A Study in Blue

Here's a shot of the trail at Lyon's Woods Forest Preserve yesterday:

The question at hand is what does that photo have to do with this one?


Mountain Bluebird (Sialia curricoides), Illinois Beach State Park,
Lake Co, IL 11/5/2011

I've read that the blue snow in the first shot is the result of the camera's white balance being off -- but that's really how it looked. Why? The simple answer: it's reflecting the sky. But... why is the sky blue? That's down to Rayleigh Scattering, where different wavelengths of light (and hence different colors) are bent by different amounts when they hit dust and other particles in the atmosphere. Since reds and yellows are only bent a little, you only see them close to the sun, where the direct light is enough to overwhelm them. Any spot in the sky away from the sun, the only light bent far enough to reach your eyes will be in the blue part of the spectrum.

So that's why this glacier is blue, right?

Northwestern Glacier, Kenai Fjords National Park, AK 8/4/2012

Not so fast. Notice that the glacier is deepest blue in the shadows. It also turns out that its almost always cloudy in the summer in Kenai Fjords National Park, where this was taken, and you can see from the photo that it was this day as well. The tour boat captain stated that the blue is deepest when there are clouds in the sky. So what gives?

Well, this one isn't the result of scattering, rather it's the result of water (even as ice) preferentially absorbing red light. (Scuba divers experience the same phenomenon.) So if there's enough water, the only light left to be transmitted will be blue. Apparently it's possible to see this effect in fresh snow, by poking a deep enough hole into the snow. But it doesn't explain the blue shadows, because the air pockets in snow reflect light back out so fast that the red light never has a chance to be absorbed.

What does this have to do with the bluebird? Birds don't have blue pigments! Instead, the feathers use a scattering and absorption effect to achieve blue colors. Incoming light is refracted by small structures on the barbules (small structures in the vanes of the feather), and blue light ends up bouncing back out, to make the feather appear blue. This wouldn't work, though, if there was nothing to absorb the reds and yellows. Eventually they would be reflected back out, and you'd end up with white feathers. So feathers on blue birds have a thin layer of melanin inside all of the structural stuff. You can see the effect of this in domesticated Budgerigars. Normally, they use a combination of yellow pigment (carotenoid pigments, actually) and structural blues to produce green plumage. However, if a gene interferes with the pigment, they end up looking blue. If a different gene interrupts melanin production, they end up yellow. Finally, if both genes are interrupted, they end up white.

So that's what blue skies, Illinois snow, Alaskan glaciers, and bluebirds have in common!

p.s. According to our captain, Northwestern Glacier actually has an Illinois connection -- apparently its named for Northwestern University!