Snow

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For other uses, see Snow (disambiguation).
Animation of snowcover changing with the seasons
Animation of snowcover changing with the seasons
Trees covered with snow
Trees covered with snow

Snow is precipitation in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by external pressure.

Contents

[edit] Occurrence

Snow covering a leaf.
Snow covering a leaf.

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 those near the Equator – have permanent snow cover on their upper portions, around 5,300 m. Examples include Mount Kilimanjaro in Tanzania and the Tropical Andes in South America; however, the only snow actually to appear on the Equator line is at 4,690 m altitude on the southern slope of Volcán Cayambe in Ecuador.

Snow in Brazil
Snow in Brazil

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. Another example of these temperate unglaciated mountains is the Kunlun Mountains, Tibet and the Pamir Range to the north of the Himalayas in Central Asia. Here, just like the Andes, mountains in Central Asia can reach to 6,300 meters and be barren of snow and ice due to the rain shadow effect caused by the taller Himalaya Range.

[edit] Benefits and problems

Snow serves as a thermal insulator conserving the heat of the Earth and protecting crops from subfreezing weather. But substantial snowfall sometimes disrupts infrastructure and services, even those 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," a day on which school sessions or other services are canceled owing 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, as those areas are ill prepared to handle any amount of snow. A mudslide, flash flood, or avalanche can occur when excessive snow has accumulated on a mountain and there is a sudden change of temperature.

[edit] Snowflake

A snowflake is an aggregate of ice crystals that forms while falling in and below a cloud. They are typically hexagonally symmetrical.[1]

[edit] Geometry

Snowflake Gallery.
Snowflake Gallery.
Snow crystal
Snow crystal

Large, well formed snowflakes are relatively flat and have six approximately identical arms, so that the snowflake nearly has the same 6-fold dihedral symmetry as a hexagon or hexagram. This symmetry arises from the hexagonal crystal structure of ordinary ice. However, the exact shape of the snowflake is determined by the temperature and humidity at which it forms.[1]. Rarely, at a temperature of around -2 °C (28 °F), snowflakes can form in threefold symmetry - triangular snowflakes. [2] Snowflakes are not perfectly symmetric however. The most common snowflakes are visibly irregular, although near-perfect snowflakes may be more common in pictures because they are more visually appealing.

Photomicrograph images of snow flakes by Vermont scientist-artist Wilson Bentley, c. 1902
Photomicrograph images of snow flakes by Vermont scientist-artist Wilson Bentley, c. 1902

Snowflakes can come in many different forms, including columns, needles, and plates (with and without "dendrites" - the "arms" of some snowflakes). These different forms arise out of different temperatures and water saturation - among other conditions. Six petaled ice flowers grow in air between 0 degrees Celsius and -3 degrees Celsius. The vapor droplets solidify around a dust particle. Between temperatures of -1 °C (30 °F) and -3 °C (27 °F), the snowflake will be in the form of a dendrite or a plate or the six petaled ice flower. As temperatures get colder, between -5 °C and -10 °C, the crystals will form in either needles or hollow columns or prims. When the temperature becomes even colder from -10 degrees Celsius to -22 degrees Celsius the ice flowers are formed again and any temperature below -22 degrees Celsius the vapors will turn into the prisms again. In special circumstances, where the crystal has started forming at around -5 °C, and is then exposed to warmer or colder temperatures, a capped column may be formed which consists of a column-like design capped with a dendrite or plate-like design on each end of the column.[1] At even colder temperatures, the snowflake design returns to the more common dendrite and plate. At temperatures approaching -20 °C, sectored plates are formed which appears as a dendrite, with each dendrite appearing flattened, like the design of a snowflake plate.[1]

There are, broadly, two possible explanations for the symmetry of snowflakes. First, 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. 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. The sixfold symmetry happens because of the basic hexagonal crystalline structure from which the snowflake grows. Surprisingly, the exact reason for the threefold symmetry of triangular snowflakes is still a mystery.

There is a widely held belief that no two snowflakes are alike, but that claim has not been proven. 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 that 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 Center for Atmospheric Research. The crystals were not flakes in the usual sense but rather hollow hexagonal prisms.

A common estimate is that a snowflake can appear in 10158 forms, under the estimate that a snowflake has 100 attributes that can vary, resulting in 100 factorial forms. However, the number of possible snowflakes per the atomic structure would be based on the number of molecules, and the former number would be very, very large.

[edit] Electron microscope gallery






[edit] Density

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

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 50 cm, then it will melt into a pool of water 50 cm 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 commonly has a density of between 5% and 12% of water. Snow that falls in maritime climates is usually denser than snow that falls in mid-continent locations because of the higher average temperatures over oceans than over land masses. 15% density snow is not uncommon in California. Cloud temperatures and physical processes in the cloud affect the shape of individual snow crystals. Highly branched or dendritic crystals tend to have more space between the arms of ice that form the snow flake and this snow will therefore have a lower density, often referred to as "dry" snow. Conditions that create columnar or platelike crystals will have much less air space within the crystal and will therefore be more dense and feel "wetter".

Once the snow is on the ground, it will settle under its own weight until it is approximately 33% water. Increases in density above this initial compression occur primarily because of changes to the crystals due to direct solar radiation. By late spring, snow densities typically reach a maximum of 50% of water.[3] Spring snow melt is a major source of water supply to areas in temperate zones characterized by a prolonged dry summer and fall, and where there are mountains high enough to catch and hold winter snow. In these locations, 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.

[edit] Records

The highest seasonally cumulate precipitation of snow ever measured was at Mount Baker Ski Area, outside of Bellingham, Washington in the United States during the 1998–1999 season. Mount Baker received 1,140 in. (29 m) of snow,[4] thus surpassing the previous record holder, Mount Rainier, Washington, which during the 1971–1972 season received 1,122 in. (28.5 m) of snow.[5]

[edit] Recreation

Building a snowman.
Building a snowman.

Some forms of recreation depend on snow

[edit] Types of snow

Hoar frost that grows on the snow surface due to water vapor moving up through the snow on cold, clear nights
Hoar frost that grows on the snow surface due to water vapor moving up through the snow on cold, clear nights

[edit] Snow precipitation

Blizzard
A long-lasting snow storm with intense snowfall and usually high winds. Particularly severe storms can create whiteout conditions where visibility is reduced to less than 1 m.
Flurry
A period of light snow with usually little accumulation with occasional moderate snowfall.
Freezing rain
Rain that freezes on impact with a sufficiently cold surface. This can cover trees in a uniform layer of very clear, shiny ice – a beautiful phenomenon, though excessive accumulation can break tree limbs and utility lines, causing utility failures and possible property damage.
Snowbird ski resort, one of the snowiest places in the U.S.
Snowbird ski resort, one of the snowiest places in the U.S.
Graupel
Precipitation formed when freezing fog condenses on a snowflake, forming a ball of rime ice. Also known as snow pellets.
Ground blizzard
Occurs when a strong wind drives already fallen snow to create drifts and whiteouts.
Hail
Many-layered ice balls, ranging from "pea" sized (0.25 in, 6 mm) to "golf ball" sized (1.75 in, 43 mm), to, in rare cases, "softball" sized or greater (­>4.25 in, 108 mm).
Hailstorm
A storm of hail. If the hail is sufficiently large, it can cause damage to cars or even people.
Lake effect snow
Produced when cold winds move across long expanses of warmer lake water, picking up water vapor which freezes and is deposited on the lee shores.
Sleet
In Britain, rain mixed with snow; in America, ice pellets formed when snowflakes pass through a layer of warm air, thaw, then refreeze on further descent.
Snow pellets
See graupel.
Snow squall
A brief, very intense snowstorm.
Snow storm
A long storm of relatively heavy snow.
Soft hail
Granules of snow or ice pellets formed when supercooled water accretes on ice crystals or snowflakes.
Thundersnow
A thunderstorm which produces snow as the primary form of precipitation.

[edit] Snow on ground

Winter in Croatia
Winter in Croatia
Artificial snow
Snow can be also manufactured using snow cannons, which actually create tiny granules more like soft hail (this is sometimes called "grits" by those in the southern U.S. for its likeness to the texture of the food). In recent years, snow cannons have been produced that create more natural-looking snow, but these machines are prohibitively expensive.
Blowing snow
Snow on ground that is being moved around by wind. See ground blizzard.
Corn
Coarse, granular wet snow. Most commonly used by skiers describing good spring snow. Corn is the result of diurnal cycle of melting and refreezing.
Snow blowing from a roof in Ottawa
Snow blowing from a roof in Ottawa
Crust
A double layer of snow in which the lower layer may be powdery dry but where the surface is frozen together into a stiff, icy surface, which often can support human weight.
Ice
Densely packed material formed from snow that doesn't contain air bubbles. Depending on the snow accumulation rate, the air temperature, and the weight of the snow in the upper layers, it can take snow a few hours or a few decades to form into ice.
Firn
Snow which has been lying for at least a year but which has not yet consolidated into glacier ice. It is granular.
Packed Powder
The most common snow cover on ski slopes, consisting of powder snow that has lain on the ground long enough to become compressed, but is still loose.
Heavy snow on pine tree
Heavy snow on pine tree
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.
Penitentes
Tall blades of snow found at high altitudes.
Powder
Freshly fallen, uncompacted snow. The density and moisture content of powder snow can vary widely; snowfall in coastal regions and areas with higher humidity is usually heavier than a similar depth of snowfall in an arid or continental region. Light, dry (low moisture content) powder snow is prized by skiers and snowboarders. It is often found in the Rocky Mountains of North America.
Slush
Snow which partially melts upon reaching the ground, to the point that it accumulates in puddles of partially-frozen water.
The textures of a snowdrift on the Long Mynd, Shropshire
The textures of a snowdrift on the Long Mynd, Shropshire
Snowdrift
Large piles of snow which occur near walls and curbs, as the wind tends to push the snow up toward the vertical surfaces.
Watermelon snow
A reddish/pink colored snow that smells like watermelons, and is caused by a red colored green algae called chlamydomonas nivalis

[edit] See also

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[edit] References

  1. ^ a b c d Klesius, Michael (2007), "The Mystery of Snowflakes", National Geographic 211(1): 20, ISSN 0027-9358
  2. ^ http://www.its.caltech.edu/~atomic/snowcrystals/class/class.htm
  3. ^ http://cdec.water.ca.gov/snow/misc/density.html
  4. ^ http://www.usatoday.com/weather/news/1999/wsnorcrd.htm
  5. ^ http://www.nps.gov/archive/mora/interp/faq.htm

[edit] External links

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