Real snow crystals grow six-fold symmetric patterns because water molecules bond together in a hexagonal lattice, a shape dictated by the geometry of hydrogen bonding between oxygen atoms. As a tiny hexagonal crystal falls through humid air, its corners protrude slightly further into the surrounding water vapor than its flat faces, so they receive a disproportionate share of the diffusing vapor molecules and grow faster — a runaway feedback known as diffusion-limited growth (a Laplacian instability) that produces the intricate branching dendritic arms. Because each arm sees essentially the same temperature and humidity at any instant, all six arms tend to branch in near-identical ways, giving the crystal its striking symmetry. The physicist Kenneth Libbrecht has spent decades photographing and growing snow crystals in the lab to study exactly how this happens, and his work, along with the earlier Nakaya diagram (which maps crystal shape against temperature and humidity), shows how sensitively the final form depends on the exact conditions the crystal experiences on its way to the ground.

Characteristics

• Six-fold symmetry comes from the hexagonal crystal structure of ordinary ice (ice Ih).
• Nakaya diagram: thin plates form near -2°C, needles near -5°C, and branched dendrites near -15°C.
• No two snowflakes are exactly identical — chaotic microscale temperature/humidity fluctuations make every growth path unique.
• Branch tip growth rate depends on the local supersaturation of water vapor around the crystal.
• Typical snowflakes are about 2-5mm across, though larger aggregates can form when crystals stick together.
• Warmer, more humid air tends to produce faster, more elaborate branching than cold, dry air.
• Simple prisms and plates form under milder conditions, while dendrites need higher supersaturation.