Science Questions

How do snowflakes maintain symmetry?

Tue, 12th Jan 2016

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Question

Harold asked:

Can you please explain to me how the individual 'arms' of a snowflake 'know' what is happening on the others so that they all grow to match and remain symetrical?

Answer

Kat Arney put this question to physicist Stuart Higgins...natural snow crystal

Stuart - Well, you need a big cloud of gas and water vapour and you need the right conditions - it needs to be nice and cold.  And so, at the start of a snowflake, you need a speck, you need a nucleus, so typically this might be something like the dust in a cloud and the water will start to solidify and crystallise around that nucleus.  And it just so happens that the way it packs is in a hexagonal structure - a regular hexagon has a symmetry of six and so thatís what gives us this kind of growing six pronged structure of the snowflake.

Kat - And are all snowflakes different? Is this true or is this a myth?

Stuart - It is true to a certain extent, so they have similar structure.  So snowflakes will share this common six pronged structure due to the fact  that itís the packing of the water structure underneath.  But what happens after that, when these arms spread out and they start to interlink and branch like tree branches, thatís more dependent on the conditions, that could be the temperature and the exact pressure and all the different factors that might affect it.  So, the chance of getting those two sets of conditions, to grow the same crystal in the same snowflake twice is very rare.

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Harold asked the Naked Scientists: Can you please explain to me how the individual 'arms' of a snowflake 'know' what is happening on the others so that they all grow to match and remain symmetrical? BTW, I love the show. What do you think? Harold, Wed, 12th Jan 2011

The snowflakes are not absolutely symmetrical, they do have hexagonal symmetry *, but the arms are not identical to each other, just similar ... http://en.wikipedia.org/wiki/Snowflake#Gallery

The six radiating arms form from the same initial conditions, at the hub, and are built according to the same rules (laws of physics) and using the same (hexagonal) building blocks, that is why they are similar. One arm doesn't "know" what the other arm is doing. The arms are variations on a theme: similar but not truly symmetrical.

o angles, originates in the way the water molecules are packed ... http://en.wikipedia.org/wiki/Close-packing ] RD, Wed, 12th Jan 2011

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