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Question:
I would like to know how the geometry of the water molecule leads to a six-sided snowflake. When someone asked me this question, I could not answer , despite years of physics.! Thanks!



Replies:
It is not so much the geometry of the water molecule that determines the shape of snowflakes as the geometry of ice - water in ice orients itself in such a way that a six-fold rotational symmetry is generated, and the very tiny ice crystals that are the beginnings of snow flakes will exhibit this six-fold symmetry. If conditions are consistently right as the snow-flake grows (so that surrounding water molecules settle on it in a symmetrical fashion) it should preserve this symmetry. However, I have seen an awful lot of snow in my life, and I do not think I have ever seen a snowflake that was really very symmetrical (they are often very flat, which is explainable in standard crystal growth theory, but retaining perfect six-fold symmetry is kind of hard to explain).

Arthur Smith



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