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Snow

Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes.[2] It consists of frozen crystalline water throughout its life cycle, starting when, under suitable conditions, the ice crystals form in the atmosphere, increase to millimeter size, precipitate and accumulate on surfaces, then metamorphose in place, and ultimately melt, slide or sublimate away.

For other uses, see Snow (disambiguation).

Snow

0.1–0.8 g/cm3

1.5–3.5 kPa[1]

3–7 MPa[1]

0 °C

0.05–0.7 W/(K·m)

1–3.2

Snowstorms organize and develop by feeding on sources of atmospheric moisture and cold air. Snowflakes nucleate around particles in the atmosphere by attracting supercooled water droplets, which freeze in hexagonal-shaped crystals. Snowflakes take on a variety of shapes, basic among these are platelets, needles, columns and rime. As snow accumulates into a snowpack, it may blow into drifts. Over time, accumulated snow metamorphoses, by sintering, sublimation and freeze-thaw. Where the climate is cold enough for year-to-year accumulation, a glacier may form. Otherwise, snow typically melts seasonally, causing runoff into streams and rivers and recharging groundwater.


Major snow-prone areas include the polar regions, the northernmost half of the Northern Hemisphere and mountainous regions worldwide with sufficient moisture and cold temperatures. In the Southern Hemisphere, snow is confined primarily to mountainous areas, apart from Antarctica.[3]


Snow affects such human activities as transportation: creating the need for keeping roadways, wings, and windows clear; agriculture: providing water to crops and safeguarding livestock; sports such as skiing, snowboarding, and snowmachine travel; and warfare. Snow affects ecosystems, as well, by providing an insulating layer during winter under which plants and animals are able to survive the cold.[1]

Light: visibility greater than 1 kilometer (0.6 mi)

Moderate: visibility restrictions between 0.5 and 1 kilometer (0.3 and 0.6 mi)

Heavy: visibility is less than 0.5 kilometers (0.3 mi)

Slab avalanches occur in snow that has been deposited, or redeposited by wind. They have the characteristic appearance of a block (slab) of snow cut out from its surroundings by fractures. These account for most back-country fatalities.

result from a deposition of fresh dry powder and generate a powder cloud, which overlies a dense avalanche. They can exceed speeds of 300 kilometers per hour (190 mph), and masses of 10,000,000 tonnes (9,800,000 long tons; 11,000,000 short tons); their flows can travel long distances along flat valley bottoms and even uphill for short distances.

Powder snow avalanches

Wet snow avalanches are a low-velocity suspension of snow and water, with the flow confined to the surface of the pathway. The low speed of travel is due to the friction between the sliding surface of the pathway and the water saturated flow. Despite the low speed of travel (~10 to 40 kilometers per hour (6 to 25 mph)), wet snow avalanches are capable of generating powerful destructive forces, due to the large mass, and density.

[60]

Operations – Addresses the application of solid and liquid chemicals, using various techniques, including prewetting of chloride-salts. It also addresses plowing capability, including types of snowplows and blades used.

Decision-making – Combines weather forecast information with road information to assess the upcoming needs for application of assets and the evaluation of treatment effectiveness with operations underway.

Personnel – Addresses training and deployment of staff to effectively execute the anti-icing program, using the appropriate materials, equipment and procedures.

On , observations of the Phoenix Mars lander reveal that water-based snow crystals occur at high latitudes.[107] Additionally, carbon dioxide precipitates from clouds during the Martian winters at the poles and contributes to a seasonal deposit of that compound, which is the principal component of that planet's ice caps.[108]

Mars

On , observations from the Magellan spacecraft reveal the presence a metallic substance, which precipitates as "Venus snow" and leaves a highly reflective substance at the tops of Venus's highest mountain peaks resembling terrestrial snow. Given the high temperatures on Venus, the leading candidates for the precipitate are lead sulfide and bismuth(III) sulfide.[109]

Venus

On 's moon, Titan, Cassini–Huygens spacecraft observations suggest the presence of methane or some other form of hydrocarbon-based crystalline deposits.[110]

Saturn

Extraterrestrial "snow" includes water-based precipitation, but also precipitation of other compounds prevalent on other planets and moons in the Solar System. Examples are:

Eskimo words for snow

The wrong type of snow

Lexicon


Notable snow events


Recreation


Related concepts


Science and scientists


Snow structures

Archived June 8, 2007, at the Wayback Machine

United Nations Environment Programme: Global Outlook for Ice and Snow

Institute of Low Temperature Science, Hokkaido University

Swiss Federal Institute for Forest, Snow and Landscape Research

National Snow and Ice Data Center of the United States

American Society of Civil Engineers ground snow loads interactive map for the continental US