Having seen more than my share of photographs of snowflakes, I recently noticed that most seem to be perfect mirror images. How can the spines on ONE side of the flake possibly know what random shapes those on the OTHER are assuming in an essentially random freezing environment? Why would not all snowflakes be nonsymmetrical masses of frozen material with absolutely NO symmetry whatsoever? –Pete Formaini, Ithaca, New York
As water vapor rises into the colder upper atmosphere it cools, and when chilled enough condenses into infinitesimal water droplets, which commonly form around particles of dust, sea salt, etc, known as nucleation sites. If the temperature keeps dropping, eventually the droplets freeze into ice crystals. These crystals start out as tiny hexagonal prisms–the hex shape resulting from the angle formed between oxygen atoms as the frozen water molecules line up. Under the right conditions, additional moisture condenses directly onto the crystal and freezes too. Since the corners where the crystal faces meet project further into the surrounding vapor than the flat sides, they accumulate ice faster, typically producing the familiar six-branched snowflake.
