Snowing

Snow
Norwegian train plowing through drifted snow
Physical Properties
Density (ρ)	0.1 – 0.8 g/cm3
Mechanical Properties
Tensile strength (σt)	1.5 – 3.5 kPa
Compressive strength (σc)	3 – 7 MPa
Thermal Properties
Melting temperature (Tm)	0 °C
Thermal conductivity (k) For densities 0.1 to 0.5 g/cm3	0.05 – 0.7 W K−1 m−1
Electrical Properties
Dielectric constant (εr) For dry snow density 0.1 to 0.9 g/cm3	1 – 3.2
The physical properties of snow vary considerably from event to event, sample to sample, and over time.

Snow refers to forms of ice crystals that precipitate from the atmosphere (usually from clouds) and undergo changes on the Earth's surface. It pertains to 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. 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 upper 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.

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; such sports as skiing, snowboarding, snowmachine travel; and warfare: impairing target acquisition, degrading the performance of combatants and materiel, and impeding mobility. Snow affects ecosystems, as well, by providing an insulating layer during winter under which plants and animals are able to survive the cold.