Scientist Explains Snowflakes

Ken Libbrecht’s quest began about four years ago when he saw snowflakes falling out of the sky.

“It just occurred to me that this was something I didn’t know anything about,” says the distinguished professor of physics at the California Institute of Technology.

So he started reading about snow crystals, “the white stuff that falls out of the sky,” as he puts it, and learned something quite surprising.

Nobody else knew much about it either.

Water on a Speck of Dust

The powdery white stuff that turns our highways into ski chutes is so common this time of year that everybody thinks they understand it, he says, but the fact is that nobody “really” understands it.

So he began collecting information and conducting experiments, and the result is a fascinating Web site that is crammed with information about the gazillions of snowflakes that fall on this planet every year.

And yes, to answer the most common question first, it’s true that no two are exactly alike.

Libbrecht’s curiosity led to the establishment of a research project at Cal Tech, where snow practically never falls, complete with “cold chambers” which can replicate the conditions that fashion snow crystals into complex structures of many different designs.

It didn’t take long, he says, to turn up a few surprises.

Most snow crystals begin with a similar design, a hexagonal prism made up of water molecules, usually clustered around a speck of dust. But during its brief lifespan, the prism at the center adds new features, sometimes making the crystal look like a flower, or a fern, or an indescribable maze of fingers reaching outward from each of the six sides of the hexagon.

How, Libbrecht wondered, could they have evolved in so many different ways, each with a slightly different form, all incredibly complex and beautiful?

Temperature Shapes Flakes

Experiments in his lab yielded the answer, at least partly. It turns out that snow crystals are highly temperature dependent, and their rate of growth can vary 100 fold by a change in temperature of just a few degrees, he says.

“This is quite a puzzle,” Libbrecht says. “I never realized the growth rate was quite as dramatic as that, and we don’t have a good explanation for why that is.”

As they blow about in a cloud, the crystals pass through various temperature regimes. If it’s cold enough, their arms grow very rapidly, and if it warms up a tad, the arms are capped, spreading outward slightly, until the next cold spell causes them to grow again.

Thus each snow crystal changes its structure over and over as it drifts down toward our driveways, but what we see outside our windows isn’t individual snow crystals, which are nearly microscopic in size. As they descend, the crystals reach warmer air and become “sticky,” the professor says. Thus they glob together and form snowflakes.

The snowflakes, or their component parts — the snow crystals — may look identical to us. But just as no two human faces are exactly the same, there is something a little different about every snow crystal that reaches the ground, because each has gone through a different growth process. Even the water molecules that make up their basic prism may be different.

Even if two crystals had identical water molecules, laid out in exactly the same way, the trip down through the cloud would have subjected each to different growth patterns. So the chances that two crystals would be identical when they reach the ground are so low that “the odds of it happening within the lifetime of the universe is essentially zero,” Libbrecht says.

So each crystal is, in some way, different from all the others. That’s astonishing, considering the fact that there are so many of them. Libbrecht estimates that each year the number of snow crystals forming in the Earth’s atmosphere is around 10 to the 24th power. That’s 1, followed by 24 zeros.

Science For the Sake of Knowing

Of course, Libbrecht’s interest extends beyond the “curiosity” that initially fired up his experiments. He hopes the research will lead to a fuller understanding of how ice crystals grow and how they interact with other materials.

“It touches on a lot of different fields, like how friction works on ice,” he says. All you have to do is touch your brakes on an icy highway to know how important that area is.

But, in the end, he says, he’s doing all this just because he wants to understand it. That’s one of the great things about being a scientist, he adds.

“When you’re in the science business, sometimes you just find a phenomenon you don’t understand and you want to explain it,” he says. After all, he adds, most basic research is driven by curiosity.

That holds true for the discipline where he spends most of his research time. Libbrecht is an astrophysicist, so he concentrates on understanding the physics that drives the universe. And that’s similar in one way to trying to figure out snowflakes.

“This country spends billions of dollars a year on astrophysics, and it also has no purpose other than it’s very interesting,” he says.

So like a true scientist, he has shifted some of his effort to understanding something closer to home. And as a “refugee from North Dakota,” the white stuff that falls out of the sky was a logical target.

We don’t have to know that every snowflake is different. But isn’t it great that we do?
