How do snowflakes form?
Snowflakes are precipitation particles made from individual or aggregated ice crystals growing in the atmosphere. Water's crystal structure underlies sixfold symmetry. Not every snowflake resembles a perfectly formed six-branched star.
How temperature and humidity shape crystals
Water vapor can deposit onto growing ice surfaces. Temperature and supersaturation influence plates, columns, and branched forms. Conditions change as a crystal travels through cloud layers. Freezing of supercooled droplets onto crystals produces riming, while aggregation combines separate crystals into larger flakes. A large aggregate is not necessarily one enormous individual crystal.
Reaching the ground and forming cover
Snow can reach the surface even when air is not below freezing everywhere; the full temperature and humidity profile matters. Snow cover compacts, melts, or sublimates after landing. Equal snow depths can contain different amounts of water, since fresh fluffy snow and dense wet snow have different densities. A single fixed depth-to-rain conversion is therefore unreliable.