To understand why the sky is blue, we first need to understand a little bit about light. Although light from the sun looks white it is really made up of a spectrum of different colours, as we can see when they are spread out in a rainbow.
We can think of light as being a wave of energy, and different colours all have a different wavelength.
When the sun's light reaches the Earth's atmosphere it is scattered, or deflected, by the tiny molecules of gas (mostly nitrogen and oxygen) in the air. Because these molecules are much smaller than the wavelength of visible light, the amount of scattering depends on the wavelength. This effect is called Rayleigh scattering, named after Lord Rayleigh who first discovered it.
Shorter wavelengths (violet and blue) are scattered the most strongly, so more of the blue light is scattered towards our eyes than the other colours. You might wonder why the sky doesn't actually look purple, since violet light is scattered even more strongly than blue. This is because there isn't as much violet in sunlight to start with, and our eyes are much more sensitive to blue.
Scattering of light At sunrise and sunset the sun is very low in the sky, which means that the sunlight we see has travelled through a much thicker amount of atmosphere. Because blue light is scattered more strongly by the atmosphere, it tends to be scattered several times and deflected away in other directions before it gets to us. This means that there is relatively more yellow and red light left for us to see.
The diagram on this page shows a simplified illustration of these effects. A person standing at position A would see a blue daytime sky, as there is plenty of blue light being scattered in all directions.
At position B, it is evening and a person standing here would see a familiar orange sunset because the blue light has mostly been scattered away in other directions, leaving the reds and yellows.
When there are clouds in the sky, the light is also scattered by the water droplets inside the cloud. Unlike the gas molecules in the atmosphere, these water droplets are about the same size as the light's wavelength which means that they scatter light in a different way called Mie scattering. Instead of the different colours being scattered different amounts, they are all scattered equally strongly. The light being scattered from the cloud is therefore the same colour as the light that is shining on it from the sun - white.
When you are underneath a thick cloud it will seem a much darker grey colour because, as well as being scattered, most of the light is absorbed before it can get down to the bottom of the cloud. However, to someone standing further away who can see it in the sunlight, the cloud will still look white.
Last updated: 4 July 2014