[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.

[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.

The physics of snow crystal development in clouds results from a complex set of variables that include moisture content and temperatures.

[4] Snow clouds usually occur in the context of larger weather systems, the most important of which is the low-pressure area, which typically incorporate warm and cold fronts as part of their circulation.

The strong convection that develops has enough moisture to produce whiteout conditions at places which the line passes over as the wind causes intense blowing snow.

In cases where there is a large amount of vertical growth and mixing, the squall may develop embedded cumulonimbus clouds resulting in lightning and thunder which is dubbed thundersnow.

This uplifting can produce narrow but very intense bands of precipitation which may deposit at a rate of many inches of snow each hour, often resulting in a large amount of total snowfall.

[11] Orographic or relief snowfall is created when moist air is forced up the windward side of mountain ranges by a large-scale wind flow.

[12] The resulting enhanced snowfall,[13] along with the decrease in temperature with elevation,[14] combine to increase snow depth and seasonal persistence of snowpack in snow-prone areas.

[40] Snowfall's intensity is determined by visibility, as follows:[41] Snowsqualls may deposit snow in bands that extend from bodies of water as lake-event weather or result from the passage of an upper-level front.

[46] A 2007 estimate of snow cover over the Northern Hemisphere suggested that, on average, snow cover ranges from a minimum extent of 2 million square kilometres (0.77×10^6 sq mi) each August to a maximum extent of 45 million square kilometres (17×10^6 sq mi) each January or nearly half of the land surface in that hemisphere.

[4] Starting as a powdery deposition, snow becomes more granular when it begins to compact under its own weight, be blown by the wind, sinter particles together and commence the cycle of melting and refreezing.

Increases in density above this initial compression occur primarily by melting and refreezing, caused by temperatures above freezing or by direct solar radiation.

Its density generally ranges from 550 to 830 kilograms per cubic metre (34 to 52 lb/cu ft), and it can often be found underneath the snow that accumulates at the head of a glacier.

They occur in three major mechanisms:[60] Many rivers originating in mountainous or high-latitude regions receive a significant portion of their flow from snowmelt.

[1] Scientists study snow at a wide variety of scales that include the physics of chemical bonds and clouds; the distribution, accumulation, metamorphosis, and ablation of snowpacks; and the contribution of snowmelt to river hydraulics and ground hydrology.

Scientists develop and others employ snow classification systems that describe its physical properties at scales ranging from the individual crystal to the aggregated snowpack.

Scientists improve storm forecasting, study global snow cover and its effect on climate, glaciers, and water supplies around the world.

[4] Given the importance of snowmelt to agriculture, hydrological runoff models that include snow in their predictions address the phases of accumulating snowpack, melting processes, and distribution of the meltwater through stream networks and into the groundwater.

Initial snowmelt models used a degree-day approach that emphasized the temperature difference between the air and the snowpack to compute snow water equivalent, SWE.

The snowplow is common to all workers, though roadways take anti-icing chemicals to prevent bonding of ice and airfields may not; railroads rely on abrasives for track traction.

The elements to the toolboxes are:[69] The manual offers matrices that address different types of snow and the rate of snowfall to tailor applications appropriately and efficiently.

Prior to takeoff, during snowstorms they require deicing fluid to prevent accumulation and freezing of snow and other precipitation on wings and fuselages, which may compromise the safety of the aircraft and its occupants.

[73] In flight, aircraft rely on a variety of mechanisms to avoid rime and other types of icing in clouds,[74] these include pulsing pneumatic boots, electro-thermal areas that generate heat, and fluid deicers that bleed onto the surface.

Where icing may affect the steel-to-steel contact of locomotive wheels on track, abrasives (typically sand) have been used to provide traction on steeper uphills.

Some agricultural areas depend on an accumulation of snow during winter that will melt gradually in spring, providing water for crop growth, both directly and via runoff through streams and rivers, which supply irrigation canals.

Rime ice may be removed manually or by creating a sufficient short circuit in the affected segment of power lines to melt the accretions.

[98] Noted winter warfare campaigns where snow and other factors affected the operations include: Plants and animals endemic to snowbound areas develop ways to adapt.

The first group may be cold hardy owing to the ability to produce antifreeze agents in their body fluids that allows survival of long exposure to sub-freezing conditions.

Among vertebrates, alpine salamanders are active in snow at temperatures as low as −8 °C (18 °F); they burrow to the surface in springtime and lay their eggs in melt ponds.