Every snowflake starts its life as a single suspended droplet of liquid water high in the atmosphere, where the temperatures are far below the freezing point of water. Only supercooled water can freeze quickly enough to form a delicate, six-sided snowflake. The temperature and humidity determine what initial form that six-sided structure will take.
In spite of the freezing cold temperatures, the supercooled water droplet will stay in a liquid state until it is jostled by something solid. However, the instant that water droplet encounters even a tiny speck of dust, the water molecules will instantly crystallize around it. Every water molecule crystallizes at a fixed angle based on its H-O-H molecular structure. That’s why snowflakes always have six sides.
As each of these frozen crystals bumps into more water molecules, they find their own place in the crystalline structure, and the future snowflake grows heavier and heavier. While these delicate crystals are still tiny, the strong winds at these high altitudes keep them aloft. Eventually, these delicate ice crystals grow heavy enough for gravity to take over. At this point, they start to fall.
There are a lot of things that can happen to a snowflake on the way down. Very few snowflakes hang onto a perfect crystal structure all the way down. This is why most snowflakes start off as one of a few general types, but by the time they reach the ground, no two snowflakes are alike.
Many snowflakes partly melt and refreeze as they pass through warmer and colder parts of the atmosphere. Some split apart almost completely, but still remain connected to each other. These changes determine a snowflake’s final shape.
A snowflake can also die at this stage. If it completely melts, it will turn to rain, or freezing rain if the temperature of the ground is below freezing. If it completely melts and then refreezes, it loses all its delicate structure and turns into sleet.
However, if only the edges melt together as rime builds up on them, a special thing happens which lets the snowflake keep most of its shape. The ice building up on these snowflakes turns into graupel, which is sometimes called ‘soft hail’ because the ice builds up on something delicate and not something hard. When you look at graupel up close, it looks a lot like a fuzzy snowflake.
You’d think the solid prisms and columns would have the best chance of surviving intact. However, it’s the delicate stellar dendrites which we see most often. These beautiful ice stars are the largest snow crystals which reach the ground. They’re so large, it’s easy to see their structure just by looking at them. Many of them are close to an inch across! That’s millions upon millions of tiny water molecules, all joined in a delicate beauty which has survived a miles-long fall to land on your coat.
When it lands on the ground, the snowflake’s time as a delicate ice crystal is almost over. The delicate edges quickly wear away. The molecules which make up the snowflake start to shift around to make a different type of crystal, which can bond electromagnetically with the previous layer of snow and make a strong snow base.
If there’s a lot of snow coming all at once, or if there’s a lot of freezing and thawing, the snowflakes don’t have enough time to shift and bond with the previous layer of snow. Instead, they become granular. This kind of snow is never stable, because later snowflakes which fall on top of that granular layer can’t bond with it at all.
What happens instead is that later snow forms a large slab on top of the granular layer. On a mountain slope, that granular layer will give way sooner or later, and the slab of snow on top will become a slab avalanche.
All snowflakes begin as supercooled liquid water high in the atmosphere. When their drifting life is disturbed, they crystallize into light, delicate ice crystals, which fall to earth when they become heavy enough. On the ground, they form over time into piles of drifting snow. They transform from marvellous delicate beauty to the awe-inspiring silence of freshly falling snow. No machine-produced snow even comes close.