The Mystique and Science Behind Snowflakes
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Everyone knows no two snowflakes are alike... 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. Therefore, snowflake growth is often symmetrical. On the other hand, every snowflake is buffeted by changing winds, sunlight and other variables, […] thereby, taking on slightly different forms.
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…It was Johannes Kepler, the German scientist and polymath, who tried to understand why flowers of snow are six-pointed… His contemporary Thomas Harriot, an English scientist and astronomer, had sought the most efficient way to stack cannonballs on ship decks. Hexagonal patterns seemed the best way to pack spheres closely together, Harriot found. 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.” It was a remarkable early insight into atomic physics, one that wouldn’t be formalized for another 300 years. Indeed, molecules of water, with their two hydrogens and one oxygen, tend to lock together to form hexagonal arrays... Because of the hydrogen bonding, and the details of how the molecules interact with each other, you have this comparatively open crystal structure... Aside from helping grow snowflakes, this hexagonal structure makes ice less dense than liquid water, which hugely affects geochemistry, geophysics and climate. If ice did not float, life on Earth would not be possible.
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In the 1930s, the Japanese researcher Ukichiro Nakaya began a systematic study of the different snow crystal types... In low humidity, stars form few branches and resemble hexagonal plates, but in high humidity, the stars grow more intricate, lacy designs. The reason for the various crystal shapes also began to come into focus after Nakaya’s pioneering work. Imagine molecules of water arranged loosely, as water vapor just begins to freeze... Freezing water molecules begin to form a rigid lattice... These crystals grow by incorporating water molecules from the surrounding air into their pattern... A thin, flat crystal (either plate-like or star-like) forms when the edges rope in material more quickly than the crystal’s two faces... However, when its faces grow faster than its edges, the crystal grows taller, forming a needle, hollow column or rod.
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According to the physicist Kenneth Libbrecht’s model, water vapor first settles on the corners of the crystal, then diffuses over the surface either to the crystal’s edge or to its faces, causing the crystal to grow outward or upward, respectively. Which of these processes wins as various surface effects and instabilities interact depends mostly on temperature.
All this happens only in ice, an unusual mineral, because of a phenomenon called “pre-melting.” Because water ice is usually found close to its melting point, the top few layers are liquid-like and disordered. Pre-melting occurs differently on the faces and edges as a function of temperature, though the details of this are not completely understood… His new model is “semi-empirical,” partly tuned to match observations rather than explaining snowflake growth starting entirely from first principles. The instabilities and the interactions among countless molecules are too complicated to unravel entirely.
