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Snow

2007 Schools Wikipedia Selection. Related subjects: Climate and the Weather

   Animation of snowcover changing with the seasons.
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   Animation of snowcover changing with the seasons.
   Snow in Holme, West Yorkshire, winter 1978
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   Snow in Holme, West Yorkshire, winter 1978

   Snow is precipitation in the form of crystalline water ice, 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 vapor undergoes deposition high in
   the atmosphere at a temperature of less than 0°C, and then falls to the
   ground. It can also be produced by hoar frost which falls from the
   surface on which it is formed, or falling particles of ice fog formed
   when the humidity in surface air freezes at very low temperatures. Snow
   is less dense than rain, which means the same amount of moisture will
   produce a greater volume of snow than it will rain. One rule of thumb
   is that the water in eight units of snow is equivalent to one unit of
   rain. See Snow density.

Occurrence

   Snow capped trees in Afghanistan.
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   Snow capped trees in Afghanistan.

   The probability of snowfall varies with season, location, and other
   geographic factors such as latitude and elevation. In the latitudinal
   area closer to the equator, there is a rather small 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.

   Permanent snow covering is affected by factors such as the degree of
   slope on the land, amount of snowfall and the force and nature of the
   winds. As temperature decreases with altitude, high mountains, even
   near the Equator, have permanent snow cover on their upper portions,
   around 5,300 m high. Examples include Mount Kilimanjaro in Tanzania and
   the Tropical Andes in South America; however, the only snow actually to
   appear on the Equator is at 4,690 m altitude of the southern slope of
   Volcán Cayambe in Ecuador ( Google Earth images).
   University Library of Graz, Austria
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   University Library of Graz, Austria

   Conversely, many regions of the Arctic and Antarctic receive very
   little precipitation and therefore experience little snowfall despite
   the bitter cold (cold air, unlike warm air, cannot take away much water
   vapor from the sea). Here, the snow does not melt at sea level. In
   addition to the dry, snowless regions of the poles, there are some
   mountains and volcanoes in Bolivia, Chile and Argentina that are high
   (4,500 m-6,900 m) and cold, but the lack of precipitation near the
   hyperarid Atacama Desert prevents snow from accumulating into glaciers.

Benefits and problems

   Luxembourg railway station.
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   Luxembourg railway station.

   Snow is used as a thermal insulator conserving the heat of the Earth
   and protecting crops from the freezing weather. On the other hand,
   substantial snowfall sometimes may 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 an unusually
   heavy snowfall.

   In areas that normally have very little or no snow, a snow day may
   occur with light accumulation or even the threat of snowfall. A
   mudslide, flash flood, or avalanche can occur when excessive snow has
   accumulated on a mountain and there is a sudden change of temperature.

Snowflake

   A snowflake is an aggregate of ice crystals that forms while falling in
   and below a cloud. They are typically hexagonally symmetrical.
   Snow crystal
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   Snow crystal

Geometry

   A snowflake always has six symmetric arms (along three symmetric axes),
   which arises from the hexagonal crystal structure of 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 or information
   transfer between the arms, such that growth in each arm affects the
   growth in each other arm. Surface tension or phonons are among the ways
   that such communication could occur.
   Photomicrograph images of snow flakes by Vermont scientist-artist
   Wilson Bentley, c. 1902
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   Photomicrograph images of snow flakes by Vermont scientist-artist
   Wilson Bentley, c. 1902

   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 other
   atmospheric conditions. 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, nonetheless, no known scientific laws which
   prevent it. In a more pragmatic sense, it's more likely, albeit not
   much more, that two 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 Society
   has reported that matching snow crystals were discovered by Nancy
   Knight of the National Centre for Atmospheric Research. The crystals
   were not flakes in the usual sense but rather hollow hexagonal prisms.

Electron microscope gallery

   The "Japanese Tsuzumi", an unusual variation of snow crystal, is named
   after the hourglass-shaped tsuzumi drum.
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   The "Japanese Tsuzumi", an unusual variation of snow crystal, is named
   after the hourglass-shaped tsuzumi drum.

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 5 m, then it will melt into a pool
   of water 5 m deep covering the same area. This is a much more useful
   measurement to hydrologists than 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 .

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

Superlatives

   The highest seasonally cumulative precipitation of snow ever measured
   was on Mount Baker, Washington in the United States during the 1998–
   1999 season. Mount Baker received 1140 in. (29 m) of snow , thus
   surpassing the previous record holder, Mount Rainier, Washington which
   during the 1971– 1972 season received 1122 in. (28.5 m) of snow .

Recreation

   Building a snowman.
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   Building a snowman.

   Some forms of recreation depend on snow
     * Many winter sports, such as skiing, snowboarding, ice skating,
       snowmobiling and snowshoeing
     * Playing with a sled or riding in a sleigh
     * Building a snowman or snow fort
     * Throwing snowballs mutually in a snowball fight or at others to
       tease them. (Humans seem to be the only being that throw their
       snowballs. Pygmy chimpanzees have 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 cannons
   may be used to produce an adequate amount for such sports. The world's
   biggest snowcastle is built in Kemi, Finland, every winter.
   Retrieved from " http://en.wikipedia.org/wiki/Snow"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
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