Definitions

snow-pellets

Snow

[snoh]
"Snowfall" redirects here. For other uses, see Snow (disambiguation) or Snowfall (disambiguation).

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 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. The METAR code for snow is SN.

Snowflakes

Snow crystals form when tiny supercooled cloud droplets (approx 10μm in diameter) freeze. These droplets are able to remain liquid at temperatures colder than 0°C because in order to freeze, a few molecules in the liquid droplet need to get together by chance to form an arrangement close to that in an ice lattice; then the droplet freezes around this 'nucleus'. Experiments show that this 'homogenous' nucleation of cloud droplets only occurs at temperatures colder than -35°C. 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 which liquid droplets form on. Clays, desert dust and biological particles may be effective, although to what extent is unclear. Artificial nuclei include Silver Iodide and dry ice, and these form the basis of cloud seeding.

Once a droplet has frozen, it grows in the supersaturated environment (air saturated with respect to liquid water is always supersaturated with respect to ice) and grows by diffusion of water molecules in the air (vapour) onto the ice crystal surface where they are deposited. Because the droplets are so much more numerous than the ice crystals (because of the relative numbers of ice vs droplet nuclei) the crystals are able to grow to hundreds of micrometres or millimetres in size at the expense of the water droplets (the Wegner-Bergeron-Findeison process). The corresponding depletion of water vapour causes the droplets to evaporate, meaning that the ice crystals effectively grow at the droplets' expense. These large crystals are an efficient source of precipitation, since they fall through the atmosphere due to their weight, and may collide and stick together in clusters (aggregates). These aggregates are snowflakes, and are usually the type of ice particle which falls at the ground. . The exact details of the sticking mechanism remains controversial (and probably there are different mechanisms active in different clouds), 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 an 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 colour.

Geometry

Ice crystals formed in the appropriate conditions can often be thin and flat. These planar crystals may be simple hexagons, or if the supersaturation is high enough, develop branches and dendritic (fern-like) features and have six approximately identical arms, as per the iconic 'snowflake' popularised by Wilson Bentley. The 6-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.

The shape of the snowflake is determined broadly by the temperature and humidity at which it forms.. Rarely, at a temperature of around , snowflakes can form in threefold symmetry — triangular snowflakes. 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 and . Between and , the crystals will form needles or hollow columns or prisms (long thin pencil-like shapes). From to the habit goes back to plate like, often with branched or dendritic features. Note that the maximum difference in vapour pressure between liquid and ice is at approx. -15C where crystals grow most rapidly at the expense of the liquid droplets. At temperatures below , the crystal habit again becomes column-like again, although many more complex habits also form such as side-planes, bullet-rosettes and also planar types depending on the conditions and ice nuclei .

Interestingly, if a crystal has started forming in a column growth regime, say at around , and then falls into the warmer plate-like regime, plate or dendritic crystals sprout at the end of the column producing so called 'capped columns'.

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 virtually 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 remains on the ground until it melts or sublimes. In colder climates this results in snow lying on the ground all winter; when the snow does not all melt in the summer they become glaciers.

The water equivalent of the snow is the thickness of a layer of water having the same content. For example, if the snow covering a given area has a water equivalent of , then it will melt into a pool of water 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".

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. By late spring, snow densities typically reach a maximum of 50% of water.

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 or before the snowfall.

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 and later Snowmobiles. Basic infrastructures such as electricity, telephone lines, and gas supply can also fail. In addition, snow can make roads much harder to travel and cars attempting to traverse 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 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

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. Large amounts of snow that accumulate on top of man-made structures can lead to structural failure.

Records

The highest seasonal total snowfall measured in the United States was at Mount Baker Ski Area, outside of the town Bellingham, Washington during the 19981999 season. Mount Baker received 1,140 inches (29 m) of snow, thus surpassing the previous record holder, Mount Rainier, Washington, which during the 19711972 season received 1,122 in. (28.5 m) of snow. 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

Types of snow

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 of 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
Supercooled 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.Needles
A class of snow flakes that are acicular in shape (their length is much longer than their diameter, like a needle). One of the 4 classes of snow flakes.Rain and snow mixed
Precipitation consisting of both snow and rain; also called "wintry mix" or "wintry shower". 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; Some Americans also refer to this as sleet, while others refer to sleet as 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.
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

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. Chopped powder
Powder snow that has been cut up by previous skiers.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.Cornice
An overhanging formation of windblown snow. Important in skiing and alpine climbing because the overhang can be unstable and hard to see from the leeward side. Crud
This covers varieties of snow that all but advanced skiers find impassable. Subtypes are (a) windblown powder with irregularly shaped crust patches and ridges, (b) heavy tracked spring snow re-frozen to leave a deeply rutted surface strewn with loose blocks, (c) a deep layer of heavy snow saturated by rain (although this may go by another term). Crud is negotiated with a even weighting along the length of the skis, and smooth radius turns started, if necessary, with a pop or jump. When an advanced skier falls over on crud, it is probably because it is 'heavy crud', q.v.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.Finger Drift
A narrow snow drift(1-3 feet in width) crossing a roadway. Several finger drifts in succession resemble the fingers of a hand.Heavy crud
See 'Crud'.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.Pillow Drift
A snow drift crossing a roadway and usually 10-15 feet in width and 1-3 three feet in depth.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.
Slush
Snow which partially melts upon reaching the ground, to the point that it accumulates in puddles of partially-frozen water.Snirt
Snow covered with dirt, which occurs most often in Spring, in Prairie States like North Dakota, where strong winds pick up black topsoil from uncovered farm fields and blow it into nearby towns where the melt rate is slower. The phenomenon is almost magical; one goes to sleep with white snow outside and awakens to black snow. Also, snow that is dirty, often seen by the side of roads and parking lots near areas that have been plowed.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.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

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