As the very first ice crystals come together in a group of fledgling snowflakes, the new flakes often look strikingly similar. That's in large part due to the fact that ice crystals typically take a hexagonal (six-sided) lattice shape because of the way hydrogen atoms bond with oxygen to make water.
Certain edges of the ice crystals are jagged. These ragged, uneven areas attract more water molecules than the smoother and more uniform parts of the hexagon. Each little arm sprouts more of the same, growing into an intricate and uniform snowflake.
If snowflake development stopped within the first few moments of birth, we'd wind up with a lot more flakes that look suspiciously alike. But snowflakes keep gathering more and more crystals, clustering together on top of one another in distinct patterns.
As those clusters of crystals continue their snowflake fiesta, other guests visit the flake-making party. They come in the form of environmental factors, notably humidity and temperature. Both play major roles in whether the snowflake gets bigger and bigger or fizzles out.
You can imagine how critical temperature is to ice crystal formation and structure. Between temperatures of 27 and 32 degrees Fahrenheit (-2.8 and 0 degrees Celsius), crystals take on a plate-like or prism appearance. These are prototypical six-armed snowflakes that lack much visual interest.
Drop the temperature a few degrees and you'll see needle-like structures. Hollow columns develop at even lower temperatures. And when it's crazy cold you'll see stars sprouting fern-like arms.
Lower humidity tends to result in flatter flakes. Higher humidity means more crystal development in edges and on corners.
Add some extra moisture at those really frigid temperatures and suddenly snowflakes may become mesmerizingly beautiful. They contain a multitude of intersecting plates and needles and spaces, minute masterpieces falling from the heavens.
They may result in tiny, granule-like flakes. Or they may add layer upon layer until they are monstrous snowflakes like the record-breaking 15-inch (38-centimeter) wide flakes that fell in Montana in 1887.