Look closely at a snowflake and it always has six arms, never five or seven. That is because of the shape water makes when it freezes. Water molecules are a bit like tiny magnets, and when they lock together into ice they always settle into a six-sided honeycomb pattern. As the flake grows, all six arms are sitting in the same little patch of air, so they grow the same way at the same time and end up matching. But every flake drifts down through its own path of air (warmer here, damper there) so no two flakes meet exactly the same air, and that is why no two are alike. Grow a flake in the simulator and slide the temperature; the shape changes but the six sides never do.
Most people think a snowflake's six-fold symmetry is just a pretty coincidence, like the saying that no two are alike. In fact the six sides come straight from the hexagonal lattice water locks into when it freezes, while the endless variety comes from each flake's unique path.
What's actually happening
There is a lovely double puzzle buried in a snowflake. The first is that they all share a strict rule: six sides, every single time. Not five, not eight — six. The second is the famous claim that no two are ever the same. It sounds like those two facts should fight each other, a rigid rule and endless variety, but they turn out to be two faces of one explanation, and it starts with the shape of a single water molecule.
A water molecule is bent, with two hydrogen atoms sticking out from an oxygen at a fixed angle, and the atoms carry slight charges that make them grab onto neighbours in a particular way. When water freezes, these bonds force the molecules to stack into a hexagonal lattice — a six-sided honeycomb repeated over and over. That hidden hexagon is what shows up at full size as six arms. So the six-fold symmetry is not decoration; it is the crystal structure of ice itself, printed large. As the flake grows, water vapour freezes fastest onto the six corners, so six branches reach out, and because all six are in the same tiny pocket of air at the same instant, they receive the same conditions and grow as near-identical siblings. That symmetry is exactly what the simulator preserves: change the air and the arms grow plates or needles or feathery ferns, but the six-fold pattern holds.
The endless variety comes from the journey. The precise shape a crystal grows (flat plates, slender needles, broad ferns) depends very sensitively on temperature and how much moisture is in the air, a relationship scientists mapped in the Nakaya diagram. A flake might start as a plate high up, sprout branches lower down where it is colder and damper, then thicken again, and every flake takes its own tumbling path through the cloud. Two flakes would have to follow the same route through the same shifting air, molecule for molecule, to come out identical — which essentially never happens. So both halves of the saying are true at once: rigidly six-sided because of how ice is built, and all different because of the road each one travelled to the ground.
A snowflake is rigidly six-sided because of how ice is built, yet endlessly varied because of the path each one fell through.
- 1Press Grow and watch a flake build outward from the centre, all six arms reaching at once.
- 2Slide the temperature and humidity to new settings and grow another — the arms turn into plates, needles, or feathery ferns.
- 3Notice that whatever shape you get, the flake always has exactly six matching arms. That is the ice lattice showing through.
Common questions
The detailed pattern depends very sensitively on temperature and humidity, and every flake tumbles through its own changing path in the cloud. Two flakes would have to follow the same route molecule for molecule to match, which essentially never happens.
All six arms sit in the same tiny pocket of air as the flake grows, so they experience nearly identical temperature and humidity and branch together as near-identical siblings.
Occasionally two crystals fuse while spinning and lock at a slight twist, making a rare twelve-sided flake. It is still built from sixes, just two of them stacked.