Snowflakes form in the sky where water droplets freeze into tiny ice crystals. As they fall to the ground, water vapor freezes onto the primary crystals, building new crystals — the 6 arms of the snowflake.

For the droplets to freeze, the temperature in the cloud must be below −30°F. If it’s higher, then, oddly enough, water alone won’t be enough to form a snowflake. Even if the air temperatures is below 32°F, the droplets will remain liquid unless a solid object appears, a nucleus like a dust or pollen particle, around which the snowflake crystallizes.

As for the arms of a snowflake, their appearance is a matter of probability. As the hexagon grows larger, the vertices become the most convenient place for the accumulation of water molecules. This encourages elongation of these vertices into arms, which symmetrically branch out from the center, forming a snowflake.

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