Snow

Snow is precipitation in the form of tiny ice crystals called snowflakes. Ice crystals form when supercooled cloud droplets freeze. In order to freeze, a few molecules in the liquid droplet need to get together to form an arrangement close to that in an ice lattice. Then the droplet freezes around this arrangement of molecules. Homogeneous nucleation of cloud droplets only happens at temperatures colder than -35°C. In warmer clouds an aerosol particle, also called 'ice nucleus', must be present in the droplet to act as a nucleus.

The crystals grow in size as they join together, becoming heavy and then dropping down through the cloud. Although there are different crystal shapes, snowflakes are always six-sided, with some having six branches sticking out of the central core. Ice crystals formed in the appropriate conditions can often be thin and flat. Planar crystals (thin and flat) grow in air between 0 °C (32 °F) and −3 °C (27 °F). Between −3 °C (27 °F) and −8 °C (18 °F), the crystals will form needles. Snow remains on the ground until it melts. In colder climates the snow lies on the ground all winter; and when the snow does not all melt in the summer, they become glaciers.