Individual snowflakes have unique, intricate patterns, but the complicated way in which they fall appears to be universal.
Normally, when an object falls to the ground it picks up speed until the forces of gravity and air resistance balance out. At this point, the object stops accelerating, reaching its terminal velocity. However, light and delicately shaped snowflakes get caught in turbulent air flows on their way down, which turns their descent into a more complicated sequence of floating and twirling. This makes the path they take, along with their velocities and accelerations, difficult to predict.
at the University of Utah and his colleagues have now found that despite these complications, snowflakes may still follow universal rules for falling.
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The researchers measured the mass, size and density of over half a million falling snowflakes on a mountain near Salt Lake City, Utah. They built a device to do this that used a sheet of laser light, a hot plate and two different cameras. The way the snowflakes blocked the light gave the velocity and acceleration, and how the snowflakes evaporated on the hot plate afterwards gave the mass and density.
The device captured vastly different snowflakes, some 20 times larger or 20 times denser than others, in very different conditions, like during more or less turbulent storms. But when the researchers analysed the data, they found surprising evidence of universal behaviour across all snowflakes in all conditions.
The way that snowflakes accelerated and their terminal velocities fluctuated followed the same statistical pattern, forming two unexpectedly simple and similar lines. Each was described by a statistical distribution called a Laplacian, a narrower and more peaked cousin to the normal distribution.
Singh says previous experiments with snowflakes couldn鈥檛 measure so many variables at once, which forced the researchers to make more assumptions about their behaviour and miss some of the patterns uncovered by the new experiment.
, also at the University of Utah and who worked on the project, says the reason for the similarity isn鈥檛 clear yet and that no one on the team expected to find a pattern that would fit so many different snowflakes.
鈥淭his is an extremely complex problem; we have irregular turbulence and irregular [snowflake] shapes. But for reasons that we don鈥檛 really understand yet, [we found that] if we give the atmosphere the freedom to express itself, it expresses itself simply,鈥� he says.
Beyond being a physics mystery, this finding could have consequences for weather and climate prediction because most precipitation, even in the tropics, starts as snow. Understanding what snowflakes do during storms factors heavily into how meteorologists model their evolutions, says Garrett.
To try to understand their new findings, he and his colleagues are planning to upgrade their device to be able to capture the snowflakes and air flow in more detail.
Physics of Fluids
