Atmospheric bluesky

Published on December 26th, 2010 | by Carl Mundy

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Why do we have blue skies, red sunsets?

It’s a question that at one point in our lives we have either been asked or asked ourselves. Why is the sky blue? Why are our sunsets red? These two questions are two sides of the same coin. Out of all the colours in the rainbow, why does the sky appear blue and not red, yellow or even green? It all has to do with the air we breath and the light from our Sun.

Why is the sky blue?

Our journey starts in the neighbourhood of our nearest star, our Sun. Light emitted from our Sun (the processes which lead to this will be explored in a later, more in-depth article) crosses the expanse of space towards our tiny blue planet and enters the Earth’s atmosphere, which is made up of air molecules, water, dust and aerosols. What is important is that the Sun gives off light in a range of wavelengths, or colours. It’s just that it emits more in the yellow and green than any other which is why we see the Sun as a yellow ball in the sky, rather than blue.The light travels through the air and eventually hits an air or water molecule or a dust or aerosol particle. For the purpose and scope of this article, we can say that light travels in straight lines but when the light hits one of these particles/molecules, it is deflected in another direction. This is called scattering. This light eventually reaches the Earth’s surface. This is the reason why the sky can still seem light even when the Sun has set below the horizon as well. How much of this light depends on two things: the size of the molecules/particles in the air and the colour of the light.

British scientist Lord Rayleigh explored this area and found that, when the wavelength of the light is much larger than the size of the particle, , then the amount of light scattered is proportional to the size of the particles divided by the wavelength, of the light, all to the forth power, as seen below. 

    \[  I \propto \left(\frac{d}{\lambda}\right)^{4} \]

Molecules in the atmosphere like Oxygen (O2) and Nitrogen (N2) have sizes of the order 10-10m while the light we see from our Sun typically has a wavelength of 10-7m – which is 3 orders of magnitude larger!

From this relation we can see that light of smaller wavelengths is scattered more than that of larger wavelengths. Blue light has a smaller wavelength than red light and so this is scattered more in our atmosphere. Essentially, more blue light reaches the surface of the Earth – where we are – than any other so we perceive the sky as blue.

Why are sunsets red?

If you are still here, then you can now find out why sunsets are red. At the time of a sunset we see light that has travelled through a thick layer of the atmosphere without scattering – we see light that has come directly from the Sun without interacting with the atmosphere. This is predominately red light and so sunsets appear red to us!

Further your knowledge

Do you want to learn more about this area? Atmospheric physics is an exciting area of research with simple physics able to explain complicated phenomena in extraordinary detail. Why do cyclones rotate in opposite directions in opposite hemispheres? It can even explain why the myth that bath water drains in opposite directions in opposite hemispheres is false. If you want to further your knowledge, read up on the following areas of interest:

  • Structure of the atmosphere
  • Coriolis force
  • Global Warming

 

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About the Author

Astronomy PhD student from the UK with a passion for astronomy and science outreach projects. Involved with weekly science-based radio programme The Science Show on University Radio Nottingham (URN).



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