Snow

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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. The process of this precipitation is called snowfall. 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 come in a variety of sizes and shapes. Types which fall in the form of a ball due to melting and refreezing, rather than a flake, are known as graupel, with sleet and snow grains as examples of graupel. A blizzard and snow storm indicate heavy snowfalls, while flurries are used for the lightest snowfall. Snow can fall as much as one meter at a time during a single storm in flat areas, and meters at a time in rugged topography, such as mountains. Lake-effect snow can lead to more localized high amounts downwind of unfrozen bodies of water. When snow falls in significant quantities, travel by foot and car becomes highly restricted, and mobility is decreased to the use of snowmobiles and skis. When heavy snow occurs early in the fall, significant tree damage occurs to trees still in leaf.

Once on the ground, snow can be categorized as powdery when fluffy, granular when it begins the cycle of melting and refreezing, and crud or eventually ice once it packs down into a dense drift after multiple melting and refreezing cycles. When powdering, snow drifts with the wind, sometimes to the depth of several meters. After attaching to hillsides, blown snow can evolve into a snow slab, which is an avalanche hazard on steep slopes. The existence of a snowpack keeps temperatures colder than they would be otherwise, as the whiteness of the snow reflects all sunlight, and the heat it absorbs goes into melting the snow rather than increasing its temperature. The water equivalent of snowfall is measured to monitor how much liquid is available to flood rivers during the upcoming spring. Snow cover can protect crops. If snowfall stays on the ground for a series of years uninterrupted, the snowpack develops into a glacier.

[edit] Snowflakes

Snowflake captured by a microscope

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 silver iodide and dry ice, and these form the basis of cloud seeding.^[3]

Once a droplet has frozen, it grows in the supersaturated environment, which is one where air is saturated with respect to ice when the temperature is below the freezing point. The droplet then grows by diffusion of water molecules in the air (vapour) onto the ice crystal surface where they are collected. Because water droplets are so much more numerous than the ice crystals due to their sheer abundance, the crystals are able to grow to hundreds of micrometres or millimetres in size at the expense of the water droplets. This process is known as the Wegner-Bergeron-Findeison process. The corresponding depletion of water vapour causes the droplets to evaporate, meaning that the ice crystals grow at the droplets' expense. These large crystals are an efficient source of precipitation, since they fall through the atmosphere due to their mass, and may collide and stick together in clusters, or aggregates. These aggregates are snowflakes, and are usually the type of ice particle that falls to the ground.^[4] Guinness World Records list the world’s largest snowflakes as those of January 1887 at Fort Keogh, Montana; allegedly one measured 38 cm (15 inches) wide.^[5]

The exact details of the sticking mechanism remains controversial. Possibilities include mechanical interlocking, sintering, electrostatic attraction as well as the existence of a "sticky" liquid-like layer on the crystal surface. The individual ice crystals often have hexagonal symmetry. Although the ice is clear, scattering of light by the crystal facets and hollows/imperfections mean that the crystals often appear white in color due to diffuse reflection of all spectrum of light by the small ice particles.^[6]

[edit] Geometry

Example of the diversity of snowflake shapes.

Ice crystals formed in the appropriate 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 approximately identical arms. The six-fold 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]

The shape of the snowflake is determined broadly by the temperature and humidity at which it is formed.^[4] Rarely, at a temperature of around −2 °C (28 °F), snowflakes can form in threefold symmetry — triangular snowflakes.^[7] The most common snow particles are visibly irregular, although near-perfect snowflakes may be more common in pictures because they are more visually appealing.

Planar crystals (thin and flat) grow in air between 0 °C (32 °F) and −3 °C (27 °F). Between −3 °C (27 °F) and −8 °C (18 °F), the crystals will form needles or hollow columns or prisms (long thin pencil-like shapes). From −8 °C (18 °F) to −22 °C (−8 °F) the shape reverts back to plate-like, often with branched or dendritic features. The maximum difference in vapour pressure between liquid and ice is at about −15 °C (5 °F) where crystals grow most rapidly at the expense of the liquid droplets. At temperatures below −22 °C (−8 °F), the crystal development becomes column-like, although many more complex growth patterns also form such as side-planes, bullet-rosettes and also planar types depending on the conditions and ice nuclei.^[8][9][10] If a crystal has started forming in a column growth regime, at around −5 °C (23 °F), and then falls into the warmer plate-like regime, then plate or dendritic crystals sprout at the end of the column, producing so called "capped columns."^[4]

No two snowflakes are alike. It is more likely that two snowflakes could become virtually identical if their environments were similar enough. 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.^[11] The crystals were not flakes in the usual sense but rather hollow hexagonal prisms.

Snowflakes

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[edit] Types

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

Types of snow can be designated by the shape of its flakes, its rate of falling, and by how it collects on the ground. Snowfall's intensity is determined by visibility. When the visibility is over 1 kilometre (0.62 mi), snow is determined to be light. Moderate snow describes snowfall with visibility restrictions between 0.5 kilometres (0.31 mi) and 1 kilometre (0.62 mi). Heavy snowfall describes conditions when visibility is restricted below 0.5 kilometres (0.31 mi).^[12] A blizzard and snowstorm indicate heavy snowfalls, with blizzards defined by having high winds during their heavy snowfall.^[13] Snow flurries are used to describe the lightest form of snow showers.^[14] Types which fall in the form of a ball due to melting and refreezing cycles, rather than a flake, are known as graupel, with sleet and snow pellets as types of graupel associated with wintry precipitation.^[15][16] Once on the ground, snow can be categorized as powdery when fluffy, granular when it begins the cycle of melting and refreezing, and eventually ice once it packs down into a dense drift after multiple melting and refreezing cycles. When powdering, snow drifts with the wind, sometimes to the depth of several meters. After attaching to hillsides, blown snow can evolve into a snow slab, which is an avalanche hazard on steep slopes.

[edit] Density

Approximate ice (and snow) coverage on the northern hemisphere.^[17] Yellow lines demarcate limit of permanent permafrost and red line where average temperature even during the coldest month makes snow melt.

Snow remains on the ground until it melts or sublimates. The water equivalent of a given amount of snow is the depth of a layer of water having the same mass and upper area. For example, if the snow covering a given area has a water equivalent of 50 centimetres (20 in), then it will melt into a pool of water 50 centimetres (20 in) deep covering the same area.^[18] This is a much more useful measurement to hydrologists than snow depth, as the density of cool freshly fallen snow widely varies. New snow commonly has a density of around 8% of water. This means that 13 inches (330 mm) of snow melts down to 1 inch (25 mm) of water.^[19]

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 by melting and refreezing, caused by temperatures above freezing or by direct solar radiation. In colder climates, snow lies on the ground all winter. By late spring, snow densities typically reach a maximum of 50% of water.^[20] 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. When the snow does not all melt in the summer it evolves into firn, where individual granular elements become more spherical in nature,^[21] evolving into a glacier as the ice flows downhill.^[22]

[edit] Regional characteristics

First snow of winter, Truckee, California

Since fresh snow reflects 90 percent or more of short-wave radiation, and radiates energy nearly completely further into the infrared spectrum, little energy from the sun is converted into heat from the new snow, and much heat is lost.^[23] Snow that falls in maritime climates is usually denser than snow that falls in mid-continent locations because of the higher average clouds 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 snowflake 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 denser and feel "wetter".

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. The world record for the highest seasonal total snowfall was measured in the United States at Mount Baker Ski Area, outside of the town Bellingham, Washington during the 1998–1999 season. Mount Baker received 2896 cm (1,140 inches) of snow,^[24] thus surpassing the previous record holder, Mount Rainier, Washington, which during the 1971–1972 season received 2850 cm (1,122 in.) of snow.^[25]

[edit] 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 overlying air mass, tree cover and clouds. Heat exchange takes place by convection between the snowpack and the overlaying air mass is governed by the temperature gradient and wind speed. Moisture exchange between the snowpack and the overlying air mass is accompanied with latent heat transfer that is influenced by vapor pressure gradient and air wind. Rain on snow can add significant amounts of thermal energy to the snowpack. A generally insignificant heat exchange takes place by conduction between the snowpack and the ground. The small temperature change from before to after a snowfall is a result of the heat transfer between the snowpack and the air.^[26]

[edit] Effects on human society

Cars stuck in the snow

Substantial snowfall can disrupt public infrastructure and services, slowing human activity even in regions that are accustomed to such weather. Air and ground transport may be greatly inhibited or shut down entirely. Populations living in snow-prone areas have developed various ways to travel across the snow, such as skis, snowshoes, and sleds pulled by horses, dogs, or other animals and later, snowmobiles. Basic utilities such as electricity, telephone lines, and gas supply can also fail. In addition, snow can make roads much harder to travel and vehicles attempting to use them can easily become stuck. The combined effects can lead to a "snow day" on which gatherings such as school, work, or church are officially canceled. In areas that normally have very little or no snow, a snow day may occur when there is only light accumulation or even the threat of snowfall, since those areas are unprepared to handle any amount of snow. In areas near mountains, people have harvested snow and stored it as layers of ice covered by straw or sawdust in icehouses. This allowed the ice to be used in summer for refrigeration or medical uses.

[edit] Agriculture

Snowfall can be beneficial to agriculture by serving as a thermal insulator, conserving the heat of the Earth and protecting crops from subfreezing weather. Some agricultural areas depend on an accumulation of snow during winter that will melt gradually in spring, providing water for crop growth. If it melts into water and refreezes upon sensitive crops, such as oranges, the resulting ice will protect the fruit from exposure to lower temperatures.^[27]

[edit] Brightness

Snow blindness (also known as ultraviolet keratitis, photokeratitis or niphablepsia) is a painful eye condition, caused by exposure of unprotected eyes to the ultraviolet (UV) rays in bright sunlight reflected from snow or ice.^[28] Fresh snow reflects about 80% of UV radiation.^[29] This condition is a problem in polar regions and at high altitudes,^[30] as with every thousand feet (approximately 305 meters) of elevation (above sea level), the intensity of UV rays increases by four percent.^[31] Snow's large reflection of light makes night skies much brighter. However, when there is also cloud cover because snow is falling, light is then reflected back to the ground. This greatly amplifies light emitted from city lights, causing the 'bright night' effect. A similar brightening effect occurs in a reduced version when no snow is falling when there is a full moon and a large amount of snow.

[edit] Damage

Damage caused by Lake Storm "Aphid" in October 2006

When heavy, wet snow with a snow-water equivalent (SWE) ratio of between 6:1 and 12:1 and a weight in excess of 9.8 pounds per square foot^[32] piles onto trees still in full leaf during the early autumn, significant tree damage occurs on a scale usually associated with hurricanes.^[33] An avalanche can occur when excessive snow has accumulated on a mountain and there is a sudden change of temperature, which causes the snow to rush downhill en masse. Preceding an avalanche is a phenomenon known as an avalanche wind caused by the approaching avalanche itself, which adds to its destructive potential.^[34] Large amounts of snow which accumulate on top of man-made structures can lead to structural failure.

[edit] Recreation

Building a snowman.

• Many winter sports, such as skiing, snowboarding, snowmobiling and snowshoeing depend upon snow. Where snow is scarce but the temperature is low enough, snow cannons may be used to produce an adequate amount for such sports.
• Children (also adults and occasionally other species) can play on a sled or ride in a sleigh.
• Snow can be used to explore unknown or uncharted areas such as dense forest, fields, and marshlands because, barring heavy snowfall or blizzards, a person's footsteps remain a visible lifeline.
• One of the recognizable recreational uses of snow is in building snowmen. A snowman is created by making a man shaped figure out of snow - often using a large, shaped snowball for the body and a smaller snowball for the head which is often decorated with simple household items - traditionally including a carrot for a nose, and coal for eyes, nose and mouth; occasionally including old clothes such as a top hat or scarf.
• Snow can be used to make snow cones, which are usually eaten in the summer months when temperatures flare above 100 degrees Fahrenheit.
• Snow can be used to build defensive snow forts for outdoor games such as Capture the flag or for snowball fights.
• The world's biggest snowcastle, the SnowCastle of Kemi, is built in Kemi, Finland every winter.
• Since 1928 Michigan Technological University in Houghton, Michigan has held an annual Winter Carnival in mid-February, during which a large Snow Sculpture Contest takes place between various clubs, fraternities, and organizations in the community and the university. Each year there is a central theme, and prizes are awarded based on creativity.
• Snowball softball tournaments are held in snowy areas, usually using a bright orange softball for visibility, and burlap sacks filled with snow for the bases.

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[edit] See also

Find more about Snow on Wikipedia's sister projects: Definitions from Wiktionary

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Learning resources from Wikiversity

[edit] References

[edit] External links

• Kenneth G. Libbrecht - Snowflake FAQ
• Snow at above freezing temperatures
• 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
• United Nations Environment Programme: Global Outlook for Ice and Snow
• Why are snowflakes symmetrical?