Croatia, Blue Skies and Sunsets

“As the sun went down it seemed to drag the whole sky with it like the shreds of a burning curtain leaving rags of bright water that went on smoking and smouldering among the estuaries and around the many islands”                     Laurie Lee – ‘As I walked out one Midsummer Morning

Yesterday I finished with a sunset and this made me think about the sky.  Throughout the journey through Dalmatia and the Croatian islands I had been in awe of the big soft blue skies and gentle billowing clouds and throughout the week this had been very changeable.  The day had started with a disappointment in the sky but by lunchtime everywhere was exceptionally blue and the clouds were magnificent, rising high like great cathedrals in the sky.

If you have ever wondered why the sky is blue this is the reason.  Light travels through space in a straight line for as long as nothing disturbs it and as it moves through the atmosphere it continues on its journey until it collides with a bit of dust or a gas molecule and then what happens to the light depends on its wavelength and the size of the thing it crashes into.

Dust particles and water droplets are much larger than the wavelength of visible light and when light hits these large particles, it gets reflected in different directions. Gas molecules however are smaller than the wavelength of visible light and when light hits them, some of it gets absorbed and then the molecule radiates the light in a different direction.  The colour that is radiated is the same colour that was absorbed but the different colours are affected differently because blues are absorbed more easily than reds.

This process is called Rayleigh scattering and is named after Lord John Rayleigh, an English physicist, who first explained it a hundred and thirty years ago.  The blue colour of the sky occurs because the absorbed blue light is radiated in different directions and gets scattered all around the sky and since we see the blue light from everywhere overhead, the sky looks blue.  It’s as simple as that!

The day finished with a sunset, so what about sunsets?  Well, as the sun begins to set, the light must travel farther through the atmosphere before it gets to us and more of the light is reflected and scattered.  As less reaches us directly, the sun appears less bright and the colour of the sun appears to change, first to orange and then to red and this is because even more of the short wavelength blues and greens are now scattered and only the longer wavelengths are left in the direct beam that we can see.

What makes it even more dramatic is that the sky around the setting sun takes on a lot of different colours and the most spectacular shows occur when the air contains many small particles of dust or water because these particles reflect light in all directions and then as some of the light heads towards us, different amounts of the shorter wavelength colours are scattered out and we get to see the longer wavelengths and the sky appears red, pink or orange.

And that explains a sunset!  But if you prefer a more scientific explanation here are the details:

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