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First, a quick word about light. . |
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Photons, or light particles, come in a rainbow of colors. The photons on the violet-to-blue end of the color spectrum cruise the universe in shorter
wavelengths, while the redder ones travel in longer undulations.
When photons encounter an object (a mirror, a black dog, an apple), they may have various reactions: They may bounce back (reflect). They may bounce sideways (scatter). They may pass right through (transmission). Or they may assault a molecule in the object, give up their energy and die (absorption). And individual wavelengths, or shades of color, may react differently to the same object: An apple looks red because most of the colors in the light spectrum are absorbed by the apple. Only some shades of red -- plus a weaker assortment of yellows and oranges, blues and violets -- bounce back. That said, why is snow white? The answer lies in snow's messy construction. A beam of white sunlight entering a snow bank is so quickly scattered by a zillion ice crystals and air pockets that most of it comes zinging right back out of the snow bank. No one wavelength is preferentially absorbed or reflected, so snow is essentially the color of the sunlight reflecting off it -- white. |
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But all snow is not so snow-white. . |
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Occasionally, walking across white snow, you'll see color flashes, like sparkles from a
diamond. In this case, a cold, dry night has probably squeezed the last moisture out of the
air and onto the top layer of snowflakes, where it has crystallized in a faceted pattern
similar to ... a diamond. These little torture chambers bend and split light into its rainbow
of wavelengths the way a prism does.
And then there are glaciers, which often look an eerie blue. Unlike snow, which is mostly air, ice is mostly ice. Without the scattering effect of air bubbles, light can penetrate much deeper into ice. And the deeper it goes, the more photons from the red end of the spectrum it loses along the way. Two meters into the glacier, most of the reds are dead. The colors in sunlight react differently to each molecule -- ice molecules, dog molecules, apple molecules. In ice, it just happens that some blue wavelengths skate the farthest -- 24 meters -- before they're finally absorbed. And until they're absorbed, they're free to scatter and reflect. Some of them eventually find their way back out of the ice to glow blue at your eyeballs. |
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Determining where the white -- or the blue -- goes when the snow melts really begs a look at the warm version: water. . |
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The surface of water reflects very little light (about 7 percent). What happens to the other
93 percent of sunlight depends on where the water trickles. Clean, glacial melt in a stream
will transmit light down to the stony stream bed, where some will be reflected and the rest
will be absorbed. In a glacial tarn, or lake, tiny bits of glacier garbage (ground-up stone
and dust) suspended in the water will impede transmission and color the water. As the water
spills into an ocean, it will change color again as it mixes with a new assortment of salts
and minerals and little life forms.
Those impurities traditionally are blamed for the color of water, says outspoken meteorology professor Craig Bohren of Penn State. He thinks differently. He says water is naturally a very, very faint shade of blue. All by itself. Without the sky reflecting in it, without blue minerals floating around in it, water, the water you drink, is blue. "You need meters and meters of the stuff in order to see it," he says. "It's like window glass: You think it's clear. But take a look at it on edge. You'll see green." |
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Who knew snow could be such a colorful subject? . |
| There is even a red algae that grows on glacial snow, for a warm, rosy effect. "It tastes (and looks) like watermelon," enthuses Antarctic expert Stephen Warren. "Try it -- it won't hurt you." Different strains of algae produce yellow, orange, green, even purple snow. The flavor of these hues -- except, perhaps the yellow -- has yet to be established. |
 Vocabulary
albedo, n.
This word, based on the Latin word for white (as in albino) refers to the percentage of light an object
reflects. The albedo of water is low; the albedo of snow is high. Try it in conversation: "My albedo's
getting kind of high. Let's go to the beach."
Check out more of "The Skinny On ..." stories:
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Hannah Holmes tastes fine, New England snow outside her home in Portland, Maine. A fresh
"Skinny On ... " appears every other Friday, adding to
Hannah's extensive work for Discovery Online. She also writes for Escape, Outside, Sierra,
Backpacker, Eco Traveler and Women's Sports and Fitness. Send her a note at skinny@online.discovery.com.
Main WebLinks Picture: Tony Clements | Copyright © 1997 Discovery Communications, Inc. |