Why are no two snowflakes the same?

From a distance, snowflakes may all look the same, but they're not. In fact, snowflakes come in lots of different shapes -- not just those classic stars you might try to replicate with paper cutouts.

If you were to look at snow under a microscope, you might find simple prisms, solid columns, sheaths, scrolls on plates, cups, hollow columns, 12-branched stars, bullet rosettes, capped columns, skeletal forms, radiating dendrites, simple needles, arrowhead twins, crossed plates, irregulars or simple stars — and that's just a sampling.

So, why do flakes have such diverse shapes?

Ken Libbrecht, a physicist at the California Institute of Technology and a snowflake expert, said first it's important to understand how flakes form in the atmosphere.

Clouds are mostly made of water droplets. A snowflake begins when a single droplet freezes and starts to grow by absorbing water vapor from the air (this water vapor is the product of other water droplets that have evaporated). About 100,000 water droplets evaporate in the process of making just one snowflake.

When the water molecules attach to one another to form snowflakes, they're actually forming crystals. In crystals, molecules line up in a nice, orderly fashion. The kind of crystals that water molecules like to form are six-sided, or hexagonal.

"A crystal is a solid in which the molecules are lined up in a regular array," Libbrecht said. "Think of them as building blocks and the blocks all stacked together a certain way."

Learn more about how snowflakes form and how a Libbrecht, who is based in southern California, makes his own flakes in the lab:

The fact that snowflakes are crystals is what distinguishes snow from sleet or hail.

"Sleet particles are not at all like snowflakes — they look like frozen drops of water, which is what they are," Libbrecht said. "Another thing that can happen is droplets can hit a snowflake and form a bunch of droplets ... That's called graupel or soft hail. What makes the snowflake special is that it forms from water vapor."

But even though snowflakes are all hexagonal crystals, it doesn't mean they all resemble stars.

This timelapse shows one of the ways Libbrecht grows snowflakes in his southern California lab:

Libbrecht says we don't exactly understand why different shapes form, but we do know it has to do with the conditions where the flakes grow.

"To make a rod or column, the ends grow faster than the middle, and the opposite is true for a plate or a star," Libbrecht said. "That behavior depends on temperature. If you want nice stellar snowflakes, those tend to form only when the temp is minus 15 Celsius. Columns grow at minus 5. Humidity also affects things. The higher the humidity, the faster the crystals grow."

So as the snowflakes fall from the cloud to the ground, the crystals continue to grow. All these variables — humidity, temperature, path, speed — are also the reason that no two snowflakes are exactly alike.

"When they grow in the clouds the way they look depends on the path they take through the clouds," Libbrecht said. "Since no two snowflakes follow exactly the same path, no two snowflakes look exactly alike."

Brains On is a science podcast for kids and curious adults produced by MPR News and Southern California Public Radio