Snowflake science

The physics behind snowflake formation

After the Beast from the East last year, and with more snowstorms on the horizon, you might be wondering about that white stuff that falls from the sky. Tiny crystals of frozen water have fascinated humanity for as long as we’ve experienced snow, and a surprising amount of physics goes into their formation.

The idea that every snowflake is unique comes from Wilson “Snowflake” Bentley, a Vermont farmer who adapted a microscope to be able to take pictures and became the first person to photograph a snowflake in 1885. He went on to take thousands of photographs of snowflakes until his death in 1931, never seeing two that looked the same. He lauded the beauty and transience of snowflakes, proclaiming that “every crystal was a masterpiece of design, and no design was ever repeated”.

“Snowflakes don’t always form in that easily-recognisable hexagonal crystal.”

Hans Verlinde, Professor of Meteorology at Pennsylvania State University, says that this cannot be definitively proven since we cannot feasibly observe every snowflake in existence. “But we can address the probability of finding two identical ice crystals, which is vanishingly small,” he says. “The bigger the crystals get, the greater the freedom for different growth paths, and the lower the probability of finding identical crystals even at the macroscopic visual level.”

Caltech physicist Kenneth Libbrecht will tell you that this is due to the conditions that go into the formation of a snowflake within a cloud. According to Libbrecht, snowflakes tend to follow a standard pattern of growth based on temperature and the water saturation of air, and that they don’t always form in that easily recognisable hexagonal crystal you can make by cutting paper (the technical term is dendrites). Near -2°C, they grow into thin, platelike forms. As the temperature drops, near -5°C, they create slender columns and needles. As the temperature approaches -15°C, they become incredibly thin plates, and at temperatures below -30°C, they create columns again. The traditional dendrite snowflakes form around 0°C and -15°C, and water saturation is at a maximum around -15°C. Lots of water in the air is a requirement for the formation of dendrite snowflakes. They need the excess water to grow as large as they do – though they may seem tiny to us, they are massive compared to some other types of snowflakes.

“In the atmosphere, it would just get bigger and bigger and thinner and thinner.”

Libbrecht describes the formation of these sharp snowflakes, which start as a small crystal with a slight bump on it. More ice begins to grow around the edge of the bump, making the snowflake larger. “As soon as the ledge gets a little bit sharper, then it grows faster, and if it grows faster, then it gets sharper still, creating a positive feedback effect,” Libbrecht says. “In the atmosphere, it would just get bigger and bigger and thinner and thinner, and eventually you’d get a really nice, beautiful snowflake.” It takes a long time swirling and crystalising in clouds before snowflakes fall, and due to the countless factors that go into their formation, their beauty lies partly in aesthetics and partly in their singularity.

So, as snow may fall from the sky and shut down the city, or as you see it on holiday, think about every little step each snowflake has to take before it lands on your face. And if it’s not -15°C, spare a thought for the non-traditional snowflakes out there that are probably the ones swirling through the sky. It won’t be long before they melt back into water, or slush, as snow is wont to do. Fulfill the tradition that has been going on for millenia and look at the snow in wonder, but wonder at how amazing the physics is. Snowflake Bentley would be proud.