Why does the sky bleed?

The above photograph shows the view from my bedroom on a winters morning. It is one of the traits of any scientist when they see something like this to stop and affirm in their mind they know what is really happening. We do not sit around basking in the beauty of such phenomena – leave that to the artists.

The reason the sky is red is not one which is of overall great complexity it is just nice to stop and think about what is really happening. Perhaps we should start with why the sky is normally blue? It seems foolish to explain why the sky is red before we know why it was blue in the first place. And then why sunlight is a warm yellow. Then finish up with red. It all comes down to light. The elusive, mysterious light. Light has wave-particle duality, but for the purposes of this explanation think of light as travelling in waves. When light hits something, one of three things can happen:

  1. It is absorbed;
  2. It is refracted; and
  3. It is reflected.

Now the earth of course has an atmosphere which is what allows us to breathe this delicious air. On a very small scale, air contains big things. There are lots and lots of tiny particles whizzing around doing their thing – as gaseous particles like to do. Light is an arrogant character and wont be considerate of others. What light tends to do is take the shortest path possible and go that way. Well of course what this means is collision. Air-light collision. Of course air isn’t really one gas at all, it is made up of lots of different gasses the most important being nitrogen and oxygen. When light hits these particles it is sort of taken in by these particles and then re-scattered in a different way.

This analogy is by no means perfect but it helps me to understand things in my mind (I would encourage you to think of it in whatever way you understand it best). I think of this a little bit like a watering can. The water is the beam of light, and the head is the atom. When it hits the head of the watering can the water is sprinkled out. If I were to put a head on the can with a different profile – different holes or different spacing then the can would sprinkle differently. Light and particles are very much like this – the light is absorbed by the particle and then scattered.

Richard of York gave battle in vain. The colours of the rainbow. Everyone knows this – red, orange, yellow, green, blue, indigo, violet. Now the colours in the second half of this progression, the greeney blues are associated with short wavelengths and high frequencies. The particles which exist in the air scatter light in all colours but not in equal proportions. Infact it is basically in reverse – most violet, then indigo, then blue etc etc. So the reason the sky looks to be the colour which it is is a result of the blend of these scatter patterns.

Okay I think that should suffice nicely for blue light. Now all this blue light has been scattered. What that means is we see Roy on Earth. That is Red, orange and yellow. Think about it carefully – light travels the shortest distance when it is directly above us – 12pm. It has longest to travel when it is sunset – because the sun has moved. All that is happening is more of the OYGBIV is being scattered – and more of the red. Sunsets (or sunrises – the earth is a sphere) scatter of more and more of the OYGBIV. What is left? Red.

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8 responses to “Why does the sky bleed?

  1. I like the watering can analogy, it works well. There is another component to the ‘why is the sky blue?’ explanation though. If we follow through your argument then the sky should actually be more of a purple colour than blue, and indeed it is, except that the human eye isn’t very good at seeing those wavelengths and so we only see the blue component of the colour.
    I would also argue that there is always time to appreciate a good sunset. Keep up the good work.

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    • Hi Jason, thank you for your insight on the subject! I have to say I hadn’t relised the last part I had just thought the mix of colours is what gave the sky its colour – fascinating to learn how it really is a case of perception!

      Liked by 1 person

  2. I am always paying attention to different lighting conditions when I take photos at different times of the day. Morning light tends to be more golden and twilight has a blue cast. It was explained to me that these colors are more pronounced in photographs because the human brain kind of corrects for these variations in light temperatures, but film or a photo sensor in a digital camera does not. But I always wondered why the sky is different colors at different times of the day, and I never knew why some sunsets are red. Now I know. Thanks!

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    • H Amanda, great to hear someone appreciate the same phenomena from a different perspective. I think there is so much value behind the sky from a scientific and an artistic perspective – glad I could help!

      Liked by 1 person

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