HOW COME? Snow makes for silent night

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Why does snow seem to make the world quieter when it first falls? asks a reader.Silent night. The snow is falling thick and fast, filling your recent footprints. When a car appears, its passing is muted. And when we awake on a winter morning, we may know it's snowed even before we get out of bed, just from the soft silence.

Scientists say that as new snow piles up on bare ground or old snow, it's the arrangement of the flakes that dampens sounds. Snow crystals come in varied shapes and sizes, from flat plates to pointy stars, and each big flake is made of dozens of crystals. And by volume, each snowflake is about 90 percent air.

So building up in a compact layer, snowflakes pile up loosely, with plenty of air-filled spaces in between. These spaces or pores in the snow absorb sound, like the pores in acoustic ceiling tiles. Sound waves that would normally hit bare ground or concrete and be partially reflected are instead mostly absorbed into the airy blanket of snow.

Sound waves passing parallel to the ground get muffled, too. According to researchers, as a sound wave travels across a layer of fluffy snow, the pressure of the wave briefly pushes air down into the spaces between flakes. In their sweep across the pore-filled snow, sound waves steadily lose energy due to friction and thermal (heat) effects. By the time sound waves reach your ears, they are weakened. And a banging trash can lid becomes a muffled whoomp.

But for some aquatic creatures, falling snow may not bring a hushed quiet. According to studies, flakes landing on the surface of a body of water create a high-pitched hissing sound, audible to porpoises and some other water-dwellers who can hear high-frequency sounds we can't.

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How does it work? Instead of surrounding a solid object, water engulfs a parcel of air. As water melts the snowflake's icy bits, a bubble is left behind. Under pressure from the water, the bubble pulsates. Like a vibrating tuning fork, the bubble emits a high-pitched sound. As flake after flake hits, the noise underwater can increase by 30 decibels. That's the loudness difference between a conversation heard from 3 feet, and a jackhammer heard from 50.

So in a snowfall, lucky us. Still, a layer of new snow -- a stack of airy flakes -- is fragile. Blowing wind flattens it. A freezing rain collapses it, adding a coating of solid ice. Once the delicate layer caves, snow's sound-muffling quality quickly disappears.

Until then, as snowflakes softly fall, enjoy the surpassing quiet.

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