Why is the Sky Blue?
Here’s one I don’t hear often enough: Why is the sky blue? It’s not as simple an answer as you might expect. The air we breathe is a mixture of oxygen and nitrogen but it sure looks invisible to us when we wave our hands through it. The clouds are white for the most part, but they’re made up of water vapor. What gives?
One way to approach this mystery is to find correlations. For example, take a look at this picture from high altitude:
What’s going on here? The sky is bluer closer to the surface but fades to black as it approaches space. How does this correlate? Well, we know that the density of the atmosphere is greatest near sea level and gradually drops off as you go up in altitude, so much so that even at 16,000 feet humans need to breathe an oxygen source or risk altitude sickness. So is it the oxygen that is making the sky appear blue? Yes and no.
What’s actually happening is the blue wavelength of light is being scattered in all directions more heavily than the other colors. Light that we see is made up of a spectrum of colors (remember the color gradient from light passing through a prism?) determined by their frequency – short frequency goes towards blue and longer frequency towards red. As light moves through the atmosphere it comes in contact with molecules in the air but the longer wavelengths tend to be unaffected while the shorter wavelengths get bounced around in all directions. So while the rest of the colors head straight to us, the blue wavelengths shoot off in all directions like sparks from a sparkler thereby creating a flood of blue light no matter which direction you look upwards to.
That’s about the most simple answer I can give, but it’s nowhere near complete. There are other complications such as how the cones and rods in our eyes bias towards blue and how the density of the atmosphere effects the scattering. But for the most part, when someone asks you “why is the sky blue?” you can say with reasonable confidence “because the air scatters the blue portion in all directions.”
Hmm, this means I’ll have to bookend this discussion with “Why are Sunsets Red?” next time…