Somehow Christmas time always feels more magical when there’s snow about. As snowflakes drift down and stack against frozen panes, you sink deeper into the sofa to sleep off the lunchtime overindulgence. And as you sit there the world goes silent as a white blanket settles leaving the landscape an undulating mass of indistinct shapes. But while we’re all familiar with these beautiful six-pointed stars of ice, have you ever stopped to think about how they form? What are the chemical and physical processes that make a snowflake grow into the shape it is?

As it is, each and every tiny snowflake has a humble beginning – a microscopic seed. This can be a speck of dust or some other particle – organic or inorganic. So, as the air temperature falls, water vapour coalesces around this seed and the snowflake begins to grow. As the water molecules freeze around the seed they form a crystal with six-fold symmetry. This symmetry is an intrinsic property of ice crystals and is due to the way the water molecules stack when they freeze. As the ice crystal grows it retains this hexagonal structure and builds upon it. This gives it its flat, plate-like structure with the six pointed shape we know so well.

Oxygen atoms are in red while hydrogens are marked out by grey bars © Ken Libbrecht

The hexagonal crystal lattice gives the snowflake its underlying structure and from there it grows by a process known as faceting. As the ice crystal grows the ‘rough’ edges with the most ‘dangling’ bonds grow fastest until only the smoother facets – geometric faces of the crystals – remain. These then grow in a uniform manner and, on their own, would result in ice crystals that were hexagonal columns. The complex branching patterns that give snowflakes their unique and beautiful shapes is down to small imperfections appearing in the facets, which are then quickly amplified by the stacking of more and more molecules of water at these points.

One man who has certainly spent a lot of time considering the shape and structure of snowflakes is Ken Libbrecht, at Caltech in the US. Libbrecht, orginally an atomic- and astro-physicist, says he became fascinated by snowflakes when he first saw a morphology diagram of how different snowflake structures form at different temperatures. If, like me, you’ve been watching the BBC’s brilliant Frozen Planet series then you’ll already be familiar with some of his work: high defintion videos of snowflakes growing.

Libbrecht says he got a call from the Frozen Planet team who told him that they wanted to use his videos of growing snowflakes on the programme. However, to date, Libbrecht hadn’t created any high quality videos. ‘Right before the BBC called it popped into my head that this is how to do [high definition videos], and when they called I said, “Great, I need a little money to do it”.’ Libbrecht went on to make a number of changes to his apparatus and produced the amazing videos that the Frozen Planet team were then able to model and superimpose on to the footage above.

© Ken Libbrecht

As a result of this collaboration with the BBC, Libbrecht says this set him off on exploring another phenomenon. As snowflakes grow – unlike other crystals – their edges actually get sharper. He thinks that this may be linked to surface melting, which occurs when an ice crystal warms to nearer to 0°C and the molecules near the surface become less tightly bound and the structure becomes more amorphous. At the sharp edge of the ice crystal there are fewer neighbouring molecules to stabilise the lattice in this region and surface melting increases, this leads to further growth of the crystal making them even sharper and so on. It’s a process of positive feedback that gives ice crystals their razor sharp edge, although why they grow faster as they get sharper is still something of a puzzle.

A growing ice crystal © Ken Libbrecht

However, Libbrecht says that building up a picture of this process will be extremely difficult. Ice crystals have a high vapour pressure so water molecules are coming and going at a prodigious rate. As the structure is in a constant state of flux it makes it very difficult to get an idea of what’s going on. He compares it with trying to take a clear picture in the midst of a hurricane. If you want to find out more about the fascinating world ice crystals and some experiments you can do then check out the Snow Crystals website.

And if all this talk of snow and ice has left you feeling distinctly chilly, then sit back and warm up by a real (virtual) log fire. Is there anything people won’t film and put on YouTube?

Patrick Walter

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