Like all precipitation, snow forms high in the atmosphere. The temperature there is below the freezing point of water, but until a water molecule is jostled, it will stay in a liquid state. Only supercooled water can freeze quickly enough to form a delicate, six-sided snowflake.
The instant the supercooled molecules of water encounter anything solid, even a tiny speck of dust, they will crystallize around it. As each of these frozen crystals bumps into more water molecules, they find their own place in the crystalline structure. Their initial temperature determines what form that structure will take.
As more water molecules are added, the snowflake gradually grows heavier. While it is still tiny, the high winds in the clouds keep it aloft, but eventually the snow 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. Many snowflakes partly melt and refreeze as they pass through warmer and colder parts of the atmosphere. Some split apart completely, but still remain connected to each other. Very few snowflakes keep a perfect structure all the way down. These changes in temperature are the second thing which determines a snowflake’s final shape.
You’d think the most solid shapes, the solid prisms and columns, would have the best chance of surviving. However, it’s the delicate stellar dendrites which we know best. These are the largest snow crystals which reach the ground. Many of them are one or two centimetres across! That’s millions upon millions of tiny water molecules, all joined together in a delicate beauty which has survived a kilometres-long fall. When these snowflakes land on your coat, you can clearly see their structure just by looking at them.
A snowflake can also die at this stage. If it completely melts, it will turn into rain. If it completely melts and then refreezes, it loses all its delicate structure and turns into sleet. However, if only the edges melt together a little as rime builds up on them, the resulting graupel will look a lot like a fuzzy snowflake. Graupel is also sometimes called ‘soft hail’ because the ice builds up on something delicate instead of something hard, without destroying that centre.
When it lands on the ground, the snowflake begins to change again. The delicate edges are worn away. The molecules start to shift around to make a different type of crystal. In well-bonded snow, the molecules link electromagnetically with the previous layer of snow, but it takes time for newly-fallen snowflakes to adjust and bond.
However, if there’s a lot of snow coming all at once, or if there’s a lot of freezing and thawing, the snowflakes can’t bond with the previous layer. Instead, they become granular. Snow which falls on top of that layer can’t bond with it either, but it can form a large slab on top of it. This type of snow is never stable. When the deep granular layer gives way on a mountain slope, it becomes the most dangerous type of avalanche, a slab avalanche.