snow

 

precipitation formed by the sublimation of water vapor into solid crystals at temperatures below freezing. Sublimation resulting in the formation of snow takes place about a dust particle, as in the formation of raindrops. Snowflakes form symmetrical (hexagonal) crystals, sometimes matted together if they descend through air warmer than that of the cloud in which they originated. Apparently, no two snow crystals are alike; they differ from each other in size, lacy structure, and surface markings. Snowfall, reduced to its liquid equivalent, is usually included in statistics on rainfall; the factors determining snowfall are similar to those affecting rainfall. On an average, 10 in. (25 cm) of snow is equivalent to 1 in. (2.5 cm) of rain. In the United States the average snowfall is about 28 in. (71 cm) per winter; the record is 1,140 in. (2,896 cm) at Mt. Baker in Washington state during the snow season of 1998–99. Snow that piles up on slopes may suddenly slide downward in an avalanche. A glacier consists of ice that was formed from compacted snow. Snow serves as an insulating blanket, lessening to some extent the extremes of temperature fluctuation to which the soil is subjected, but it also brings about a rapid cooling of the overlying atmosphere, giving rise to polar air masses. Snow lessens loss of water by dormant plants. The sudden melting of snow is a primary cause of floods. Snow necessitates the building of snowsheds over rail lines and highways in certain mountain localities where a heavy fall is likely to impede travel; the use of snowplows to clear sidewalks, streets, and roads; the use of snow fences to prevent drifting over roads; and the use of skis, snowshoes, toboggans, snowmobiles, and sleds for travel. It is a primary factor in the location of winter sports centers and so has great economic value to certain areas. In some ski resorts machines are used to make artificial snow. As in the case of rainfall, snowfall has been produced artificially by introducing dry-ice pellets into supercooled clouds, that is, clouds containing unfrozen water droplets at temperatures below freezing.

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Snow is a type of 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.

Snowflakes

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

Geometry

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.

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 °C (32 °F) and -3 °C (27 °F). 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 (23 °F) and -10 °C (14 °F), the crystals will form in needles or hollow columns or prisms. When the temperature becomes even colder (from -10 °C to -22 °C) the ice flowers are formed again, and at temperatures below -22 °C, the vapors will turn into prisms again. If a 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. The exact reason for the threefold symmetry of triangular snowflakes is still a mystery ^{[citation needed]} although trigonal symmetry is a subsymmetry of hexagonal.

There is a widely held belief that no two snowflakes are alike. Strictly speaking, it is extremely unlikely for any two macroscopic 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 in Wisconsin in 1988 by Nancy Knight of the National Center for Atmospheric Research. The crystals were not flakes in the usual sense but rather hollow hexagonal prisms.

Snow on the ground

Snow fall remains on the ground until it melts. In colder climates this results in snow lying on the ground all winter; when the snow does not all melt in the summer it becomes glaciers.

This is often called snowpack, especially when it does persist a long time. The deepest snowpacks occur in mountainous regions. It is influenced by temperature and wind events which determine melting, accumulation and wind erosion.

The water equivalent of the snow is thickness of the layer of water having the same content. For example, if the snow covering a given area has a water equivalent of  cm ( in), then it will melt into a pool of water  cm ( in) 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 15% 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. 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 (largely due to differential evaporation) until its density is approximately 30% of water. Increases in density above this initial compression occur primarily melting and refreezing, caused by temperatures above freezing or by 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 near mountains that catch and hold winter snow, especially those with a prolonged dry summer. In such places, 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.

Many rivers originating in mountainous or high-latitude regions have a significant portion of their flow from snowmelt. This often makes the river's flow highly seasonal resulting in periodic flooding. In contrast, if much of the melt is from glaciated or nearly glaciated areas, the melt continues through the warm season, mitigating that effect.

Energy balance

The energy balance of the snowpack is dictated by several heat exchange processes. The snowpack absorbs solar shortwave radiation that is partially blocked by cloud cover and reflected by snow surface. A longwave heat exchange takes place between the snowpack and its surrounding environment that includes overlaying air mass, tree cover and clouds. Convective (sensible) heat exchange between the snowpack and the overlaying air mass is governed by the temperature gradient and wind speed. Moisture exchange between the snowpack and the overlaying air mass is accompanied with latent heat transfer that is influenced by vapor pressure gradient and air wind. Rain on snow could induce significant heat input to the snowpack. A generally insignificant conductive heat exchange takes place between the snowpack and the underlying ground. ^[4]

Impact on human society

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.

Records

The highest seasonal total snowfall 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 inches (29 m) of snow,^[5] thus surpassing the previous record holder, Mount Rainier, Washington, which during the 1971–1972 season received 1,122 in. (28.5 m) of snow.^[6] Guinness World Records list the world’s largest snowflakes as those of January 1887 at Fort Keogh, Montana;. allegedly one measured 15 inches (38 cm) wide.

Recreation

Some forms of recreation depend on snow

• Many winter sports, such as skiing, snowboarding, 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.
• 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, the SnowCastle of Kemi, is built in Kemi, Finland every winter.

Types of snow

Snow falling

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.

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 lake's shores.

Pukak

A layer at the bottom of old snowpack, consisting of coarse, sugar-like ice crystals and air. Subnivean animals live in the pukak layer because its temperature is generally stable at a few degrees below freezing and it can be tunneled through with relative ease.

Sleet

In Britain, rain mixed with snow; in America, ice pellets formed when snowflakes pass through a layer of warm air, partially 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.

Snow on the ground

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.

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.

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 and in Niseko, Japan.

Slush

Snow which partially melts upon reaching the ground, to the point that it accumulates in puddles of partially-frozen water.

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

See also

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.scientificjournals.org/journals2007/articles/1118.pdf
5. ^ http://www.usatoday.com/weather/news/1999/wsnorcrd.htm
6. ^ http://www.nps.gov/archive/mora/interp/faq.htm

External links

• Snowpack energy and mass balance An article that contains detailed analysis of snowpack energy and mass balance.
• Ultra-high resolution images of snowflakes, hosted by the Electron Microscopy Unit of the USDA Beltsville Agricultural Research Center
• Snow at above freezing temperatures
• Kenneth G. Libbrecht - Snowflake FAQ
• United Nations Environment Programme: Global Outlook for Ice and Snowbe-x-old:Сьнегpdc:Schneeeml:Naivnds-nl:Sneezh-yue:雪

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