Why Snowflakes Have Six Sides

On a winter walk across the Charles Bridge in Prague in 1611, Johannes Kepler watched a snowflake land on his coat. He noticed it had six points. So did the next one. So did every one. He wrote a short essay called De Nive Sexangula — "On the Six-Cornered Snowflake" — wondering whether the symmetry came from cold itself, from some hidden geometric tendency in matter, or from something he could not yet name. He guessed it had to do with how spheres pack most efficiently. He was about three centuries early, but right.

Water molecules are bent. Two hydrogens stick off a single oxygen at roughly 104.5 degrees, and the hydrogens carry a small positive charge while the oxygen carries a small negative one. When water freezes slowly enough for molecules to find their lowest-energy positions, each oxygen ends up bonded to four others arranged at the corners of a tetrahedron. Tile that arrangement out across a plane and you get a hexagonal lattice. The six-fold symmetry of the molecule's bonding becomes the six-fold symmetry of the crystal.

The outline of any individual flake — whether it grows long needles, fat plates, or fern-like dendrites — depends on the temperature and humidity of the exact air it falls through. A flake at -2 °C grows plates. At -5 °C it grows needles. At -15 °C and high humidity, the most baroque dendrites. Wilson Bentley, a Vermont farmer, started photographing flakes through a microscope in 1885 and ended up with more than 5,000 images. None of them are identical, but every one has six arms.