Quiet after a snowstorm

In this blog we are returning to some of the early columns of Ask Your Science Teacher,  published in 2009. The columns are updated and revised. Many originally ran in The Tomah Journal

Why is it so quiet after a snowstorm? Ah, what a keen observer you are! Yes, it is the epitome of serenity and peacefulness after a fresh snowfall. The newly fallen snow absorbs the sound. A dog’s distant bark seems muted and subdued. Passing cars, trucks, and snowplows are barely heard. This snow-induced silence may last for an entire day.

Snow is soft and acts as an effective sound damper. The snow behaves the same way that a carpeted room will be quieter than a room with bar wood floors.

It is the arrangement of the flakes that dampens sound. Snowflakes come in all shapes and sizes with varying weight and water content. Some crystals have pointy stars, some are flat plates, and each snow flake consists of dozens or hundreds of these crystals.

Snowflakes don’t fit together like a jig saw puzzle. They loosely pile up and have plenty of air spaces in between. It is these hole, spaces, or pores that absorb the sound, just like the pores in acoustic ceiling tile.

Even sound travelling parallel to the snow is absorbed because the pressure of a passing sound wave pushes air down into the spaces between the flakes. Much of the sound wave energy is lost.

Later, as the snow compacts by gravity and some of the surface crystals melt, it no longer muffles sound effectively. Blowing wind, sunlight, and a tad of rain can quickly render sound absorption useless. You no longer hear that softer muffled sound.

Soon the surface of the snow is more like a whiteboard on the schoolroom wall rather than the overhead acoustical tile. The hard-packed snow acts like bare ground, or blacktop, or concrete, reflecting and transmitting sound. Noise level return to normal.

John Greenleaf Whittier’s 1866 epic poem “Snow-Bound” alluded to the quiet of the countryside after a classic New England snow storm.

The Army is interested in this question. They can use sound waves to measure snow depth to an accuracy of one inch. The Forest Service, Bureau of Land Management, Interior Department, and ski resorts all use this information. Sound waves have increased the accuracy of spring runoff and flooding predictions.  Ice thickness in Arctic and Antarctic regions are monitored.


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