Snowfall

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 pertains to frozen crystalline water throughout its life cycle, starting when it precipitates from clouds and accumulates on surfaces, then metamorphoses in place, and ultimately melts, slides or sublimates 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, and causes 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, and such sports as skiing, snowboarding, and snowmachine travel. Snow affects ecosystems, as well, by providing an insulating layer during winter under which plants and animals are able to survive the cold.

Snow develops in clouds that themselves are part of a larger weather system. The physics of snow crystal development in clouds results from a complex set of variables that include moisture content and temperatures. The resulting shapes of the falling and fallen crystals can be classified into a number of basic shapes and combinations, thereof. Occasionally, some plate-like, dendritic and stellar-shaped snowflakes can form under clear sky with a very cold temperature inversion present.