Snow is a form of precipitation, but whether flakes form is dependent on a variety of atmospheric conditions. Temperature is an important factor. If it is very cold, long ice crystals may form. If it is too warm, sleet, graupel (frozen water droplets), or clumps of connected partial flakes may form. The ideal temperature for snowflake formation is -10 degrees Celsius. Humidity is also a factor.
Varieties
Snowflakes can have hexagonal or triangular symmetry. They may also form as less glamorous ice needles or tubes. Some can even form in an hourglass shape. The six armed or hexagonal variety is more common. Triangular snowflakes are rare, and form when the ambient air temperature is about -2 degrees Celsius. Snowflakes aren't always perfectly symmetrical either. On the molecular scale, this would be almost impossible. Micrographs of snowflakes have revealed artifacts or defects in symmetry.
Snowflake Growth
Since snowflakes are crystals they grow. A central droplet of frozen water attracts other water molecules. The molecules attach, and other molecules subsequently layer on top. It is generally thought that similarity in dendrite arms is due to similar conditions in the air around the flake. The dendrites grow at roughly the same rate, in roughly the same conditions. This hypothesis is supported by the fact that not all snowflakes have identical dendrites. This probably rules out the possibility that information is being transferred somehow (electromagnetically, mechanically, or otherwise) between the dendrites to govern their formation.
Molecular Structure
The growth of snowflakes and other ice crystals is determined by the molecular structure of water itself. Water is made of one oxygen atom and two hydrogen atoms. The two hydrogen atoms attach chemically to the oxygen atom at angles making a V-shaped molecule. Three V's can connect together to make a molecule with six arms. Actually, the crystal growth is messier and more complex than this, but crystals have self-similarity. This means that the atomic geometry of any crystal is reflected in it's macroscopic structure.
