Snow Bright, Snow Light
Everyone knows, of course, that snow is bright. Just how bright didn't really dawn on me until the day I prepared to climb my first big glaciated peak, Washington's Mt. Rainier. My friend and rope team leader watched as I slathered on sunscreen. But before I put the tube away, he suggested, "Make certain you've done the insides of your nostrils." The insides of my nostrils? Unless I'm standing on my head, that's one of the few places where the sun never shines. "Hey," he explained, "You're going to be spending the day on a mirror." Not quite, as it turns out, but close enough.
The scientific measure of a surface's reflectivity is called "albedo." A perfectly reflective surface would have an albedo of 1.0; the earth taken as a whole is about 0.4. But nice clean snow? Brand new dry snow has an albedo of 0.9-it reflects more than 90% of incoming sunlight.
"It's because of the crystalline structure," explained Arthur Judson, who began his career with the Forest Service as a snow ranger doing avalanche control and over the next 40 years helped develop the science of avalanche forecasting. Judson-who grew up on the East Coast's snowiest belt, New York's Tug Hill Plateau, and who lives now in Steamboat Springs amid Colorado's snowiest passes-points out that when snow first falls, the crystals are at their sharpest. Classic skiers know that's important because it means the flake's sharp edges will grab a layer of kick wax and not let go. But it also contributes to the snow's brightness.
"Those new crystals have all kinds of reflecting surfaces on them," said Judson. Those delicate branching tips are like a mine full of diamonds-a drift of new snow is really a collection of angles and facets, all of them scattering and reflecting incoming light. That's why you can ski so easily without a headlamp on a full moon following a new snow. Even though sunlight is six magnitudes brighter, the snow reflects enough of the moonlight to illuminate the woods.
As the snow ages, however, "there's a vapor transfer from the tips-from the sharpest points-towards the center," Judson explained. "It gradually goes from sharp cornered plates to more rounded grains. When you get down to wet snow, all those tips pretty much disappear, and you're left with nearly spherical grains." Those grains have fewer reflective surfaces, and hence absorb more solar radiation. By the time snow is old and dirty, its albedo may have dropped as low as 30 percent.
This albedo effect has global implications. Snow is one of the forces that keeps our planet cool-it acts as a mirror, bouncing incoming solar radiation back out to space. But as we dump our carbon dioxide into the atmosphere and begin the implacable heating of our globe, the extent and length of snow cover begins to diminish. And that melting sets off a vicious cycle-the less snow there is, the more the brown earth absorbs heat, and the hotter it gets. In turn, that means less snow.
Even the conversion of snow-covered tundra to boreal forest is enough to change the albedo. All those spruces soak up solar heat that the snow would just bounce back. Across the Arctic Ocean, computer models indicate that ice and snow cover may entirely disappear by mid-century. The blue ocean that will replace them is, by comparison, a heat sponge. This same principle explains why ski tracks melt as they do come spring. If a twig or rock is sticking up through the white, it absorbs the sun's heat instead of reflecting it back. That heat is passed down the stem of the plant, or the body of the rock, starting to melt out even the parts below the snow line.
But if snow's brightness has deep global consequences, it also has strong effects on us as individuals. Seasonal Affective Disorder, or SAD, leads many people to feel depressed during the winter months, apparently due to the lack of light. According to one theory, the brain's pineal gland increases production of the hormone melatonin when it gets dark; this hormone in turn causes sleepiness.
No one is diagnosed SAD at the equator, where the amount of light stays the same year-round. But incidence seems to increase steadily as latitudes move north. Two percent of northern Europeans suffer badly, with as many as ten percent showing some symptoms. Sufferers are advised to sit in front of special bulbs for 45 minutes a day, keeping their eyes open so the light gets in.
But snow diminishes the effects of SAD. In snowy latitudes, the incidence of SAD becomes less common, even though you're moving farther north and further into the dark. The brightness of the snow-nature's own light box-seems enough to ward off the winter blues, as long as you get outside into the natural light.
Snow's brightness may explain why you start feeling grumpy if you don't get to ski for a few days, and why those Decembers when the snow just won't come are so unbearably depressing. It may also explain a little bit of the March syndrome, SAD in reverse. "I get depressed in the spring as soon as the snow starts to melt," said Judson. "It's been that way for 50 years."
Snow bright, snow light…bring on that albedo!