I thought a lot about this, am equally frustrated by the 'explanations' given in pop science, and have tried to research it without getting great answers, but I

I thought a lot about this, am equally frustrated by the 'explanations' given in pop science, and have tried to research it without getting great answers, but I have some ideas and points on the matter: 1) Most ice crystals are not symmetric! The pretty ones you see in pictures are the cream of the crop or even artificially grown like these. Even artificially grown ones are often imperfectly symmetric, see the stunted arm in one of the pairs in this article, for example. 2) Many snowflakes are thought to form around dust particles or such. This makes their symmetry more remarkable since their base is not symmetric! 3) A stable structure that forces its pattern out sounds like something that should be invariant under a lot of changing conditions, but the sheer diversity of snowflakes argues that it actually depends heavily upon many parameters. So it's extra remarkable that such structure is both sufficiently stable to form symmetry across the millimeter scale and sufficiently unstable to form many different patterns 4) But the things it's sensitive to appear to be humidity, temperature, etc. that are essentially constant across the length of a snowflake even though they change significantly across the duration of the snowflake's growth.