Why is the sky blue? And other annoyingly rhetorical questions

I am often bemused by people who use ‘why is the sky blue’ as rhetoric – often to symbolise some question for which there is no answer.

Actually there is an answer, and we’ve known it since 1871: ‘Rayleigh scattering’. It’s also why sunsets look red and orange. The effect is named after John William Strutt, Third Baron Rayleigh (1842-1919), the British physicist who discovered it.

The phenomenon works like this: incoming sunlight, which contains light of all wavelengths and hence colours, is scattered by molecules in the upper atmosphere. The increasing density of atmosphere itself also acts as a scattering mechanism.  The wavelength of light mostly scattered (technically, absorbed and re-emitted) is at the shorter end – blue and green, creating the diffuse glow across the whole sky which, to the human eye, usually looks light blue.

Other wavelengths are scattered when the light comes at a direct angle, which is why the Sun appears yellow (but don’t look – it will damage your eyes).

This scattered light is also polarised. That’s why a polariser on your camera produces such a dark blue at certain viewing angles relative to the sun.

Oriental Bay, Wellington – an image I took with full polarising, creating false-colour blue in the sky.

When the sun angle lowers, and the light is passing through a thicker layer of atmosphere, more of the blue wavelengths are scattered and only the longer wavelengths are obvious – orange and red – hence the colours of sunset, gradiating to a darkening blue above.

Blue sunset on Mars – for the same reason skies are blue on Earth. An approximately true colour image by the Spirit rover at Gusev Crater, 2005. Photo: NASA/JPL, public domain.

This scattering effect is true everywhere – not just on Earth. It varies slightly because atmospheric compositions differ, and the oxygen in our atmosphere is a factor. However, if you were dangling from a balloon in Jupiter’s atmosphere and looked up, you’d see blue sky there, even though the air is mostly a poisonous mix of hydrogen, helium and traces of other stuff like phosphene. Even the sky of Mars is blue – we’ve imaged that blue slice-wise through the upper air. It appears pink from lower down, looking up, because suspended dust in the atmosphere scatters the longer wavelengths. That’s still Rayleigh scattering. And Martian sunsets are blue – for exactly the same reason.

Mars imaged in 1995 by the Hubble Space Telescope – with blue cast due to Rayleigh scattering. Cool. Photo: NASA, public domain.

Earth’s sky appears blue, I might add, to us. Humans are lucky; our colour vision is based on three receptors. Many animals use two, which reduces the palette of colours they can see. (Of course, most of them also have much better night vision: swings and roundabouts).

So there you have it. Next time anybody idly gets rhetorical and asks ‘why is the sky blue’, you can go all Sheldon on them with an annoying literal answer. Or talk about Martian sunset colours, but I suppose that comes to the same thing really.

Copyright © Matthew Wright 2013