Why is the Sky Blue?

Jay,

This seems like something everyone should know, right? Well, it's actually not that simple. First of all, the sky is not just blue. It's Carolina Blue.

As light from the Sun travels through Earth's atmosphere, it runs into things. When light travels through water droplets or hits dust particles, it gets bounced off in different directions. This doesn't explain why the sky is blue, though, because every color of light is similarly affected by dust and water droplets because they are so much larger than the wavelength of visible light (400-700 nanometers).

When light hits gas molecules, which are smaller than wavelength range of light, it is scattered. This effect is called Rayleigh Scattering. Different wavelengths of light are scattered at different angles. Red, orange, and yellow light are mostly unaffected because they have longer wavelengths. Blue light, though, is scattered greatly because of its shorter wavelength.

If you look closer to the Sun in the sky (never look directly at the Sun!) you'll notice that the sky gets more pale or white in color because your eyes are receiving more of all the wavelengths of visible light. The rest of the sky looks blue because of the Rayleigh Scattering of the shorter wavelengths of light, especially when look farther away from the Sun in the sky. If you look closer to the horizon, though, the sky looks pale again because you're looking through more atmosphere, and the blue light is scattered too much by the extra atmosphere.

Rayleigh Scattering also explains why we have a orange-red sunsets. When the sun is close to the horizon, only the longer wavelengths of light are able to pass straight through the greater amount of atmosphere, while the shorter wavelengths are scattered out.

Hopefully this helps! So, if you want to answer the bumper sticker question "If God isn't a Tar Heel, then why is the sky Carolina Blue?," you can respond, "God is actually just a fan of Rayleigh Scattering."

Jesse Richuso
Morehead Planetarium and Science Center