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SALT LAKE CITY -- Curiously, one thing you will never find on the side of a snow covered mountain or in the last snowman you built is an eight-sided (or four- or five-sided) snowflake. What makes snowflakes, those tiny but beautiful ice crystals, so unique and so symmetrical?
Well, before you can answer that question, you have to know a little about how snowflakes form in the first place. There are actually several kinds of snow crystals (they’re listed below). The most common form, the kind people most often think of when they think of snowflakes, is the stellar or star snow crystal. Stellar snowflakes will grow six main branches. As the snow crystal continues to grow, side branches may form on the snowflake’s main branches.
Snowflakes only ever have six or 12 arms, though theoretically a snowflake can also have 18 or 24 sides - any multiple of six. But this has never been observed, and once you know the process by which a snowflake is formed, you understand why that never happens.
A common misconception about snowflakes is that they are made from frozen raindrops. This is not quite true - it's more like a tiny piece of a frozen cloud. A snowflake forms when water vapor solidifies into solid ice. Once the tiny snow crystal forms, more water vapor attaches to it and the snow crystal continues to grow so long as there is water vapor available.
Kenneth Libbrecht explains it all very well in his book, "The Snowflake." Molecules form chemical bonds between themselves, and these bonds, due to their chemical structure, have preferred orientations, which mandates how the molecules stack up. Water molecules like hexagons.
A snowflake begins as a single water molecule, and as more water vapor attaches to it, the snowflake grows and grows, always maintaining the hexagonal structure, according to Libbrecht.
As the snowflake arms grow, they develop imperfections. Any point along their length that protrudes even a little more than another area will attract water molecules to it quicker than surrounding flat surfaces. This is how snowflake arms develop branches, called dendrites.
There are six basic kinds of snow crystals. But each category of snow crystal may have several additional subsets. The six types are stellar (or star), dendrites, columns, plates, columns capped with plates and needles.
Japanese physicist Ukchiro Nakayo, working in the 1930s, discovered that a snow crystal’s shape and growth is heavily dependent on temperature and humidity it finds itself in, with temperature determining shape and humidity determining complexity.
But why is a snowflake symmetrical? It seems illogical that all six arms would be exactly symmetrical. Yet they are. Nakaya also answered this question.
The snowflake’s six arms grow independently of each other, but their growth is coordinated by the crystal’s movement through the cloud. As it passes through the cloud, it will experience varying levels of temperature and humidity. It’s nothing more complex than this: the arms are identical because they started growing from the same source and shared an identical growth history.
And it's possible that more than one snowflake has shared an identical growth history as well. According to Paul Yeager, a meteorologist and author of "Weather Whys," an estimated 10^24. snowflakes fall throughout the world per year. With that many snowflakes being made yearly throughout the eons of time, Yeager says it’s likely that there have been identical snowflakes.
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