Why is the sky blue?

Or perhaps, why does the sky appear to be blue? Because the atmosphere (air) itself is not blue, so why should we look up and see a blue heavens above us?

This is the stereotypical “why” question. The sky’s colour is something we’re very used to and take for granted. But we do have a perfectly good explanation for why it’s blue, and why it changes colours around sunset and sunrise.

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Why blue?

Light from the sun appears to be white, but it is made up of a combination of all the rainbow colours that appear white all together. Each colour of light has a different wavelength which corresponds to a different energy.

The gas molecules (mainly nitrogen and oxygen) that make up Earth’s atmosphere are what this sunlight encounters before reaching our eyes here on Earth. When the white/multi-coloured light hits gas molecules, it is scattered. This means that all the individual light colours are, quite simply, scattered, rather than staying together as a beam of white light.

Blue and violet light have a shorter wavelength, so they are higher energy light waves and are scattered more by gas particles (blue light is scattered nearly 10 times more than red light). This means that we see the blue light in the sky, whereas the other colour of light make it down to Earth’s surface more directly.

And while violet light has an even shorter wavelength than blue, there is less of it to start with and it is scattered so much that very little actually reaches our eyes, so the sky is definitely blue rather than violet.

 

Sunrise and sunset

When the sun is near the horizon, its light must pass through a greater distance of atmosphere. Through this process, the blue light is scattered and rescattered many times. In the end, very little of it actually reaches your eyes, while the reds and yellows take a more direct path straight to you.

So quite simply, the sky appears all shades of red, pink, yellow and gold!


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