Out in the Woods

Photo by Francesco Ungaro

Why is the Sky Blue? No Simple Answer

By Kevin McKeon, Maine Master Naturalist

Earth’s surface is mostly water, so the sky must be the blue water’s reflection from Earth’s atmosphere, right? Turns out it’s a little more complicated than that. The sky looks blue due to something called the Rayleigh scattering phenomenon.

It begins with how humans perceive light. Light hits our eyes after refracting through and reflecting from objects. It then gets transferred to electrical signals which get processed into various colors to form images in our brains. Our sense of vision spans from the shorter violet wavelengths to the longer red ones. The combination of all those colors forms what we perceive as white. Shorter wavelengths have higher energy: The very short waves — microwaves — heat our coffee and pop our corn! (A deep dive into waves can be found here.)

Earth’s 60-mile-thick atmosphere of gases, dust, soot, pollen, and oceanic salt forms a mostly transparent shell, held in place by gravity. It’s relatively thin and can be compared to a pillowcase tightly wrapped around a basketball. As light waves pass through it, the various particles and air densities cause a multitude of refractions, giving us red sunsets, blue skies, and rainbows.

Light waves are also affected by the medium through which they travel, like Earth’s various atmospheric levels, the oceans, prisms, and the vacuum of space. The sky on the moon looks black to its human visitors because there is no atmosphere, thus there are no particles from which light can reflect to our eyes. Stars generating high energy can appear to be blue-white, and low energy stars, orange-red. NASA describes the sun as a 4.6-billion-year-old yellow dwarf. It will eventually expand into a red giant and engulf Earth and then, around its 8 billionth birthday, begin to run out of fuel — collapsing into a white dwarf.

Back on Earth: As the sun lowers to the horizon at dusk, its light must travel through more atmosphere and more particles to reach our eyes, increasing the number of light wave refractions from these particles. Also increased are the scattering and absorption of the short, high-energy waves, to the point where only the longer, orange-red waves are perceived by our eyes, giving us the reddish sunsets. This has recently been illustrated by the many photos of the aurora borealis published online. The more colorful ones are time-lapsed, with both the longer exposures and more sensitive camera lenses able to reproduce more colors.

Now for our blue skies. The higher energies of blue light waves hit more atmospheric stuff than the red ones, so get refracted and scattered more than the red ones. John William Strutt, 3rd Baron Rayleigh, aka Lord Rayleigh, found that the shorter, stronger wavelengths of blue light get scattered in all directions by various atmospheric particles — overpowering our retinas with the resultant, much more prevalent, blue light waves. So, these refractions are described as Rayleigh scattering, and explain why the sky is blue. Also, some refractions from elements’ molecules change the wavelength — thus the color. So, elements can be identified according to the color that’s reflected from them. This is called Raman effect and is the basis for spectroscopic analysis. It’s how scientists can determine the elemental makeup of materials, like soil samples, crime scene evidence, and even the stars and planets!

Blue sky video: https://www.youtube.com/watch?v=0b1fqodmZJ0

Dive into waves: https://science.nasa.gov/ems/02_anatomy/

Editor’s note: Did you see something unusual last time you were out in the woods? Were you puzzled or surprised by something you saw? Ask our “Out in the Woods” columnist Kevin McKeon. He’ll be happy to investigate and try to answer your questions. Email him directly at: kpm@metrocast.net