A crystal’s shape starts with a six-sided framework
Snowflakes form when water vapor in a cloud attaches directly to an ice crystal. Water molecules settle into a repeating arrangement that gives ordinary ice its sixfold symmetry. That is why many snow crystals begin with a hexagonal plate or column, rather than an arbitrary shape.
As the crystal grows, its edges and corners do not all collect vapor at the same rate. Temperature affects which parts grow fastest, while humidity determines how much water vapor is available. In especially moist air, growth can become unstable: tiny differences along an edge let some spots capture more vapor, pushing them outward into branches.
A changing cloud keeps rewriting the pattern
A falling crystal passes through air that varies in temperature and moisture. Even a modest change can shift its growth from a flat plate toward a column, or encourage new branches and side branches. The six main arms share the same broad conditions because they belong to one crystal, but small local differences make their fine details diverge.
This makes a snow crystal a record of its route through the cloud—not a deliberate design, but the accumulated result of where and when vapor froze onto it. Two crystals can look remarkably similar, especially at a glance, yet matching every tiny branch and ridge would require their growth conditions to align in minute detail. A flake’s symmetry comes from the physics of ice; its intricate differences come from the changing air that built it.
