By: Steve Roark
Tennessee Division of Forestry
September often provides some brilliant blue skies as the seasons change. As an amateur naturalist, the basic question of why the sky is blue came to me, which required some research to figure out. I thought I would share, so prepare yourself for a short physics lesson.
The sun emits light, which is a form of energy that exists as bands of electromagnetic waves. The length of these waves is what gives us the colors we see. The color violet has the shortest visible wavelength, while red has the longest. All other colors fall somewhere in between. Light travels from the sun to the earth in tiny energy particles called photons.
The earth’s atmosphere is mostly nitrogen, oxygen, and carbon dioxide gas. When photons from the sun strike these gases, they absorb most of them. But light containing the blue wavelength doesn’t get absorbed, but instead bounces off the atmosphere and come racing down to earth and enters your eye, which sends the information to your brain. Your brain interprets the wavelength and tells you: “That’s blue… pretty!”
Rainbows are proof that all the other colors are up there. The raindrops reflect more of the different wavelengths, which bounce down to our eyes so we can see violet, indigo, blue, green, yellow, orange, and red bands of color arching across the sky.
What’s cool about the sky is that it’s never the same color blue every day. When it’s heavy laden with humidity, it’s pale and subdued. Other times after a storm front has cleared out the air, the sky is a deep blue it defies description. Morning skies are different from evening skies. Whatever shade it is, pause in your busy day and take a look.
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