Snow crystals form when humid air is cooled to the point that molecules of water vapor start sticking to each other. In the clouds, crystals usually start forming around a tiny microscopic dust particle, but if the water vapor gets cooled quickly enough the crystals can form spontaneously out of water molecules alone. Over time, more water molecules stick to the crystal until it gets heavy enough to fall…

When they fall and grow in nature, they experience a variety of humidity and temperature conditions along their paths, creating their famously unique shapes. But when they grow in a lab under controlled conditions? Identical twin snowflakes.

In this episode of Deep Look, we peek inside the CalTech lab of professor, physicist, and snowflake expert Ken Libbrecht. He’s made a chamber that mimics cloud conditions so that he can grow, catch, and study their hexagonal shapes. From KQED:

Each water molecule is each made out of one oxygen atom and two hydrogen atoms. As vapor, the water molecules bounce around slamming into each other. As the vapor cools, the hydrogen atom of one molecule forms a bond with the oxygen of another water molecule. This is called a hydrogen bond. These bonds make the water molecules stick together in the shape of a hexagonal ring. As the crystal grows, more molecules join fitting within that same repeating pattern called a crystal array. The crystal keeps the hexagonal symmetry as it grows.

Read more about Snowflake science and snow crystal morphology at Libbrecht’s site.

Plus: More Deep Look and more ice crystals! Make your own in a bottle, go on a snowflake safari, or see them form in time lapse through a microscope.

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