Why The Sky Is Blue…And Red

Crepuscular rays from the setting sun at the Phoenix park, Dublin. These rays are due to Mie Scattering and appear white. Image: Author
Crepuscular rays from the setting sun at the Phoenix park, Dublin. These rays are due to Mie Scattering and appear white. Image: Author

Ok, a lot of the time the sky is grey in Ireland, but we do get our fair share of blue skies and spectacular dawns and sunsets. But why does the sky have a blue/orange/red colour? Why not white, or black? The truth is it contains all of these colours, it just depends on where you view it from.

Before explaining the reasons why, however, let’s take look at what light actually is. Light is electromagnetic radiation of wavelengths that are visible to our eyes. Electromagnetic radiation occurs when electrons in an atom receive enough energy to cause them to vibrate billions of times a second, each time emitting photons (parcels of energy) of a particular energy and hence wavelength. This wavelength depends on the atom (i.e. substance) and on which electrons within that atom are doing the vibrating.

The visible light spectrum, showing the wavelengths of the colours of the rainbow. Image: Wikipedia
The visible light spectrum, showing the wavelengths of the colours of the rainbow. Image: Wikipedia

Far off in our sun, where nuclear fusion of hydrogen to helium generates temperatures of around 5,500 °C on its surface, radiation is emitted over a wide range of wavelengths, from gamma rays to x-rays to radio waves. It takes sunlight around 8.5 minutes to cover the 150-million kilometre journey to Earth, travelling at 300,000 km per second. Around 50% of this radiation reaching the Earth’s atmosphere is infra-red (longwave), around 40% is visible (white light) and around 10% ultra-violet (shortwave). Much of the ultra-violet is absorbed by the ozone layer but enough still makes it to the surface, where it is absorbed and re-emitted as longwave radiation, warming the air above the surface. Of course it also affects humans, causing skin-ageing, sunburn, cataracts, etc.

Taking the visible portion of sunlight, this has a range of wavelengths of around 380-720 nanometres (a nanometre is one-millionth of a millimetre), and appears white to our eyes. The violets, indigos and blues are at the short-wavelength end whereas the yellows, oranges and reds are at the long-wavelength end.

Our eyes are most sensitive to the blue, green and red wavelengths.  As sunlight passes through the atmosphere the tiny air molecules (mostly nitrogen and oxygen) scatter the blue wavelengths through what is known as Rayleigh Scattering. Because this scattering is inversely proportional to the 4th root of the wavelength (if you halve the wavelength the scattering is 24=16 times stronger), blue is scattered much more than the red (by about a factor of 10). We therefore see this blue light and is this is the reason why the sky is “blue”.

So why does it appear orange or red at dawn and sunset? If you think of it, the sunlight is now passing through a lot more atmosphere along its shallow path to your eyes than if it were higher in the sky. By the time it reaches you most of blue has already been scattered out of it whereas the longer wavelengths have not. We therefore see these longer wavelengths as red, orange and yellow.

Of course definition of the word “sky” depends on what you mean. If you’re in space then the sky is black as there is no atmosphere to scatter light and no particles to reflect any. And back on Earth, you may notice that the sky looks increasingly paler blue or even white the lower down towards the horizon you look. This is due to Mie Scattering, which is like Rayleigh Scattering but is caused by much larger particles (dust, haze, etc.) in the atmosphere. It is not so wavelength-dependent, therefore all the wavelengths are scattered fairly equally, giving a white appearance. This white effect is most noticeable if you look towards the sun, as Mie scattering is greatest in the forward direction.

And of course the sky is black at night. We all know that!

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