Snow is a type of precipitation in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. The process of precipitation is called snowfall.

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. The METAR code for snow is SN.

Snowflakes

Once a droplet has frozen, it grows in the supersaturated environment (air saturated with respect to liquid water is always supersaturated with respect to ice) and grows by diffusion of water molecules in the air (vapour) onto the ice crystal surface where they are deposited. Because the droplets are so much more numerous than the ice crystals (because of the relative numbers of ice vs droplet nuclei) the crystals are able to grow to hundreds of micrometres or millimetres in size at the expense of the water droplets (the Wegner-Bergeron-Findeison process). The corresponding depletion of water vapour causes the droplets to evaporate, meaning that the ice crystals effectively grow at the droplets' expense. These large crystals are an efficient source of precipitation, since they fall through the atmosphere due to their weight, and may collide and stick together in clusters (aggregates). These aggregates are snowflakes, and are usually the type of ice particle which falls at the ground. . The exact details of the sticking mechanism remains controversial (and probably there are different mechanisms active in different clouds), possibilities include mechanical interlocking, sintering, electrostatic attraction as well as the existence of a 'sticky' liquid-like layer on the crystal surface.

The individual ice crystals often have an hexagonal symmetry. Although the ice is clear scattering of light by the crystal facets and hollows/imperfections mean that the crystals often appear white in colour.

Geometry

The shape of the snowflake is determined broadly by the temperature and humidity at which it forms.. Rarely, at a temperature of around , snowflakes can form in threefold symmetry — triangular snowflakes. The most common snow particles are visibly irregular, although near-perfect snowflakes may be more common in pictures because they are more visually appealing