Why Is the Sky Blue?

Nothing says "fair weather" like clear, blue skies. But why blue? Why not green, purple, or white like clouds? To find out why only blue will do, let's explore light and how it behaves.

Sunlight: A Melange of Colors

blue sky
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The light we see, which is called visible light, is actually made up of different wavelengths of light. When mixed together, the wavelengths look white, but if separated, each appears as a different color to our eyes. The longest wavelengths look red to us, and the shortest, blue or violet. 

Usually, light travels in a straight line and all of its wavelength colors are mixed together, making it appear nearly white. But whenever something intercepts light's path, the colors are scattered out of the beam, changing the final colors you see. That "something" could be dust, a raindrop, or even the invisible molecules of gas that make up the atmosphere's air.

Why Blue Wins Out

As sunlight enters our atmosphere from space, it encounters the various tiny gas molecules and particles that make up the atmosphere's air. It hits them, and is scattered in all directions (Rayleigh scattering). While all of the light's color wavelengths are scattered, the shorter blue wavelengths are scattered more strongly -- roughly 4 times more strongly -- than the longer red, orange, yellow, and green wavelengths of light. Because blue scatters more intensely, our eyes are basically bombarded by blue.

Why not violet? 

If shorter wavelengths are scattered more strongly, why then isn't the sky appear as violet or indigo (the color with the shortest visible wavelength)? Well, some of the violet light is absorbed high up in the atmosphere, so there is less violet in the light. Also, our eyes aren't as sensitive to violet as they are to blue, so we see less of it. 

50 Shades of Blue

blue sky-beach
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Ever noticed that the sky directly overhead looks a deeper blue than it does near the horizon? This is because the sunlight that reaches us from lower in the sky has passed through more air (and therefore, has hit many more gas molecules) than that reaching us from overhead. The more molecules of gas the blue light hits, the more times it scatters and re-scatters. All of this scattering mixes some of light's individual color wavelengths together again, which is why the blue appears to be diluted.

Now that you have a clear understanding of why the sky is blue, you  may wonder what happens at sunset to make it turn red...