On the first day of Christmas...

Chris Smith and Graihagh Jackson, joined by a host of festive luminaries including Hugh Hunt, Alan Calverd and Amy Thomas are exploring the scientific side of the festive period...

Graihagh - This week we’re celebrating each of the 12 days of Christmas with a science experiment.

Chris - And how’s your singing, Graihagh?

Graihagh - Let’s find out with day 1. A partridge in a pear tree. Ready everybody? I’ll lead us in.

Everyone sing: on the first day of Christmas my true love gave to me, a partridge in a pear tree

Graihagh - So we have a beautiful pear tree (sort of) here in the studio! Well actually it is a lavender with white flowering the odd pear hanging from it, but it will suffice! I thought we would start by making our own snowflake decorations for the tree. So in front of you you should have some paper and scissors. Let’s get cutting! It’s like being back at school and making paper chains... So Hugh, you’re a physicist, could you start off by telling us how snowflakes form?

Hugh - Snow is made of water, and water molecule H2O has an oxygen with two hydrogens, and the angle between the hydrogen atoms is about 120 degrees. But it’s only approximately 120 degrees, isn’t that right Alan

Alan - Yes, it’s actually about 110 degrees, which is possibly why no two snowflakes look exactly the same.

Graihagh - I was going to say, does that matter the angle? Is that what causes them to be like fingerprints and be completely individual?

Alan - I think the old idea was that it was the hydrogen bond angle that determined the hexagonal shape but because the hydrogen bond angle doesn’t make a hexagon, we’ve got to think of some other reason why the come out that shape.

Hugh - You can kind of think that this 120 degree angle, it’s like the angle you get in a boomerang. So you can imagine three boomerangs connecting together makes this kind of hexagon shape and that’s the kind of base building block of a water crystal. And if you imagine drawing a snowflake, you might start by drawing three dissecting lines like an asterisk, and then at the end of each line you might draw another asterisk, and then at the end of those little lines you can draw another asterisk. You keep on adding…

Graihagh - You just keep going for ever and every.

Hugh - And if you ask five different people to draw using those instructions, you’re going to get five different snowflakes because they’re going to draw their asterisks at randomly different places. So every snowflake's going to look different.

Graihagh - But why do they stop growing, is there a moment when they stop growing?

Hugh - Well the water is coming out of the atmosphere, and they’ll only grow when there’s water vapour in the atmosphere around the snowflake, and once the snowflake has formed and the air is just a little bit drier around. Snowflakes need very particular conditions to grow in and amongst those conditions don’t exist anymore they stop growing.

Graihagh - Yeah. I suppose it’s hoping for a white Christmas this year, what do you reckon?

Hugh - I think the weather forecast is a bit blustery for Christmas I think.

Graihagh - I haven’t actually looked… I don’t know. I always want a white Christmas because I always want to go outside sleighing. And I remember thinking we did this every year when I was a kid but, actually, that never seems to happen any more. I thought climate change was supposed to be happening. I mean, why isn’t there any snow?

Hugh - Well, you could do what you do in Australia at Christmas time because it doesn’t snow then. So it make snow out of mashed potato.

Graihagh - How much mashed potato do you need to…

Hugh - I was only kidding!

Graihagh - I was going to say that must be a colossal amount of potatoes.

Chris - Amy, tell us about the psychology behind all this -what makes an aesthetically pleasing snowflake because I want to win this competition? So I haven’t started cutting yet actually, so I might start now.

Amy - I think as we’ve heard about how they grow and crystallise and the hydrogen bonds are able to form sort of hexagonal shape. And hexagons are quite prevalent in nature and symmetrically, obviously, is something we’re quite in tune with as humans.

Graihagh - Why are we particularly fond of…?

Amy - I think it because there’s some mathematical reasoning behind hexagons. When they fit together you’re maximising your area but minimising your perimeter. So, if you have like a beehive and you’re making honeycomb, it’s more energetically favourable for the bees to make hexagonal shapes by reducing their perimeter but maximising their area.

Graihagh - Okay. So I want something as symmetrical as possible, basically?

Amy - that’s what you should go with.

Graihagh - And that’s how I’m going to be onto a winner.

Amy - Symmetrical and hexagons, I think that’s where you should start off from.

Chris - It’s a bit late for me now. I wish I’d known that five minutes ago.

Graihagh - And Alan - very briefly, I’ve heard that all snowflakes are different, just like a fingerprint. Is this true?

Alan - Probably. I think it must have been about a hundred years ago, a couple of Canadian scientists collected snowflakes and photographed them. And they published books containing thousands of snowflake photographs, no two of which are alike.

Graihagh - Wow! And they had to trawl through every single one of them to identify the fact that they’re not alike?

Alan - I remember this was an exam question in crystallography 101, which said “the average snowflake is less symmetrical than the average photograph of a snowflake” and I think there may be a lot of selection bias in what actually goes into the photograph of a snowflake.

Graihagh - Okay - mine’s not gone very well. I’ve only just barely cut a hexagon. Oh wow that’s amazing.

Chris - Some amazing results here!