Why Is the Sky Blue? Rayleigh Scattering in Plain English

It's one of the first science questions most of us ask as children, and plenty of adults still can't answer it with confidence. The good news is that the explanation's simple once you hold on to one idea: light travels as waves, and each colour has a different wavelength.

I've pulled this together from the Met Office and NASA's Space Place, so you can explain it to a curious child, a class or yourself. Here's how sunlight, air and your own eyes work together to colour the sky.

Sunlight is a mix of colours, not plain white light

Sunlight looks white, but the Met Office explains that it's made up of many colours, each with its own wavelength. NASA's Space Place points out that a prism can split sunlight into a rainbow, which is the easiest way to see that hidden mix for yourself.

Red has the longest wavelength in the visible spectrum, while blue and violet have much shorter ones. NASA also notes that light travels in straight lines unless something reflects it, bends it or scatters it. As soon as sunlight enters our atmosphere, it starts meeting gas molecules, and that's where the colour of the sky comes from.

Rayleigh scattering: how air molecules make the sky blue

The effect is called Rayleigh scattering. According to the Met Office, sunlight hits gas molecules in the atmosphere, mainly nitrogen and oxygen. These molecules are much smaller than the wavelengths of visible light, and because of that they scatter shorter wavelengths much more than longer ones.

So blue light gets bounced around in all directions, right across the sky, while more of the red, orange and yellow light carries on its way. When you look up at any patch of sky away from the Sun, you're seeing scattered blue light arriving from every angle. Here's the chain of events in order:

1. Sunlight, carrying every colour, reaches the top of the atmosphere.
2. It meets tiny molecules of nitrogen and oxygen.
3. Short wavelengths, blue and violet, are scattered strongly in all directions.
4. Longer wavelengths, such as red and orange, are scattered far less.
5. Scattered blue light reaches your eyes from all over the sky, so the whole sky looks blue.

Why the sky isn't violet when violet light scatters even more

This is the twist that catches people out. Violet has an even shorter wavelength than blue, so it's scattered even more strongly. The Met Office gives two reasons we don't see a violet sky:

• Our eyes are more sensitive to blue light than to violet.
• There's less violet in sunlight to begin with.

Put those together and blue wins. There's still scattered violet light up there, but the mix of sunlight and the way our eyes respond mean the sky reads as blue rather than purple.

Why the blue sky fades to pale near the horizon

Look straight up on a clear day and the blue is at its deepest. Look towards the horizon and it fades to a paler, almost white shade. The Met Office explains that light reaching you from low in the sky has passed through much more air, so it's been scattered and scattered again along the way. That extra scattering mixes in more white light and makes the blue less intense.

NASA's Space Place adds that the Earth's surface also reflects and scatters light, which mixes the colours further and adds to that whiteness near the ground.

Why sunsets and sunrises turn the sky red and orange

The same process explains a red sunset. At sunrise and sunset the Sun sits low, so its light passes through a much thicker layer of atmosphere before it reaches you. Over that longer path, most of the blue is scattered out of the beam, leaving the longer red and orange wavelengths to reach your eyes.

NASA's Space Place notes that dust, pollution and other particles in the air can scatter blue light too, which is why the whole western sky sometimes glows red. As the Met Office puts it, blue skies and red sunsets are two results of the same scattering, seen at different angles and times of day.

Why clouds look white against a blue sky

If air scatters blue light, why aren't clouds blue as well? The Met Office explains that cloud droplets are much larger than gas molecules, and they scatter all colours almost equally. Sunlight starts out white, the droplets scatter every colour by about the same amount, so the light stays white and so do the clouds.

Cloud bases are often grey, and the Met Office says that's down to the same scattering: light inside the cloud is mostly sent upwards or sideways, so less of it reaches the bottom. Rain clouds have larger droplets that scatter even more, so even less sunlight gets through to their undersides, which gives them that dark, brooding look. Seen from a plane, the tops of clouds look brilliant white because they're lit directly by the Sun.

What colour is the sky on Mars compared with Earth?

Mars makes a lovely comparison. NASA's Space Place explains that Mars has a very thin atmosphere, mostly carbon dioxide, with fine dust that scatters light differently from the air on Earth. During the day, the Martian sky is orange or reddish. At sunset, the sky around the Sun turns blue-grey, the opposite of what we see at home. Photos from NASA's Mars Pathfinder lander show both.

Sky feature	Earth	Mars
Daytime sky	Blue	Orange or reddish
Sky around the setting Sun	Red and orange	Blue-grey
Main scatterers	Nitrogen and oxygen molecules	Fine dust in a thin, mostly carbon dioxide atmosphere

Explaining the blue sky to children and curious minds

If you're explaining this to a child, here's how I'd put it: sunlight is a mix of every colour, and the air is full of tiny particles that bounce blue light around far more than red. So wherever you look in the sky, blue light is heading your way. The prism NASA mentions is a handy way to show that white light hides a rainbow.

Sky questions tend to lead to space questions. If the next one you're asked is how far away the stars are, my guide to what a light year measures picks up the story.

Science communicators can find science blogs that publish contributed explainers here.

Frequently asked questions

Is the sky blue because it reflects the sea?

No. The Met Office and NASA both put the colour down to sunlight being scattered by gas molecules in the air, not to reflection from the oceans.

What is Rayleigh scattering in simple terms?

It's the scattering of light by particles much smaller than its wavelength, such as nitrogen and oxygen molecules. Short wavelengths like blue are scattered far more than long ones like red, which the Met Office says is why the sky looks blue.

Why don't we see a purple sky?

Violet light is scattered even more than blue, but our eyes are more sensitive to blue and sunlight contains less violet, so the sky looks blue to us.

Why is the sky red at sunset?

When the Sun is low, its light crosses much more atmosphere, so most of the blue is scattered away before it reaches you. The reds and oranges are what's left.

Is the sky blue on other planets?

Not always. NASA says the daytime sky on Mars is orange or reddish, while the sky around the setting Sun there looks blue-grey.