Blue Skies and Red Sunsets
Blue sky above Perth. Notice how the sky goes to almost white at the horizon.
We probably take for granted the most colourful thing in our daily lives, the blue sky. When it goes away on rainy days we appreciate it more. But we're so used to it few would consider why it's blue, rather than green, or red, or that if we were on the moon, where there is no atmosphere, the sky would be black, the colour of deep space.
The black sky on the moon due to the lack of an atmosphere.
The light from the sun is white, a combination of all the colours of the rainbow and the earths atmosphere is full of air which is combination of many types of molecules which are much smaller than the wavelength of light. Sometimes the air has water drops or ice crystals in it. These are clouds and the stuff that make up clouds are larger than the wavelength of light, and scatter all the wavelengths equally well. Thus clouds look white, having scattered all the components of white light equally well. But in the case of air, where the molecules are much smaller than the wavelength of the light, the different colours are not scattered equally. The shorter the wavelength the more likely the light will be scattered. This is called Rayleigh scattering. Blue light is the shortest wavelength of the colours in white light, while red is the longest. Thus the sky is blue because when you look up into the air you are more likely to see scattered blue light than any other colour.
The earth's blue atmosphere seen from space.
This phenomena known as Rayleigh Scattering, after Lord Rayleigh who first came up with the model which describes it, explains other aspects of light in the sky other than the colour blue. The model describes how scattering of light by small molecules in the atmosphere goes as a forth power of the wavelength. For example, blue light with a wavelength of 450 nanometers, compared to red light with a wavelength of 600 nm is (600/450)^4 times more likely to be scattered. That is 3.2 time more likely. While blue light is more likely to scatter, it says all colours will be scattered but with shorter wavelengths more likely. The intensity of the light will depend on the amount of atmosphere you are looking at. Looking directly overhead, at the apex, one is looking through the least atmosphere and the intensity will the lowest. Directly overhead is where the sky usually looks bluest as it has the lowest component of other colours and is backed by the blackness of space. Looking towards the horizon one is looking through the most atmosphere and enough off all the colours can be scattered that the light looks almost white and is brighter because there is much more light scattered.
For the same reason the sky is bluest looking directly overhead is why the sky looks darker and bluer at altitude, because the observer is looking through less air, resulting in a deep blue sky. However at altitude on a mountain an observer looking towards the horizon will look through an even thicker cross section of the atmosphere than an observer at sea level, resulting in an even whiter appearance towards the horizon.
Blue sky as seen on Mauna Kea, Hawaii. Notice the sky is almost white at the horizon.
The complementary affect of the blue sky is the red sunset or sunrise. At sunset the light from the sun reaching the observer has traveled through the most atmosphere possible and has had most of the blue component of the white light scattered resulting in predominately red light reaching the observer or illuminating clouds that the observer can see. The light at sunset is more likely to be red than sunrise because the turbulence in the atmosphere during the day can mix more particles into the atmosphere which enhance the scattering.
Sunset at Casey Station Antarctica
The redish light of sunset reflected off the surface of the earth as seen from the STS-093.
Scattering of blue light by the atmosphere can also alter the colour of objects seen in the distance. This is the effect that causes mountains in the distance to appear blue.
Blue sky seen through a hole in extremely clear blue glacier ice.