water 20 inches deep covering the same area. This is a much more useful measurement to hydrologiststhan snow depth, as the density of even freshly fallen snow widely varies. New snow often has a density of around 12% of water, and even under cold conditions, the same snow will settle under its own weight until it is approximately 33% water. More snow on top of this will compress it even further. By late spring, snow densities often exceed 50% of water [1].

Water equivalent is of great interest to water managers wishing to predict spring runoffand the water supplyof cities downstream. Measurements are made manually at marked locations known as snow courses, and remotely using special scales called snow pillows.

Types

Flurries

A period of light snow with usually little accumulation with occasional moderate snowfall.

Sleet

Snow which has partially thawed while falling.

Soft hail

If this re-freezes on further descent, the resulting small icy pellets or granules of snow are called soft hail.

Freezing rain

A rainstorm, where the rain and ground are sufficiently cold that the raindrops freeze on impact. This can cover trees in a uniform layer of very clear, shiny ice, a beautiful phenomenon.

Snizzle

Extremely small snow/ice particles that fall lightly, almost like drizzle that is frozen. This kind of snow is very common in Michigan.

Snow storm

A long storm of relatively heavy snow.

Snow squall

A brief, very intense snowstorm.

Blizzard

A long-lasting snow storm with intense snowfall and usually high winds. Particularly severe storms can create whiteoutconditions where visibility is reduced to less than 1 m.

Snowdrift

Large piles of snow that occur near walls and curbs, as the wind tends to push the snow up toward the vertical surfaces.

Ground blizzard

Occurs when a strong wind drives already fallen snow to create drifts and whiteouts.

Packing snow

Snow that is at or near the melting point, so that it can easily be packed into snowballs and hurled at other people or objects. This is perfect for snow fights and other winter fun, such as making a snowman, or a snow fort.

Slush

Snow that has been melting for some time, to the point that it has become large, very cold puddles with chunks of snow and ice floating in them.

Hail

Many-layered ice balls, ranging from "popcorn" sized, to "nickel", "golf ball", "baseball" and "softball" sized hail.

Hailstorm

A storm of hail. If the hail is sufficiently large, it can cause damage to cars or even people.

Artificial snow

Snow can be also manufactured using snow cannons, which actually create tiny granulesmore like soft hail (this is sometimes called "grits" by those in the southern U.S.for its likeness to the textureof the food). In recent years, snow cannonshave been produced that create more natural looking snow, but these machines are prohibitively expensive.

See also

• Avalanche
• Sleigh
• Skiing
• Snow sculpture
• Snowball
• Snowboard
• Snowman
• Snow cannon
• Frost
• Snowshoe
• Snow removal
• Igloo
• Grit bin
• Lake effect snow
• List of countries receiving snowfall
• Eskimo words for snow

Media

External links

• National Science Digital Library - Snowflake
• Kenneth G. Libbrecht's Snowflake FAQ
• http://www.its.caltech.edu/~atomic/snowcrystals/photos/photos.htman:Nieu

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It uses material from the http://en.wikipedia.org/wiki/Snow Wikipedia article Snow.