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What determines the shape of a snowflake?

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Chris Westbrook Chris Westbrook | 10:00 UK time, Thursday, 20 January 2011

d ~ 51'456'000 km: day 20 of Earth's orbit

The UK snowfall this past winter gave us a great opportunity to see what snowflakes really look like, rather than just admire them as patterns on Christmas cards. It's often surprisingly easy to observe the shapes of the ice crystals by eye as they fall onto your coat. About a week before Christmas it was snowing heavily in my garden and I took some photos of the crystals which were landing:

Snowflakes shot by Chris Westbrook

Chris Westbrook

I caught the crystals on an old black tin which I keep in my shed (so the surface of it was nice and cold). Most digital compact cameras have a 'macro' mode now, and this is what I used here. The first thing that struck me from these pictures was how much the shape varies from flake to flake. So why is that?

Well, the biggest influence on a snowflake as it grows is temperature. This was first unravelled by the Japanese physicist Ukichiro Nakaya in the 1930s, who was the first to grow snowflakes artificially in a lab. The incredible fact that he discovered is that ice grows in completely different ways depending on what the temperature is. At -2C it forms small thin hexagon shapes; go down a few more degrees to -5C and it forms long slender needle-shaped crystals. Go colder still to -15C and you get the classic six-arm stellar snowflakes we all know and love: these are often 100 times as wide as they are thick. A bit colder still at -20C we get crystals shaped like pencils. Below -25C it gets even more complicated, with many individual pencil or hexagon crystals growing out from a single point to form messy complex structures.

So the shape of a snow crystal depends on the temperature at which it formed in the atmosphere right? Well this is often kilometres above us in the atmosphere: so by the time they fall to earth they've been falling for a couple of hours. During this time they will have passed through a whole range of different temperatures, modifying the shape of the crystal as it grows. To complicate things further, as the crystals fall through the cloud they inevitably collide with each other, sticking together to form clusters. The closer the temperature is to 0C, the 'stickier' the surface of the ice gets - that's why you get the really huge fluffy flakes when the temperature is hovering close to the freezing: these big flakes are dozens of individual crystals stuck together like the one in the centre of my photo.

So how can researchers possibly observe this complicated evolution of snowflakes as they grow, fall and stick together? Well one way is using radar. Just as aircraft reflect radio waves, so believe it or not, do snowflakes. Our research group at the University of Reading uses radars based at the Chilbolton Observatory to unravel the evolution of snowflakes as they fall through clouds, monitoring their shape, fall speed and number. This may seem a bit academic, after all many of us in the UK only get a few days of snowfall each year. But of course, even if the temperature at the ground is above 0C, the temperature several kilometres above us is well below freezing. So clouds of snow crystals are above us throughout the year - it's just that if they fall to earth, they melt into raindrops first. In fact, the majority of rain in the UK forms this way - by melting snow formed many kilometres up in the atmosphere.

Chris Westbrook is based in the Department of Meteorology at Reading University

Have you got any interesting observations of snow crystals over the winter? Let 23 degrees know here.

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