As summer heats up, I keep my nose in snowflakes to get the book done on time. :) Here's the latest edit, and please zoom in
to see the geometry in the center!
Most snowflakes start with a hexagonal structure in the center, and then at a certain point they branch outward from the corners of the hexagon. This is called the branching instability, and makes sense when you look at what is actually happening:
The crystal is gathering water vapour from the surrounding air and adding it to it's crystal structure. The more exposure to this vapour, the faster the growth! It happens that the corners of the hexagon are exposed to more vapour than the edges, making them grow faster. As the corners grow faster, they protrude further and get an even greater exposure to the water in the air.
This is also why branches get side-branches, and the growth pattern continues. Tons of fun science and photography are going into my upcoming snowflake book, currently available for pre-order!
Take a look: http://skycrystals.ca/product/pre-order-sky-crystals-unraveling-the-mysteries-of-snowflakes-book/