Why Is the Sky Blue? Rayleigh Scattering Explained

Why is the sky blue? It is one of the first science questions children ask, and one of the hardest for adults to answer well. The common reply, that the sky reflects the sea, is wrong: the sky is blue over deserts too, and the sea is partly blue because it reflects the sky.

The real answer involves sunlight, the molecules of the air and the human eye. The same physics explains why sunsets glow orange and red, and why clouds are white. Here is how it works.

Sunlight is a mixture

Sunlight looks white, but it is a mixture of colours, from red, with the longest wavelengths of visible light, through orange, yellow, green and blue, to violet, with the shortest. A prism or a rainbow separates them.

On its way through the atmosphere, that mixture meets countless molecules of nitrogen and oxygen. What happens next depends on the wavelength of each colour.

Scattering by small particles

When light meets particles much smaller than its wavelength, such as individual gas molecules, some of it is scattered in all directions. This is called Rayleigh scattering, after the British physicist Lord Rayleigh, John William Strutt.

The key fact is that this scattering depends very strongly on wavelength: it is inversely proportional to the fourth power of the wavelength. Halve the wavelength and the scattering increases sixteen times. Blue light, with its shorter wavelength, is therefore scattered much more than red.

Why the whole sky glows

Because blue light is scattered so strongly, it bounces around the atmosphere and reaches our eyes from every direction, not just from the direction of the Sun. Look anywhere in the daytime sky away from the Sun and you are seeing sunlight that has been scattered towards you, mostly the blue part.

Scattering removes a significant share of sunlight from the direct beam, and much of that reaches the ground as diffuse skylight. That is why shadows on a clear day are not completely dark: they are lit by the blue sky.

Why not violet?

If shorter wavelengths scatter more, violet should scatter most of all, so why is the sky not violet? The answer, as scientists describe it, is a combination of three things: the spectrum of sunlight itself, the scattering by air molecules, and the way human vision responds to the result.

Our eyes do not see the scattered light as a pure violet or pure blue. They perceive the mixture as if it were a combination of blue and white light, which is the pale blue we know. The colour of the sky is partly a fact about the atmosphere and partly a fact about us.

Red sunsets

At sunrise and sunset the Sun is low, and its light travels a much longer path through the atmosphere to reach us. Along that long path, most of the blue and green light is scattered out of the direct beam.

What is left is the longer-wavelength light: orange and red. That is why the Sun and the clouds around it glow warm colours at the ends of the day. Ozone high in the atmosphere also absorbs some light near sunset and during twilight, which helps the sky overhead keep its blue.

Why clouds are white

Clouds are made of water droplets that are much larger than the wavelength of light. Particles that size do not favour blue; they scatter all colours roughly equally. Scattering by larger particles is described by a different theory, called Mie theory.

When all colours are scattered equally, the result looks white, so clouds appear bright white in sunlight, and light passes through them much as it passes through frosted glass. Thick storm clouds look grey or dark because so little light gets through them at all; under the thickest, daylight can fall to around a thousandth of direct sunlight.

Tyndall and Rayleigh

The explanation took time to find. In 1869 the Irish physicist John Tyndall observed that light passing through a fine suspension of particles was scattered with a faint blue tint. In 1871 Rayleigh worked out the mathematics of the effect, and he later showed that individual gas molecules were enough to produce it.

It is a good example of how science answers an everyday question: a careful observation, a mathematical explanation, and then a test of whether the explanation fits everything we see, from the blue overhead to the red at the horizon.

Checking the easy answers

The idea that the sky reflects the ocean is a reminder that a plausible answer is not always a correct one. Testing it is simple: the sky is just as blue far inland. Our guide to how to evaluate a claim covers the habits that catch explanations like that.

The sky is also where some of nature's most dramatic physics happens. For another, louder example, read our explainer on how lightning works.