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Ask a science teacher: Why is snow white, and where does its color go when the snow melts?

Larry Scheckel/Reuters

Snow is a bunch of ice crystals stuck together. It’s a very complex arrangement. To understand why snow is white, we must be familiar with what happens to light when it strikes any material. The color of anything, including snow, depends on how light interacts with it.

Visible light consists of a rainbow of colors, the ROY G BIV colors of red, orange, yellow, green, blue, indigo, and violet that were assigned by Isaac Newton. When photons of light strike an object, they may bounce back (reflection), bounce to the sides (scattering), pass right through (transmission), or give up their energy (absorption). Grass is green because it reflects the green light to our eyes and absorbs all the other colors. Red apples reflect red light to our eyes and absorb the wavelengths of all the other colors.

When light goes into snow, it hits all those ice crystals and air pockets and bounces around, and then some of the light comes back out. Snow reflects all the colors; no it doesn’t absorb, transmit, or scatter any single color or wavelength more than any other. The “color” of all the light wavelengths combined equally is white. So all the colors coming out are the same colors that go in, combining to make white light.

A few years ago, I visited the famous Mendenhall Glacier in Juneau, Alaska. The glacial ice looked bluish. Ice is just very compact snow, without a lot of light-scattering bubbles. Light can penetrate much deeper into ice than into snow. The deeper the light goes, the more the longer wavelengths, toward the red end of the spectrum, get scattered out, and eventually the reds dissipate, leaving only blue colors to be reflected back to us. So the ice takes on a beautiful, eerie blue tone.

The record snowfall for any one year in the United States is 1,140 inches (95 feet) at Mount Baker Ski Area in northwestern Washington, during the 1998–1999 snow season.

Snow is beautiful. It coats everything in a pure white blanket. It helps farmers and is good for the land, because it has a ton of air pockets. Even though the snow itself is cold, the air that they hold in insulates the ground, protecting seedlings and preventing the frost from going too deep. Snow that falls in the mountains later melts and helps fill the depleted reservoirs of the American West.

You may have noticed how quiet it is outside after a fresh snowfall. In addition to making snow fluffy those air pockets absorb sound, just like the ceiling tile in my classroom. After a few days, sound travel returns to its normal pattern. Many of the fluffed-up ice crystals melt and compact somewhat, so the tiny pockets that absorbed sound are gone.

When I was a kid on the farm, my dad planted fields of oats in April. One year the oats were up about three inches when we got one of those late-spring snowstorms with four or five inches of snow. I thought all the oats would be dead. Strangely, my dad didn’t seem to be concerned. Turned out those were some of the best oats we ever had. I remember him mentioning something about the snow adding nitrogen to the soil, which makes sense because moisture helps plant seedlings fix nitrogen in the soil.

From the book, "Ask a Science Teacher: 250 Answers to Questions You’ve Always Had About How Everyday Stuff Really Works"; Copyright © Larry Scheckel, 2013. Available December 17 wherever books are sold.