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HOW COME? From water vapor to snowflakes

How do snowflakes form in winter? asks Shawn Park, a student in Woodside, NY.

How the snowflakes landing on your glove took shape depends on where they fell from in the sky -- and what happened on the way down.

In high clouds, where temperatures can hover at minus 40 degrees F., supercooled water vapor can suddenly freeze into icy crystals. But in lower, warmer clouds, a snow crystal needs a nucleus for ice to grow around.

A surprising list of nuclei candidates will fit the bill. Freezing water will happily glom onto bits of windblown clay or mineral dust, soot from fires and smokestacks, wafting pollen, drifting volcanic ash, floating fungal spores and even airborne bacteria. (Using powerful microscopes, scientists have seen some of the particles hidden in snow crystals. In one batch most crystals had hearts of clay.)

Water vapor begins to freeze onto the random blowing bits when the temperature drops to 32 degrees F. or below. The resulting snow crystal will be formed from up to one quintillion (a million trillion) individual water molecules.

On a growing crystal, water molecules tend to attach at depressions in the semiliquid surface. As water molecules link up, they freeze into a six-sided lattice. And as trillions upon trillions of water molecules hook into place, the lattice gets bigger.

Each new ice crystal is a miniature hexagonal prism. The prism can be short and stubby or long and chandelier-worthy. It can also be nearly flat, like a six-sided glass plate.

The shape of the finished snow crystal depends mainly on the temperature in the region of the cloud where it formed. Humidity also plays a role. When humidity is high, snow crystals tend to grow into lacy, star-spoked shapes. Icy branches sprout when the hexagon's jutting-out corners accumulate freezing water molecules faster than the rest of the crystal. As crystals fall through a cloud -- sometimes sideways -- their contours change. A snow crystal spinning like a top has the best chance at remaining symmetrical.

But the perfect, single-crystal snowflakes we see in pictures aren't what we usually see falling outside our window. Real snow is messy. Huge flakes dropping from a winter sky may be collections of hundreds of individual crystals. They form when falling snow crystals collide and clump, creating fluffy flakes called "aggregates."

Meanwhile, other bunches of snow crystals may run into very cold water droplets in the air on their way down. As the supercooled water freezes on the snowflakes, they accumulate an icy coating. The end result? Icy pellets instead of fluffy flakes.

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