Why Is the Sky Blue?

Quick answer

Sunlight contains all colours. When it enters the atmosphere, gas molecules scatter it in every direction, and shorter wavelengths such as blue are scattered much more strongly than longer ones such as red. The blue light arrives at your eyes from all over the sky, so the sky looks blue.

Rayleigh scattering: The scattering of light by particles much smaller than its wavelength, such as the gas molecules in air, where shorter wavelengths scatter far more strongly than longer ones.

Sunlight is a mixture of colours

Sunlight looks white, but a prism or a rainbow shows that it holds every colour. Each colour corresponds to a different wavelength: NASA's explanation notes that light travels in waves and that blue light has shorter, smaller waves than red light. The colours of the rainbow, from red through to violet, run from the longest wavelengths to the shortest.

Rayleigh scattering: what the atmosphere does to that light

When sunlight meets the gas molecules that make up the air, it bounces off them in all directions. NASA's wording is that blue is scattered more than other colours because it travels as shorter, smaller waves. In the simplest account, the strength of this scattering rises steeply as wavelength falls, so blue light is scattered several times more strongly than red.

The result is that the direct beam from the Sun loses some of its blue, while the rest of the sky glows with scattered blue light. Look at any patch of sky away from the Sun and the light reaching you has been redirected toward your eyes by air molecules along the way.

Why the sky is not violet

Violet has an even shorter wavelength than blue, so it is scattered even more. Several other factors reduce the violet that reaches the ground: the Sun emits less violet than blue, some of it is absorbed high in the atmosphere, and human eyes are less sensitive to violet than to blue. The mix that reaches the eye is perceived as blue. NASA's page does not discuss violet, so the point rests on general optics rather than on that source alone, and it is a commonly given part of the full explanation.

Why sunsets are red and orange

When the Sun is low in the sky, its light crosses a much longer stretch of air before reaching you. NASA explains that along this longer path more of the blue light has been scattered out of the beam, so the remaining light is dominated by reds and yellows. Dust, smoke and other airborne particles also scatter blue light, which is why sunsets after fires or volcanic eruptions can look especially vivid.

Why the sky changes colour with the Sun's position
Situation	Midday, Sun high	Sunset or sunrise, Sun low
Path of light through air	Short	Long, grazing through the lower atmosphere
What dominates the overhead sky	Scattered blue light	Light that has lost much of its blue
Colour near the Sun	White to pale yellow	Orange to red

Why the sky is paler near the horizon

Look toward the horizon on a clear day and the blue fades to white. Light from low in the sky has travelled through more air, so it has scattered several times and the colours blend together. Reflection from the ground adds more mixed light. NASA describes the same effect: the sky lightens near the horizon because of repeated scattering.

Other skies

The same physics explains other sights. Clouds look white because their water droplets are large enough to scatter all colours about equally. The sky on Mars looks butterscotch or reddish in daytime because the thin carbon dioxide atmosphere carries fine dust, though NASA notes the sky near the Sun at sunset can look blue-grey, nearly the reverse of Earth's.

Not the reflection of the ocean

A common belief is that the sky is blue because it reflects the sea. The explanation runs the other way round: the sea often looks blue partly because it reflects the sky, and it also absorbs red light. The sky's colour comes from scattering in the air itself.

If you want to see the same idea at work in everyday life, notice how the beam of a torch in a dusty or misty room becomes visible: tiny particles redirect part of the light toward you. That visible beam is scattering, the same process on a smaller scale. For another everyday light-and-water effect, see Why Condensation Forms.