Comments on snowflakes

The earliest preserved illustration of snowflakes is by Olaus Magnus in 
1555. Others interested in snowflakes were Descartes (in Discours de 
la Methode, 1635) and Leuwenhoek (and there were, and are many more).

It seems that at present there is no accepted explanation why many
snowflakes have a very symmetric hexagonal structure though the details
are quite different for different flakes. In fact, there is still research
going on in this field. The basic structure comes from the properties of
the water molecules, no doubt.  The problem is that the aggregation
process from the supercooled steam of the cloud is plausibly a local one.
Hence there is no obvious reason why it should conserve the symmetry of
the growing snowflake.

Some starting points for finding out more.

The standard bible on snowflakes:

Nakaya, Ukichiro 
Snow crystals, natural and artificial 
Harvard UP., Oxford UP 1954 

A book on symmetry groups with a few pages on the symmetries of snow
crystals with more references:

Hargittai, I and Hargittai, M
Symmetry through the eyes of a chemist: 2nd ed.
New York: Plenum Press 1995

The picture book of snowflakes:

W.A. Bentley and W. J. Humphreys 
Snow crystals
New York: Dover 1962 

A link to active research on the growth of snow crystals at Caltech,
with more links.