Dendritic solidification on a hexagonal lattice — the Reiter model
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Reiter's cellular automaton (1985) simulates snowflake growth on a hexagonal grid.
Each cell has a vapor density; cells with density >= 1 are frozen.
At each step: (1) frozen-cell neighbors accumulate vapor; (2) diffuse vapor smoothly; (3) add background vapor rho.
The six-fold symmetry of ice crystal lattice (hexagonal close-packed oxygen bonds) explains why snowflakes always have 6-fold symmetry.
Different rho and kappa values produce plates, dendrites, needles, and stellar crystals.