SNOW

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

Animation of snowcover changing with the seasons

Trees covered with snow

Snow forms when water vapor condenses directly into ice crystals, usually in a cloud. These crystals typically have a diameter of several milimetres and have six lines of symmetry. A snowflake is an aggregate of such ice crystals and may be several centimeters large.^[1]. The term "snowflake" is also used below for the symmetrical ice crystals themselves. Snowflakes are white in color unless contaminated by impurties.

Geometry

Symmetry of an idealized snowflake

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 regular 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, bricks 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 (14 °F) to −22 °C (−8 °F) the ice flowers are formed again, and at temperatures below −22 °C (−8 °F), the vapors will turn into prisms again. If a crystal has started forming at around −5 °C (23 °F), 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 (−4 °F), 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 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.

Snowflake Gallery.

Snow on the ground

Snow in the city of Dorval, part of the Island of Montreal, in Quebec, Canada

Snow 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 50 centimetres (20 in), then it will melt into a pool of water 50 centimetres (20 in) deep covering the same area. 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 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".

First snow of winter, Truckee, California, United States

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. That is the reason there is a small temperature rise after snowfall.^[4]

Effects on human society

• Activity: 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. Basic infrastructures such as electricity, telephone lines, and gas supply can also fail. 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 ill-prepared to handle any amount of snow.
• 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.
• Conservation:
  

  Section of an icehouse.
  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.
• Damage: 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

Building a snowman.

• Many winter sports, such as skiing, snowboarding, snowmobiling and snowshoeing depend on 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 can play on a sled or ride in a sleigh.
• Snow can be sculptured into snowmen, used to trace the motion of a person's body (snow angels), or formed into snowballs for throwing or for having snowball fights.
• Snow can be used to build defensive snow forts for outdoor games such as Capture the flag.
• 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.

Types of snow

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

vector graphic image of stylized snowflakes

Falling Snow

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.

Columns

A class snow flakes that is shaped like a six sided column. One of the 4 classes of snow flakes.

Dendrites

A class of snow flakes that has 6 points, making it somewhat star shaped. The classic snow flake shape. One of the 4 classes of snow flakes.

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.

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.

Needles

A class of snow flakes that are ascicular in shape (their length is much longer than their diameter, like a needle). One of the 4 classes of snow flakes.

Rimed snow

Snow flakes that are partially or completely coated in tiny frozen water droplets called rime. Rime forms on a snow flake when it passes through a super-cooled cloud. One of the 4 classes of snow flakes.

Sleet

In Britain, rain mixed with snow; in America, ice pellets formed when snowflakes pass through a layer of warm air, partially or completely thaw, then refreeze upon passing through sufficiently cold air during further descent. The British term for the American definition is "Ice Pellets."

Snow pellets

See graupel.

Snow squall

A brief, very intense snowstorm.

Snow storm

A long storm of relatively heavy snow.

Soft hail

See graupel.

Thundersnow

A thunderstorm which produces snow as the primary form of precipitation.

Snow on the ground

Snow covering a leaf.

Snow blowing from a roof in Ottawa

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 layer of snow on the surface of the snowpack that is stronger than the snow below, which may be powder snow. Depending on their thickness and resulting strength, crusts can be termed "supportable," meaning that they will support the weight of a human, "breakable," meaning that they will not, or "zipper," meaning that a skier can break and ski through the crust. Crusts often result from partial melting of the snow surface by direct sunlight or warm air followed by re-freezing.

Depth Hoar

Faceted snow crystals, usually poorly or completely unbonded (unsintered) to adjacent crystals, creating a weak zone in the snowpack. Depth hoar forms from metamorphism of the snowpack in response to a large temperature gradient between the warmer ground beneath the snowpack and the surface. The relatively high porosity (percentage of air space), relatively warm temperature (usually near freezing point), and unbonded weak snow in this layer can allow various organisms to live in it.

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.

Panked snow

This is a Yooper term that is used in the Upper Peninsula of Michigan and other areas around Lake Superior, which get a lot of snow. Some areas, such as the Keweenaw Peninsula in Michigan, receive an average of 180 to 250 inches (460 to 640 cm.) ^[7] of snowfall a year. The term "pank" (which rhymes with "bank" or "tank") was coined to describe what happens to older snow that has settled, which is said to have "panked". "Panking" may also be accomplished by pushing the snow down to firmness with a snow shovel, thus making it easier to walk on.

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, typically 4 - 7% water content) powder snow is prized by skiers and snowboarders. It is often found in the Rocky Mountains of North America and in Niseko, Japan.

The textures of a snowdrift on the Long Mynd, Shropshire

Pukak

See Depth Hoar

Slush

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

Snirt

A colloquialism for a combination of snow and dirt/street litter found usually found in urban areas or along the sides of roads.

Snit

A colloquialism for a combination of snow and dog feces found usually found in urban areas.

Snowdrift

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

Surface Hoar

Faceted, corn-flake shaped snow crystals that are a type of frost that forms on the surface of the snow pack on cold, clear, calm nights. Subsequent snow fall can bury layers of surface hoar encorporating them into the snowpack where they can form a weak layer. Sometimes referred to as hoar frost.

Snud

A colloquialism for a combination of snow and mud usually found in urban areas or along the sides of roads.

Watermelon snow

A reddish/pink colored snow that smells like watermelons, and is caused by a red colored green algae called chlamydomonas nivalis

Wind slab

A layer of relatively stiff, hard snow formed by deposition of wind blown snow on the leeward side of a ridge or other sheltered area. Wind slabs can form over weaker, softer freshly fallen powder snow creating an avalanche hazard on steep slopes.

See also

References

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 Snow
• Michigan Technological University Winter Carnival
• Snowflake Graphics