The sky isn’t just blue – airglow makes it green, yellow and red too

The distinct colours of airglow are the result of atoms and molecules releasing particular amounts of energy in the form of light.

Look up on a clear sunny day and you will see a blue sky. But is this the true colour of the sky? Or is it the only colour of the sky?

The answers are a little complicated, but they involve the nature of light, atoms and molecules and some quirky parts of Earth’s atmosphere. And big lasers too – for science!

So first things first: when we see a blue sky on a sunny day, what are we seeing? Are we seeing blue nitrogen or blue oxygen? The simple answer is no. Instead the blue light we see is scattered sunlight.

The Sun produces a broad spectrum of visible light, which we see as white but it includes all the colours of the rainbow.

When sunlight passes through the air, atoms and molecules in the atmosphere scatter blue light in all directions, far more than red light.

This is called Rayleigh scattering, and results in a white Sun and blue skies on clear days.

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