Day 13 – @TAThompson16

Every day until Christmas Dr Helen Czerski will be finding some cool, hidden science in pics of people’s every day lives. To get involved, tweet your pic to @helenczerski and @cosmicshambles with the hashtag #ShamblesAdvent

Today’s picture is from @TAThompson16 , and this image is all about the beautiful blue sky.   The question of why the sky is blue is a common topic at science events, but I’ve rarely heard anyone explain why the whole sky isn’t all the same shade of blue.  In this picture, we can see that the sky at the top of the picture is a nice bright blue, but lower down (just above the clouds) it’s almost white.  Why is that?

It looks here as though the sun is relatively low in the sky and off to the right.   Light is travelling from the sun across the scene above our heads, and as it passes through the molecules in the atmosphere some of it is scattered in different directions.  Blue light is more likely to be bounced sideways down to Earth than red, so when we look up, we see the blue light that’s been redirected towards us.  That’s why the sky is blue.   

But it’s a bit more complicated than that. A human artist starts from a blank white canvas.    For Earth, the default is black (the colour of the universe) so any other sky colour requires a way to add light to that black background.  And although blue light is scattered most strongly, red and green light are also redirected by the atmosphere, just not as much.  The colour that we see depends on the mix – if it’s almost all blue light, the sky obviously looks blue.  But if the other colours of the rainbow are mixed in too, it will start to look white.   And if there’s only a bit of blue and none of the other colours, we’ll get a dark inky blue dusk.   

When we look sideways, we’re looking through much more air – the air is more dense lower down, and the low angle cuts through far more of the atmosphere. So light may be scattered more than once.   But also, the lower atmosphere contains plenty of tiny particles and dust.   When light is deflected by air molecules (higher up), the blue light is far more likely to be re-directed than red and green.  But when light is scattered by these tiny particles, the colour doesn’t matter.  All of them respond in the same way, so the scattered light is white, not blue.   The fun thing here is that the colour of the sky is giving us information about its structure, even though the individual building blocks (the molecules and dust) are far too small for us to see, even if they were right in front of our nose.

Read all of the other entries of Helen’s #ShamblesAdvent here

Dr Helen Czerski is a physicist, first and foremost, but she’s acquired a few other labels along the way: oceanographer, presenter, author and bubble enthusiast. A regular on The Cosmic Shambles Network, she has also presented a number of acclaimed documentaries for the BBC and her first book, Storm in a Teacup, which looked at the physics of every day things, was a bestseller. Recently she was awarded the prestigious William Thomson, Lord Kelvin Medal and Prize from the Institute of Physics.

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