Snowflakes can form as plates or columns, but no one understood why — until physicist Kenneth Libbrecht investigated. His theory is the result of twenty years of making snow in the lab.
Humans
Snow is considered very rare in Pasadena, California, but at the California Institute of Technology, Kenneth Libbrecht can create snow using the most advanced snow-making equipment in the world.
Libbrecht’s Interest in Snow
As a physicist, Libbrecht has dealt with some big questions, such as the nature of gravitational waves and the internal processes of the sun. But he also has a fascinating hobby in the science of snow, which holds much more complexity and mystery than one might think.
The Big Question About Snow
The standard snowflake you see in holiday decorations — hexagonal, thin, and flat — well, that’s not how they always form. In addition to thin, hexagonal plates, snowflakes can grow into long hexagonal columns, like a small pencil shape. These two types of snow were discovered in Japan in the 1930s, where plates form at around -2 degrees Celsius, and columns form at around -5 degrees Celsius, then plates form again at about -15 degrees Celsius. It’s a very strange pattern that oscillates this way. I really wanted to know why that happens, but it turned out there was no answer — it’s a complete mystery.
Libbrecht’s Investigation into This Mystery
I decided that the way to answer this question was to grow lots of snowflakes under different conditions and measure their growth systematically. That was twenty years ago, and for several years, I faced problems and made no progress. Eventually, I concluded that the experiment had to be completely enclosed in a box. You add water vapor, but the conditions like temperature and pressure have to be adjusted precisely. I have little rods that go into the box, and I use them to push things around and turn things on and off. After that, I can grow the crystals — mostly, I grew crystals smaller than the width of a human hair and studied them under a microscope. If they are too big, they become too complex in shape and difficult to study.
Libbrecht’s Grand Unified Theory of Snow
People have always thought that if you have a flat surface in a crystal, it grows the same way under certain conditions. What I discovered is that the size of the surface matters a lot in snowflakes. If you envision a triangular hexagonal snowflake similar to a plate, it has two wide surfaces and six thinner surfaces around the edges. It turns out these thinner surfaces grow faster than the wider surfaces, and this creates an accelerating effect where the plates become thinner and thinner. The interesting thing is that this trend reverses — at certain temperatures, the wider surface is the one that grows faster, and thus you get vertical crystals.
The Effect of Snow on Other Planets
The spread of water molecules through the air affects the growth of snow in a way that enhances the molecular effects to create the visible thin edges in plate-like crystals and empty columns. When I grow crystals in a vacuum, none of this happens. So, yes, there will be differences in snow on other planets. If there is no atmosphere, you would get cubic crystals, but under high pressure, you would get very thin crystals. Of course, atmospheric chemistry can also change this in unpredictable ways.
Growing Two Identical Crystals
This process began when I realized, early in my work on snow, that there were no good videos of snow growth. I wanted to make a video showing a crystal that looks perfect. In the end, I figured out how to grow crystals on a stable substrate and film them. They looked much better than the crystals that fell from the sky. They were clearer and sharper. Real snow has had a tough life, falling through the atmosphere and colliding with other crystals. They also begin to evaporate, so the edges are always a bit rounded.
Questions
Unanswered Questions About Snow
My model is very complex. But it offers a lot of predictions, and I want to test them and see what happens, hoping to improve the model. One of the predictions is that exciting things may happen to snowflakes near the melting point. You might see what is called early melting, where most of the snowflake is solid crystal, but the molecules on the surface become irregular. I’ve tried to explore that. I’m also trying to grow larger crystals; I would like to make the largest snow crystal in the world. There is no specific reason… there is always something new to try.
Libbrecht’s View of Snow Today
I grew up in North Dakota, where the weather is very cold and we have a lot of snow. I used to see large six-pointed star crystals, but I didn’t know anything about it. Now I know better what to look for. I go out and carry a magnifying glass to look for different types of crystals – like the capped columns, for example, which resemble the specific hybrid between the plate and the column. I call it snow watching. The funny thing is that my wife is a botanist, and when we go out together, she is always looking at the different plants – but they all look like weeds to me.
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