Toward a Grand Unified Theory of Snowflakes
Toward a Grand Unified Theory of Snowflakes | Quanta Magazine

    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.
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.

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.

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

        
    

The earliest recorded musings on these delicate shapes date to 135 B.C. in China, according to Libbrecht’s research. “Flowers of plants and trees are generally five-pointed, but those of snow, which are called ying, are always six-pointed,” wrote the scholar Han Yin. But the first scientist to try to understand why this happens was probably Johannes Kepler, the German scientist and polymath.
In 1611, Kepler offered a New Year’s gift to his patron, the Holy Roman Emperor Rudolf II: an essay called “The Six-Cornered Snowflake.” Kepler writes that he noticed a snowflake on his lapel as he crossed Prague’s Charles Bridge and could not help but muse on its geometry. “There must be a cause why snow has the shape of a six-cornered starlet. It cannot be chance,” he wrote.
He would have recalled a letter from his contemporary Thomas Harriot, an English scientist and astronomer who, among many roles, served as a navigator for the explorer Sir Walter Raleigh. Around 1584, Harriot sought the most efficient way to stack cannonballs on Raleigh’s ship decks. Hexagonal patterns seemed the best way to pack spheres closely together, Harriot found, and he corresponded about it with Kepler. Kepler wondered if something similar was taking place in snowflakes, and whether their six sides could be pinned on the arrangement of “the smallest natural unit of a liquid like water.”
