No two snowflakes are ever the same

No two complex, branching snowflakes are likely to be identical, though simple prisms can look exactly alike. Not one matching pair in roughly a septillion (10^24) crystals a year - and the count has been running since the planet had a sky.

The reason hides in the geometry of the molecule itself. Water freezes into six-sided crystals because of how hydrogen bonds lock between molecules, but the arms of that hexagon grow through air that is never the same twice. A snowflake takes about an hour to fall, drifting through layers of slightly different temperature and humidity at every micron of its descent. Each arm grows in sync with the others, maintaining symmetry as the entire flake experiences the same atmospheric changes, building branches, plates, and needles to a different recipe with every degree it passes through.

Multiply six arms by thousands of micro-decisions, and the number of possible final shapes outruns the number of atoms in the observable universe. Two flakes can be near-identical at the start; they cannot stay that way for long.

So the next snowfall is not a repetition of the last one. It is the only one of its kind, and the only one there will ever be.