Why is the sky blue ?

Every day, when we go outside, we can see that the sky is blue. But, why blue ?

The blue sky you would like to see everyday

To summarize, the sunlight is made up of all colors of the rainbow. Those colors are defined by a specific wavelength, each color has its own.

When all colors are mixed, it seems white. But it is also possible to break them down to observe colors separately.

Our atmosphere is composed of an incredible number of molecules, dust and minuscule water droplets. Sunlight interacts with our atmosphere, without him, the sky would be black. It is the case in the moon, where stars, Earth and sun are visible in a black sky.

All colors don't interact in the same way with our atmosphere, according its wavelength. The shortest wavelength (blue color) is the most scattered by our atmosphere. That is why the sky is blue.

How blue the sky is depends on the air quality. As dust and water droplets don't scatter the sunlight, the blue will be mixed with white light when the air quality is polluted or humid. On the contrary, in mountainous areas and during sunny days, the sky will be deep blue because of the really pure air quality.

In the evening, when the sun is low, the sunlight is slanting and wavelengths have to cross a bigger atmosphere surface. In this case, wavelengths corresponding to red and orange colors are the most scattered and some of the blue light is scattered away. That is why sunsets are red and orange.

The blue sky mystery is now solved!

You can also have a look at this video published by Brain Stuff - HowStuffWorks on Youtube for further information.

Bonus: Why is the ocean blue ?

When sunlight hits the ocean, the water absorbs long wavelength (red color) and very short wavelength (purple) like a filter, and blue light is absorbed the last. So the light that we see is mostly made up of blue wavelengths.

Water in a glass is clear because there aren't enough water molecules to absorb the light.

I hope you enjoyed this article!