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It’s winter, which means it’s time get out and play in the Sectored Plates, Stellar Dentrites and Bullet Rosettes!

Or, shall we say, snow?

We all learned in grade school that no two snowflakes are alike. But why? In a single snowstorm, billions of snowflakes can fall – and that’s just in your town – yet it’s unlikely, even impossible, for two identical snowflakes to tumble to the earth.

To make it easy on us, the International Commission on Snow and Ice puts snowflakes into a simple classification system that names seven snow crystal types. That’s where we get fun names like “plates,” “dendrites” and “capped columns,” and it keeps scientists from having to name billions of snowflakes.

To learn more about how a snowflake gets its shape, we stopped by David Flory’s office. It happened to be a snowy day, and heavy wind was blowing the large, flat flakes horizontally across campus. It was cold – around 10 degrees.

Flory, a senior lecturer in the Department of Geological and Atmospheric Sciences, explained that snowflakes owe their unique shapes to temperature and humidity. So on that frigid day, the flakes we saw were flat because it was very cold, and there wasn’t much humidity. Warmer temperatures would have produced the more familiar lacy snowflakes, called dendrites.

From sky to sidewalk

“Snowflakes start out unorganized, usually between 3,000 and 5,000 meters up and at between -12 and -15 degrees Celsius (between about 3 and 10 degrees Fahrenheit),” Flory said. “A single snowflake begins to form when a cold water droplet freezes onto a floating dust particle, creating a crystal. As it falls through cold layers in the atmosphere, water vapor freezes to the crystal, which eventually builds a snowflake.”

All snowflakes start out as hexagons because they take on the molecular characteristics of water. This is why all snowflakes have six arms: since the six points of the hexagon are the most exposed to humid air, branches form on each of the points as they pick up water vapor. As the snowflake continues to fall, more water vapor (humid air) latches onto the branches, making each flake more elaborate.

The roles of temperature and wind

“In warmer temperatures, snowflakes get sticky. That helps produce those big, beautiful flakes, because dendrites start sticking together.” Flory said.

The more moisture there is in the air, the more intricate the snowflake will be. Drier conditions tend to turn out simpler shapes, such as sectored plates. Temperature also matters – warmer temps create extensive branching patterns while frigid temperatures produce simple patterns.

Wind is what guarantees no two snowflakes are alike.

“The wind is the artist,” Flory said. “Snowflakes develop based on the conditions they see, and the wind carries each snowflake on a different path, through different dust particles and water vapor, and different atmospheric temperatures. The reason why every branch of a snowflake is identical is because they are all carried through that same path.”

The cold facts:

It’s never too cold to snow. Even at incredibly cold temperatures, if there is moisture in their air, snow can fall.

Fresh, non-compacted snow is a great insulator. That’s because undisturbed snow is made up of a high percentage of air that trapped among the lattice structure of the ice crystals. Since the air can barely move, heat transfer is greatly reduced.

Fresh, fluffy snow can make it harder to hear, since sound waves are more readily absorbed into the surface. Old, hard snow helps sound travel farther and clearer, since it bounces sound waves off its surface.