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.

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