The American Chemical Society has a great two-minute video explaining the basic chemistry behind snowflake formation, including how each degree of temperature difference adds to the branches of the flake. Are no two snowflakes really alike? It depends on if we’re talking about big snowflakes or little ones.
On a side note, the incredible snowflake photo above is the work of master snowflake photographer Ken Libbrecht, author of The Art of the Snowflake: A Photographic Album and Ken Libbrecht’s Field Guide to Snowflakes, both treasured books in my little library. Geek out some more on snow and read more about his special photography rig.