As snowflakes gently meander past your living room window this winter, take a break from your hot cocoa and consider nature’s ice-cold architecture. When water molecules chill down, they assemble into a myriad of spectacular shapes from simple hexagons to star-shaped dendrites. Inspect these frozen fractals and you’ll find a recurring theme: the number six. Six sides, six edges, six branches — ice crystals seem six obsessed.
In 1611, German mathematician Johannes Kepler speculated in a New Year’s gift to a friend that this numerical repetition stemmed from the microscopic scale. Water molecules freeze as hexagons, he proposed, which then stack in alternating rows that form more hexagons as additional water molecules join the crystal. Scientists now know that H2O takes on a six-sided structure because of the way hydrogen bonds link water molecules. Kepler’s crystal ponderings laid the groundwork for the field of crystallography, which famously helped reveal the architecture of DNA and now investigates the structure of everything from diamonds to viruses.
Ice arises in an ordered manner, but conditions dictate what shape a crystal takes, and those shapes can vary dramatically. In 1966, Japanese meteorologists Choji Magono and Chung Woo Lee established 80 unique classifications of ice crystals, including cups, needles, bullets and scrolls. As ice piles up, the classification of tiny crystals can become a matter of life and death. Supercooled water droplets can latch on to ice crystals creating a bumpy surface. Snow layers composed of smooth ice crystals without these bumps are typically unstable and don’t stack up easily. A snow layer made up of bumpy crystals, however, can hold itself together long enough to form a massive slab that can slip down a mountainside as a catastrophic avalanche.
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Next time you catch a snowflake on your tongue, think about how geometry, meteorology and the quirky properties of water molecules have conspired to create one of nature’s most spectacular set of minisculptures.
Interested in diving deeper into the mysteries of water, crystals and the rest of the universe? Keep reading Science News!
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