Snowflakes or a single ‘snow crystal’ can take on a variety of sizes and structures, based upon several key factors, but they always have six sides. Water molecules form with one oxygen atom and two hydrogen atoms. When they connect, water molecules form a hexagonal lattice, resulting in the six-sided symmetrical object.
Temperature and moisture also play a factor, with low humidity generating simpler crystalline structures. More humidity results in more branches. The chart, at right, shows a good breakdown of the variable morphology of snow crystals as a function of temperature and moisture.
Image: PBS, The Science of Snowflakes
Source: Nantel M, ‘Snowflake Structure, Formation, and Energy Possibilities‘ U20 Science