OneAndOneIs2

A lot of them started falling from the sky yesterday, and for the first time in ages we have a half-decent layer of snow. Schools have closed and I'm not planning on trying to drive anywhere any time soon.

And everybody knows that snowflakes are all different and all six-sided, but possibly not WHY this is so..

So. First off, the six-sides. This is because water is made up of H2O - Two hydrogens joined to one Oxygen.

Typically, in this kind of chemical, you expect a linear molecule - the two Hydrogens get as far apart as possible. But because of Oxygen's electron arrangement, it acts like it has FOUR atoms joined to it, which gives you a tetrahedral shape. With two missing points. i.e. a bent molecule.

And when these stick together, they stick together in a hexagonal pattern:

And that hexagonal pattern at the molecular level carries right on through to the snowflake structure. Hence the six points.

So that leaves why they're symmetrical and different. This is all down to how ice forms differently under different conditions. Humidity, temperature, and pressure all play a part. Under some conditions, you get hexagonal plates. Under others, needles. Under yet others, branching 'feathers'.

So if a flake were to form in just one of those conditions, you'd get a plain hexagon; a plain six-pointed star; or six feathers joined together.

But instead, the flakes form slowly and swirl through the clouds, and so experience many different conditions. And so they form differently, but because each branch experiences the same conditions at the same time, they form symmetrically.

If you're interested, this is quite a good diagram once you work it out:

And that's why snowflakes are the way they are. The first snowflake looks like it started out in hexagonal conditions, then sectored plates, then dendrites.

Simple as that!