Snow

Snow is a type of precipitation within the Earth's atmosphere in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. 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.

Snowflakes come in a variety of sizes and shapes. Types which fall in the form of a ball due to melting and refreezing, rather than a flake, are known as graupel, with ice pellets and snow grains as examples of graupel. Snowfall amount and its related liquid equivalent precipitation amount are determined using a variety of different rain gauges.

The process of precipitating snow is called snowfall. Snowfall tends to form within regions of upward motion of air around a type of low-pressure system known as an extratropical cyclone. Snow can fall poleward of their associated warm fronts and within their comma head precipitation patterns, which is called such due to its comma-like shape of the cloud and precipitation pattern around the poleward and west sides of extratropical cyclones.

Source: de.wikipedia.org/wiki/Snow - 26.05.2010