On most sunny days around here (midwest USA), the cloudless sky is what I would call a light blue color at the zenith, fading toward nearly white at the horizon. Even in the absence of obvious clouds, any kind of humidity or haze tends to make the blue less saturated, until you get to shades of gray.
A few years ago, I was on a bike ride in a rural area here around midday in high summer on a perfectly cloudless day, and was struck by how dark blue the sky was - verging on violet directly overhead, but still dark blue even down toward the horizon. I actually stopped to stare, and tried to take pictures of the sky, but they did not really capture the effect (I’m not surprised that a cheap digital camera image of an unbroken field of uniform color didn’t get the color balance right). I initially stopped mostly because the effect was so eerie: bright sunlight and brightly lit objects contrasted against a dark cloudless sky. Living as I do in tornado country, it triggered some part of my brain that was yelling ‘bad storm’s coming!’ But I digress.
What conditions would have been in effect for the clear mid-day sky to be so dark blue? If it’s just low humidity and dust-free air, are there places in the world where the sky is normally this dark? Antarctica, for example, or some other high desert? Did the fact that this was near noon, with the sunlight impinging as close to vertical as possible contribute? What about local surface albedo? Could it have been a purely perceptual phenomenon, like my eyes being tricked by some combination of colors in the terrestrial surroundings?
I don’t recall having seen a clear sky so dark blue in the ten years or so since this happened, so either such conditions are rare or I’m not particularly perceptive.
the standard answer to “Why is the sky blue?” by an optics student is “Rayleigh scattering”, which is fine as far as it goes, but doesn’t explain the range of blues that you see. There’s actually quite a lot going on with scattering in the sky, even in the absence of complicating factors like weird particulates in the atmosphere. Jearl D. Walker listed a lot of references in his book the Flying Circus of Physics ages ago, as he’s been adding to it. Here’s the relevant chaspter online:
I have a suspiciuon that Walker started out his book by sitting down with all the back issues of the American Journal of Physics and making a card file of the articles. There do seem to be a lot from there. But for the sky, his big references are M. Minnaert’s classic the Nature of Light and Color in the Open Air and W.J. Humphry’s Physics of the Open Air. In essence, you have to consider multiple scattering, scattering from dust as well as air molecules, partial polarization of the light (classic experiment is to look at the blue sky through polarizers, as al photographers know, it’s polarized), and the effects due to contrasts that you mention. There’s a lot going on there beyond that simple first-order scattering.
I’m not wondering why the sky is blue, I’m wondering why the sky would have been such an unusually (IMHO) deep, dark shade of blue on a particular day. And, by extension, whether there are places on Earth where such conditions are more probable.
I suspect that it’s because the air was particularly dry and clean, and perhaps because the incident light was coming from a high angle, but I don’t know if that’s true.
As a matter of fact, I did, and furthermore the lenses were brown, but when I stopped I took them off and the effect persisted. I was riding along an E-W road, and the sun was in the southern sky. With the glasses on, the sky was notably darker in the east and west, at 90 degrees to the sun (so directly ahead and behind me) but even with the glasses off the sky as a whole was a much darker blue than ‘normal’. Certainly enough so to keep me standing in the ditch for a few minutes, looking around at the sky like a mooncalf. I’ve seen the polarization effect in the sky before, though, and I agree that it also makes the sky a deeper blue.
Many things affect the “blueness” of the sky. Dust particulates in the sky and humidity are two. The sky is generally bluer right after a cold front passes through. Similarly, the sky is bluer at higher altitude, though, since you specified the Midwest, that’s clearly not the factor at issue here.
I would tend to agree with this as I’ve noticed a bluer sky the day after a storm front moves through, especially after it clears a period of high humidity out.
Yes, dry air scatters less light, so it will appear darker blue. The sky tends to be a deeper blue here (UK) in winter than in summer, as we are more prone to get cold, dry Arctic air rather than milder and moister stuff from the Atlantic.
There is also evidence that the sky is getting whiter overall due to the tiny ice crystals left by air traffic: Is aviation 'whitening' the sky?
I first noticed a different color sky when I was traveling from my home state (Indiana) to Minnesota. The sky was noticeably darker blue than any average day in northern IN. It can’t be just transient conditions since my sky is never that blue.
I find the sky to be dimmer over the open plains than it is in an area with a lot of mountains. There is a certain heaviness to the sky without mountains to hold it up.
One thing I’ve noticed is that the blue part of the sky above will often seem darker if there are a lot of bright white clouds up there too (to serve as contrast-there are several optical illusions out there which work on a similar principle).
“Brightness”. Hmm. Guess it depends on what you mean by that.
I have spent time in some of the most remote areas of Oregon imaginable. In the area in the SE that’s a big black nothing on satellite pics I have seen the bluest skies ever. One solid sea of blue from horizon to horizon.
No humidity, no dust, no contrails, nothing to diffuse the sunlight.
