We've all heard that each and every snowflake is unique. But in a lab in sunny southern California, a physicist has learned to control the way snowflakes grow. Can he really make twins?

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California's historic drought is finally over thanks largely to a relentless parade of powerful storms that have brought the Sierra Nevada snowpack to the highest level in six years, and guaranteed skiing into June. All that snow spurs an age-old question -- is every snowflake really unique?

“It’s one of these questions that’s been around forever,” said Ken Libbrecht, a professor of physics at the California Institute of Technology in Pasadena. “I think we all learn it in elementary school, the old saying that no two snowflakes are alike.”

--- How do snowflakes form?
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.

--- Why do snowflakes have six arms?
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.

--- Is every snowflake unique?
Snowflakes develop into different shapes depending on the humidity and temperature conditions they experience at different times during their growth. In nature, snowflakes don’t travel together. Instead, each takes it’s own path through the clouds experiencing different conditions at different times. Since each crystal takes a different path, they each turn out slightly differently. Growing snow crystals in laboratory is a whole other story.

---+ Read the entire article on KQED Science:
https://ww2.kqed.org/science/2017/04/...

---+ For more information:

Ken Libbrecht’s online guide to snowflakes, snow crystals and other ice phenomena.
http://snowcrystals.com/

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