22 déc. 20193 min de lecture

The Science Behind Why No Two Snowflakes Are Alike

Kenneth Libbrecht is that rare person who, in the middle of winter, gleefully leaves Southern California for a place like Fairbanks, Alaska, where wintertime temperatures rarely rise above freezing. There, he dons a parka and sits in a field with a camera and a piece of foam board, waiting for snow.

Specifically, he seeks the sparkliest, sharpest, most beautiful snow crystals nature can produce. Superior flakes tend to form in the chilliest places, he says, like Fairbanks and snowy upstate New York. The best snow he ever found was in Cochrane, in remote northeastern Ontario, where there is little wind to batter snowflakes as they fall through the sky.

Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.|||

Ensconced in the elements, Libbrecht scans his board with an archaeologist’s patience, looking for perfect snowflakes and other snow crystals. “If there’s a really nice one there, your eye will find it,” he said. “If not, you just brush that away, and you do that for hours.”

Libbrecht is a physicist. His lab at the California Institute of Technology has investigated the internal structure of the sun and developed advanced instruments for gravitational-wave detection. But for 20 years, Libbrecht’s passion has been snow — not only its appearance, but also what makes it look the way it does. “It’s a little embarrassing when stuff falls out of the sky, and it’s like, ‘Why does it look like that? Beats me,’” he said.

Kenneth Libbrecht, a physicist at the California Institute of Technology, in Cochrane, Ontario in 2006. When a high-quality snow crystal lands on his foam-core board, he picks it up using a small paintbrush, places it on a glass slide, and puts it under the microscope for further inspection.Courtesy of Kenneth Libbrecht

For 75 years, physicists have known that the tiny crystals in snow fit into two prevailing types. One is the iconic flat star, with either six or 12 points, each decorated with matching branches of lace in a dizzying array of possibility. The other is a column, sometimes sandwiched by flat caps and sometimes resembling a bolt from a hardware store. These different shapes occur at different temperatures and humidities, but the reason for this has been a mystery.

Over the years, Libbrecht’s painstaking observations have yielded insights into the snow crystallization process. “He surely is the pope in the domain,” said Gilles Demange, a materials scientist at the University of Rouen in France who also studies snow crystals.

Now, Libbrecht’s work on snow has crystallized in a new model that attempts to explain why snowflakes and other snow crystals form the way they do. His model, detailed in a paper that he posted online in October, describes the dance of water molecules near the freezing point and how the particular movements of those molecules may account for the panoply of crystals that form under different conditions. In a separate, 540-page monograph, Libbrecht describes the full body of knowledge about snow crystals. Douglas Natelson, a condensed matter physicist at Rice University, called the new monograph “a tour de force.”

“As a piece of work,” Natelson said, “boy, it’s gorgeous.”

Six-Cornered Starlets

Everyone knows no two snowflakes are alike, a fact that stems from the way the crystals cook up in the sky. Snow is a cluster of ice crystals that form in the atmosphere and retain their shape as they collectively fall upon Earth. They form when the atmosphere is cold enough to prevent them from fusing or melting and becoming sleet or rain.

Although a cloud contains multitudes of temperatures and humidity levels, these variables are as good as constant across a single snowflake. This is why snowflake growth is often symmetrical. On the other hand, every snowflake is buffeted by changing winds, sunlight and other variables, notes Mary Jane Shultz, a chemist at Tufts University who published a recent essay on snowflake physics. As each crystal submits to the chaos of a cloud, they all take on slightly different forms, she explains.

Illustration: Lucy Reading-Ikkanda/Quanta Magazine, adapted from Kenneth Libbrecht

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The Science Behind Why No Two Snowflakes Are Alike

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