How do snowflakes form?

High in cold clouds, water vapour condenses and freezes around a speck of dust or pollen, forming a tiny ice crystal. Because water molecules bond in a hexagonal pattern when they freeze, the crystal grows with six-fold symmetry. As it falls through air of different temperatures and humidity, it grows arms, plates or needles. Around −15 °C with high humidity, the classic branching star shapes called dendrites form. Each flake takes a slightly different path, so no two complex snowflakes are likely to be exactly alike.