Tuesday, April 23, 2013

Why is the Sun yellow, the sky blue, and the clouds white?

The Sun, sky and clouds over Birling Gap on
Christmas day 2012
A few weeks ago my 4 year-old nephew asked his mother why the Sun was yellow. Of course this got passed on to me, and I found myself trying to explain the wave nature of light to a bemused toddler in a moving car while turned around in my seat.

To a 4 year-old color is very important, hey it’s still important twenty years later.
To understand why the Sun is yellow, the sky blue, and the clouds white we first need to understand how color works.

The different colors we see are the result of different wavelengths of light being filtered out before reaching our eyes. Red light has longer wavelengths, and blue light has shorter wavelengths. By combining all the wavelengths of light together, red, orange, yellow, green, blue and purple, you get white light.

NASA SDO AIA 304 solar image
Just like waves on the ocean, light will always travel in straight line; that is until something gets in its way. It then has three options,
       Refracted (or bent)


Each of these mechanisms can be seen as a result of the Earth getting in the way of the Suns light as it makes its way out across space, but as you will see it is predominantly the scattering of sunlight that produces the colors of the Sun, sky, and clouds as we see them.

If you were to look at the Sun from space without Earth’s pesky atmosphere in the way it would actually look white, shortly before your eyes are burnt out. Even solar astronomers are in denial though coloring the sun yellow or orange in a number of their space based observations. We humans are creatures of habit after all.

Astronaut Sunita Williams holding the white Sun in her hand
whilst on a spacewalk
As light enters our atmosphere it is interrupted on its journey by particles of gas. These particles are much smaller than the wavelength of the light that it is interrupting. This results in something called Rayleigh scattering. Rayleigh scattering resorts in the light being redirected on its path towards the surface and is strongly wavelength dependent. Meaning that the shorter the wavelength, or the more blue the light is, the more it is scattered, while the longer the wavelength, the more red the light is, the less it is scattered. Leaving the sky its beautiful blue color.

That beautiful blue sky is accompanied by a strange yellow orb so bright that you cannot look at it directly even under the protection of our atmosphere. The reason for its vibrant yellow color is directly related to the blueness of the sky. As the atmosphere scatters out the blue light the remaining spectrum of the sun is shifted towards the yellow part. As the Sun moves towards the horizon at sunset the light passes through more and more of the atmosphere. This means that more and more of the shorter wavelength light is scattered away making the Sun appear redder as it sets in the evening.

Setting Sun (image credit Monika Landy-Gyebnar)

If you live in the UK, like I do, you will most likely be disappointed by the amount of time that the bright sunshine and the beautiful blue sky is blocked by big white fluffy looking objects.
No not sheep, but clouds.
Clouds are drops or frozen crystals of water and other chemicals in the Earths atmosphere. These droplets or crystals scatter the light in all directions regardless of the wavelength making them appear white. Dark clouds are created when they pass into the shadow of another cloud, or when the top of that cloud casts a shadow on its own base. They also look darker in contrast to a brighter sky. So a dark cloud will not always mean rain. Though if you are in the UK it is the most likely result.

Rainbow over the
University of Exeter
Double rainbow over the
University of Exeter
Other than scattering and reflection, we can also observe the refraction in the atmosphere. As larger droplets of water bend sunlight a rainbow forms. They appear in the sky opposite the sun as the light is bent entering water droplets in the air and then reflected inside the back of the droplet. This refraction, like in a prism, causes the light to be split into different wavelengths and thus different colors; red, orange, yellow, green, blue, indigo, violet.  Reflecting the light multiple times inside the water droplet can create double rainbows, where the order of the colors are reversed.

Although our vision is restricted to just a tiny portion of the whole electromagnetic spectrum the wave nature of light puts on remarkable displays for all to see. So look up and enjoy the show because, hey, even on a cloudy day the light is dancing.


Here are just a few helpful websites and blogs that also answer these questions.