Why the sky is blue

In short: Sunlight contains all colours. The molecules in the air scatter short-wavelength blue light much more strongly than red light, so blue light reaches us from every part of the sky.

In more detail: This effect is called Rayleigh scattering. It happens when light meets particles much smaller than its wavelength, such as nitrogen and oxygen molecules. The scattering becomes much stronger the shorter the wavelength is.

In detail

• White light: Sunlight is a mixture of all the colours of the rainbow, from red with a long wavelength to violet with a short one.
• Rayleigh scattering: The strength of scattering depends on the fourth power of the wavelength. Blue light is therefore scattered several times as strongly as red.
• Why not violet: Violet is scattered even more, but the Sun emits less of it, part of it is absorbed high in the atmosphere, and our eyes are more sensitive to blue.
• Red sunsets: When the Sun is low, its light travels a long way through the air. The blue is scattered away along the path, leaving yellow, orange and red.
• White clouds: Water droplets are much larger than air molecules and scatter all colours roughly equally. That is why clouds look white or grey.
• Haze: Dust and tiny droplets make the sky look paler and whiter, especially near the horizon.
• In space: Without air there is no scattering. On the Moon the sky is black even during the day.
• Other planets: On Mars, fine dust tints the daytime sky reddish, while sunsets there look bluish.
• The bigger picture: The colour of the sky is a good example of how light interacts with matter (licht-und-farben, elektromagnetisches-spektrum).