I’d also like to just mention that when I did my thesis defense, one of the professors on the panel asked me to explain why the sky was blue, and I pretty much blew it. There were three professors, and each asked me a single question, and that’s the one I tanked on. So I basically failed 33% of my ultimate final exam. Technically. Not that I didn’t understand the question, or I didn’t know the answer… it was that I got so flustered out of nervousness that I actually answered the question completely wrong (I swapped the two kinds of scattering), and went on for about 3 minutes explaining it (incorrectly). I got to the end of my explanation and said, “you know what? I’m just now realizing that’s all completely wrong. Just assume I explained one thing that DOESN’T have anything to do with why the sky is blue.”
Thankfully the professors were good sports about it, and let me start over with the correct explanation.
SO. Why is the sky blue? I’ve had this post in a draft form for about three weeks and still can’t figure out a decent way to explain it that isn’t astoundingly boring, so let’s take a few more tangents beyond the few I’ve already gone on, and throw them in for good measure and see if we can’t get somewhere. Why’s the statue of liberty green? Why is snow white? Basically, the color of anything you see is the product of wavelength scattering. Interestingly enough, it’s not just limited to the wavelengths you see, all spectrums of wavelengths are reflected and scattered. Have you ever walked onto a sidewalk on a summer night, barefoot, and felt the heat? There are satellites overhead that measure and track the wavelengths that that warm heat is indicative of. You might also be interested to know, those satellites primarily function at night, because daylight data capture wouldn’t provide anything meaningful, as it would be all washed out – much like how taking a picture at night with a camera with no flash is pretty much pointless… the sensors in the camera won’t pick up any meaningful data.
But back to the sky being blue: it’s blue because of Rayleigh* scattering. Raleigh scattering is simply the effect that light has on molecular and atomic gases and solids as it passes through our atmosphere. Those gases and solids don’t change their atomic nature, but how they behave does. Those gases and molecules move at the same frequency, and take on a uniform color indicative of the wavelength that’s primarily being scattered. It’s not just why the sky is blue, but it’s also why the sun seems yellow, and why sunsets seem red, or tornado clouds seem green, etc., etc. But the XKCD comic illustrating the post raises a great question: “so why isn’t the sky PURPLE?” In case you’re not sure why exactly that’s such a great question, on the visible color spectrum, the only visible color with a shorter wavelength than blue, is purple. If blue light dominates so much because it is so short, it stands to reason that purple light should dominate even more, because it’s even shorter.
And you’d be right. The sky SHOULD be purple. At least by that logic. In fact, on a bright, cloudless sunny day, when the sun is high in the sky, I want you to observe the whole sky. The closer to the sun the sky is, the paler the blue seems to be. The closer to the horizon it is, the deeper and darker it seems to be. That, my friends, is the result of Mie** scattering. Mie scattering has the effect of “whitening” the sun’s wavelengths, because the particles in the atmosphere are scattering light more or less the same across all wavelengths. It’s really more a matter of where the observer is, than truly equal scattering. Because the purple wavelength is so short, Mie scattering has the effect of readily “flushing” it out, thus allowing blue to dominate. As the sun sets and Mie scattering diminishes, the blue transitions to purple (in areas further away from the sun) as the Rayleigh scattering effect becomes even more dominant. But since the angle at which we're observing the sun is changed, we also see more of the red spectrum in the atmosphere closer to the sun. Once our position passed completely into the earth’s shadow, all visible wavelengths are blocked. And that seems like a good stopping point because after that we're just hopping onto another tangent and onto another topic.***
And that’s why the sky is blue.
* Pronounced ”ray-lee.” But I definitely prefer the proper spelling. It just looks right.
** Pronounced “me.” Not nearly as cool, but definitely way more confusing.
*** You know, if there's a topic you'd love to see discussed, leave a comment somewhere. Here, facebook,, instagram, email, text... I'm not a hard guy to get a hold of.