This article is provided as a public service by the Geophysical Institute, University of Alaska Fairbanks, in cooperation with the UAF research community.
There's something stirring in the brief Alaska autumn -- trees and bushes wearing vivid colors, the musky smell of ripening highbush cranberries, and chilly mornings that give way to bright, crisp days.
Punctuating it all is the rich, deep blue of the sky.
The cobalt blue isn't just an illusion Alaskans conjure up to ward off winter. According to Glenn Shaw, an atmospheric scientist at the Geophysical Institute, fall skies offer some of the deepest blues of any Alaska season.
Before being able to grasp Shaw's theories, I had to find out the answer to a question often thrown at puzzled parents by curious 3-year olds: Why is the sky blue? After talking with Shaw and opening one of my favorite sources, the text book Meteorology Today, I learned the sky is blue because of the effects on light of Earth's atmosphere, the 20-mile thick blanket of gases that covers the earth.
Much of the sun's energy is emitted as light vibrating at frequencies that stimulate the human eye. The sun gives off light in all colors of the rainbow from purple to red. Sunlight appears white at midday because the wavelengths of all colors are stimulating the eye and no one color dominates.
When light energy enters the atmosphere, air molecules deflect light in all directions like a rock hit by a waterfall. It turns out air molecules are just the right size to scatter one color of light better than others -- blue. Thus, we see the fallout of countless collisions between light energy and air molecules as blue sky. If the earth weren't surrounded by atmosphere, the sky above would be as black as the sky above the moon.
So why is the fall sky a deeper blue than the spring sky? Shaw says there are two major factors: snow cover and arctic haze.
The shade of the sky's blue depends in large part on how reflective the surface beneath it is. For this reason, the sky looks bluer over the ocean (which, when calm, reflects only about 10 percent of the sunlight that strikes it) than it does over an ice- or snow-covered surface. In spring, much of Alaska is still covered in snow.
Because snow reflects about 90 percent of the light that hits it, light energy is bounced back up to the atmosphere from the snowy surface of the earth. Once bounced skyward, light energy is again scattered by air molecules. That additional scattering saps the energy of the blue light waves, making the sky appear paler than sky over a darker, less reflective surface. In fall, the snowless ground bounces back only about 10 percent of the light that touches it.
Arctic haze is a wave of pollution transported by the wind from Eastern Europe and Asia that hovers over arctic regions every spring. Made up of particles such as sulfuric acid that are larger than air molecules, arctic haze doesn't scatter blue light as effectively as pure air does. So when arctic haze shows up every spring over Alaska, it has the effect of slightly dulling the blue of the sky.
Fall is not only the time for berry picking, moose hunting and wood chopping. It's also a good time to preserve on film the bluest skies known to Alaska. The resulting photos may be useful in de-bluing the spirit on a cold winter's night.
Ned Rozell is a science writer at UAF's Geophysical Institute.
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