You have probably heard that no two snowflakes are alike. This assertion is likely true, as each snowflake is comprised of some quintillion water molecules. This means that there is a (nearly) endless number of combinations. Joe Hanson, of PBS Digital Studio's "It's Okay to be Smart" explains, "The number of possible arrangements of the 10^18 water molecules [in a snowflake] is such a large number that it dwarfs the number of atoms in the universe many, many times over."
Snowflakes are formed from frozen water during extremely cold weather (it has to be colder than 23 degrees Fahrenheit or -5 Celsius). The initial formation of the flakes takes place in the atmosphere, where water mixes with dust particles to create ice crystals. As the crystals fall to the ground, more water freezes to the initial ice crystals, filling in the spaces available to make the characteristic six-sided snowflake.
The shape of the snowflake (and its pattern) depends on the atmosphere conditions that the flake encounters as it falls (things like temperature and humidity). As you may be aware, the ice crystals that make up snowflakes are symmetrical (or patterned). This is because the ice crystals that form reflect the internal order of the water molecules as they arrange themselves in predetermined spaces (known as “crystallization”).
In the video below, you can watch a microscopic formation of a snowflake. It's truly stunning.
[mom_video type="vimeo" id="87342468"]