Snowflakes hide scientific mysteries in their delicate, crystalline beauty. And there’s no one better to explain them than Caltech physicist Ken Libbrecht. He’s one of the world’s foremost experts on snowflakes, or as he likes to call them, snow crystals. So here’s Ken Libbrecht answering five snowflake mysteries.

Why do snowflakes only have six-sides?

six sided snowflake PHOTO: Ken Libbrecht

“You start out with a little round piece of ice. As the water molecules bind together, it’s growing. They're not a hexagon yet, so there are some rough areas. Additional water molecules are drawn to the molecular energy of these rough patches, filling in the structure. Until we are left with a six-sided crystal, with those nice crisp, straight facets. This process is how the geometry of the water molecule, is transferred to the geometry of a large crystal, through this process of faceting. And when we grow little tiny crystals in the lab we get these little hexagonal prisms. And this also tells you why there’s no four-sided snowflakes or five- or seven-, or eight-. You may cut those out of paper, but in practice, in nature, there is only six-sided snowflakes and it’s because this is the way it works with ice."

Why do snowflakes take the shapes they do?

snowflake PHOTO: Ken Libbrecht

"What you have is a hexagon. And the water molecules have to sort of diffuse through the air to get to it. There’s a lot of water out there, not so much near the crystal because it’s kind of sucking all the water out of the air. And so the water molecules are drifting in. And the corners stick out a little further, and because they stick out a little further into the humid air, they grow a little faster. So, the corners grow a little faster. Then there’s this instability, there’s this positive feedback. What happens is they stick out a little bit, so they grow faster. Well, that makes them stick out even more, so they’re going to grow faster, and they stick out more, etc. So if you start with a hexagon, pretty soon you can sprout branches. And so when this process starts it takes off pretty fast because of this positive feedback. And that’s a growth instability, this positive feedback. And then you get a branched crystal and then with time you can see it make side branches also. And this is how snowflakes get most of their structure through this process of branching and side-branching.”

Why can no two snowflake ever be the same?

snowflakePHOTO: Ken Libbrecht

“It’s falling through the clouds so it will see different temperatures and humidities as it falls and each time it sees a different set of conditions the growth changes. And so for a while you get branching and then maybe you get plates growing on the ends of the branches and then more branching. And so it goes as the crystal falls. And no two crystals follow exactly the same path and therefore they all look different. So you have these crystals that are symmetrical but also different. The chances of seeing two that are exactly alike is almost zero. Because they're just complicated. And it's like seeing two trees that are exactly, exactly the same - every branch, every leaf - it just can't happen because there are too many possibilities for the different ways to make a tree. And the same is true of a snowflake. And at some level you can even calculate how many possible ways there are to make a large snowflake. And the total number is larger than the number of atoms in the universe.”

Why are snowflakes so symmetrical?

snowflakePHOTO: Ken Libbrecht

“The crystal had a complex history. The temperature and humidity it saw as it fell was a very complex function, but each arm saw the same history, because they all travel together. So they all grow in synchrony.The six branches all grow in synchrony so it has this symmetry."

Why do snowflakes shapes change with different conditions?

snowflakePHOTO: Ken Libbrecht

"We understand pretty well why they branch with the humidity and the structure of that branching. It gets harder when we start looking at how the growth changes with temperature. And in particular why we get plate-like crystals at some temperatures and long, thin ice needles and columns at other temperatures. That's been a puzzle for 75 years and it still, it's still a puzzle."

Ken Libbrecht has a wealth of resource material at his site SnowCrystals.com.  

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