A listener asks: Why do snowflakes appear so similar to each other?
Nature’s icy symmetry. I’m Bob Hirshon and this is Science Update.
We hear a lot about how every snowflake is unique, but Science Update listener Istvan Prazsac emailed us from Hungary, asking why snowflakes are so similar—each with exactly six sides. We asked Miriam Rossi, professor of chemistry at Vassar College. She says it all comes down to the way water molecules group together as they cool.
Snowflakes are symmetrical because they reflect the internal order of the water molecules as they arrange themselves in the solid state which is known as crystalization.
And the easiest way for them to bond is in groups of six. So you might wonder why every snowflake isn’t just a perfect hexagon. Rossi says that changes in temperature and humidity as the flake grows can result in a variety of patterns, giving us flakes that are all built on a hexagonal foundation, but with myriad forms. I’m Bob Hirshon for AAAS, the science society.
Making Sense of the Research
Most people have probably grown up hearing that "no two snowflakes are alike." They are, however, similar in that they all have six sides—so they're hexagons. A snowflake, or snow crystal, begins with the formation of a hexagonal plate. This hexagonal plate is formed by the arrangement of the water molecules in the ice crystal. It so happens that even though atoms and molecules can hook up in different ways, in the case of water molecules, they like to hook up into a hexagonal plate.
The process of forming the hexagonal plate is called faceting, which causes flat surfaces to appear on the crystal as it grows. Water molecules in the air strike the crystal surface and stick, but some stick more readily than others. The water molecules stick especially well to rough spots on the surface, where there are lots of available chemical bonds.
Many, but not all snowflakes, also have branching. The branching involves the way in which water molecules in the air move toward the crystal. The branches you may see when you look at a snowflake sprout from the six corners when the crystal grows large enough. The size of the branches is determined by how the crystal falls through the clouds because as it does so it experiences changes in temperature and humidity that can make the branches grow a bit differently on different crystals. Since the six branches on a crystal all take the same path, though, they all experience the same changes at the same time and grow in synchrony. This process yields the symmetic shape you see in a snowflake.
Finally, there is sharpening, which causes the crystal to grow thin, flat plates or slender, hollow columns. When a corner of a snow crystal grows, it produces narrow faceted terraces. A narrow terrace grows faster than a wide terrace, and the growth then adds more terraces that are even narrower than before. The result is an edge-sharpening growth.
Now try and answer these questions:
- How many sides are there to a snowflake, or snow crystal?
- What causes a snow crystal to form this number of sides?
- Briefly explain the three different processes that form a snowflake?
- Have you ever looked closely at a snowflake? If so, what did you see?
You can learn more about snow crystals by going to our Mystery Image Contest from December 2013, which shows an image of a snow crystal as captured by a scanning electron microscope.
You can follow up this Science Update by listening to Dangerous Hail, which discusses why hailstorms, which pelt the ground with hard balls of ice, never seem to hit human targets.
You can use this Science Update to supplement a unit on molecules or even on the various phases of water, from gas to solid. It could be a good motivation for a lesson on snow crystals or it could help students begin their own research project on the topic.