How snowflakes get their shape

As winter settles in, many of us are getting an up-close look at more snowflakes than we might care to. Let's take a moment to look at just what forces are shaping (literally) our winter weather.

Top image: Alexey Kljatov

Earlier we took a very, very close look at a series of snowflake close-up shots taken by photographer Alexey Kljatov's. Commenter raiju filled us in on why some consistent geometric patterns were emerging:

Essentially, water molecules have a positive and negative poles, which causes them to naturally line up in a hexagon shape when they crystallize (it's also the cause of a lot of water's cool properties, like surface tension). The center of a snowflake is almost always a hexagon (although not always a perfect hexagon, as you can see from the superman-logo like shaped one). Once it reaches a certain size, the branches can start to appear because it becomes too large to keep its shape. These arms always follow a rough 6-way symmetry, since they are attached at the equidistant poles "sticking out" from the central hexagon.

Of course, that still leaves the differences between individual snowflakes to explain. And for that, we turn to commenter Hypnosifl:

The basic explanation is that the formation of the branches is affected by subtle variations in things like temperature and humidity in the atmosphere as the snowflake makes its trip down, and each branch on a single snowflake experiences the same history of variations as it falls, but different flakes don't.