When the sun is low in the sky, and especially at twilight, the sky grows more and more blue towards its zenith. Why is the color blue so vivid at the apex of the sky? And what color would it be if it weren't blue?

When you look up into the sky, often you'll notice that certain parts are bluer than the rest. Generally when the sun is high, the sky becomes a deeper blue closer to the horizon. When the sun is low in the sky, the zenith of the sky comes a rich blue while the sky near the sun is pale blue, or almost white.


The deep blue zenith of the sky is known as zenith blue enhancement. The ozone layer takes a bite out of the yellow-red wavelengths of light, absorbing them. Meanwhile, it scatters the blue wavelengths. When the sun is low in the sky, any light that gets to us from the zenith of the sky has taken a long path to get to us, and so has had the most yellow and red light strained out of it. This is why the sky, while still luminous, shines blue.

If the ozone layer were stripped away, the sky would turn a very different color. The yellow and red wavelengths would linger as light faded away, and the zenith of the sky would turn an unpleasant gray-green color at sunset. After the sun dipped below the horizon, it would light up yellow, before the last of the light receded and it would turn black.

Image: Rodrigo Nuno Bragança

[Via Riddles in my Teacup, NCBI, Skyrenderer]