Both the big fluffy flakes that drift slowly down outside your window to the little crystals that settle on your hat are shaped by natural forces. While there's some research that suggests that two snowflakes can actually be alike, the conditions that make these pretty shapes ensure that there's a nearly infinite variety of them.
Snow is not frozen rain. It forms when the water vapor in clouds condenses into ice and crystallizes. Water molecules tend to join up in hexagons, which is why most snowflakes will have a six-sided heart, or six-part symmetry. As the snow crystal grows, it sprouts branches from the corners of this basic hexagonal shape. Snow crystals can also join up with other crystals to make even more complex shapes.
As the snowflake is forming, the temperature and humidity levels can affect how the crystal grows and its eventual pattern. Scientists like California Institute of Technology researcher Kenneth Libbrecht, who wrote a comprehensive review of snowflake physics published in 2005, have grown snowflakes under carefully controlled laboratory conditions can see clear patterns.
At around 28 degrees Fahrenheit, you get thin hexagonal plates and simpler star shapes; 23 degrees Fahrenheit gives you needles and columns. Between about 15 degrees Fahrenheit and around -5 degrees Fahrenheit, you will get anything from thin plates to the big, many-branched shapes called 'dendrites.' When you rachet the temperatures down even further, you start getting mostly columns and plates again.
“Why snow crystal shapes change so much with temperature remains something of a scientific mystery,” Libbrecht writes on his website. “The growth depends on exactly how water vapor molecules are incorporated into the growing ice crystal, and the physics behind this is complex and not well understood.”
In 2007, Ritsumeikan University physicist Jon Nelson's research challenged the old adage that 'no two snowflakes are alike,' at least for very simple flakes. Nelson's work showed that the early stages of snowflakes, where they are still relatively plain six-sided prisms, can persist for a while. Later on they may start branching and begin developing a unique shape. But at the beginning, there may be more matched pairs of snowflakes than previously thought.
"How likely is it that two snowflakes are alike? Very likely if we define alike to mean that we would have trouble distinguishing them under a microscope and if we include the crystals that hardly develop beyond the prism stage—that is, the smallest snow crystals," Nelson told LiveScience in 2007.
But even if tiny twin snowflakes are sure to exist, it would be close to impossible for people to find them, given the staggering number of flakes that fall every year.
"Even if there were only a million crystals and you could compare each possible pair once per second—that is, very fast—then to compare them all would take you about a hundred thousand years,” Nelson told LiveScience.
But once you get beyond simple shapes, the old adage likely holds true. Every snowflake is subject to unique conditions as it forms and falls. Even different branches can experience slightly different environments that shape them slightly differently than the rest of the snow crystal. Perfectly symmetrical snowflakes are actually quite rare.
There's also variability within the building blocks of the snowflake itself. Most water molecules consist of an oxygen atom and two hydrogen atoms, but some molecules swap one of the hydrogen atoms for deuterium, a variant of hydrogen. Those little differences can make all the difference.