Blue sky quick facts
- NOAA’s scientists say pure air scatters violet light three to four times more effectively than it scatters longer waves of light.
- At noon, with the Sun high overhead, sunlight looks white, not yellow.
- Clean air, not pollution, gives the brightest sunrises and sunsets. Haze and smog usually make the colours duller.
- The sea is blue mainly because water soaks up red light, not because it reflects the sky.
- On Mars the daytime sky is butterscotch, and the sky around the setting Sun turns blue.
Sunlight is a mix of colours
Light from the Sun looks white. In fact, it is a mix of every colour of the rainbow, all travelling together.
You can see this with a prism, a block of glass or crystal with a special shape. When white light shines through a prism, it spreads out into red, orange, yellow, green, blue and violet. Isaac Newton used a prism shaped like a triangle to show that white light is a mixture of all these colours.
A rainbow shows the same thing. Drops of rain spread sunlight out into its colours, much as a prism does.
Here is the first surprise. At noon, when the Sun is high overhead, its light looks white, not yellow. The Sun only looks yellow, orange or red when it sits low in the sky. The reason is the same one that makes the sky blue. Never look straight at the Sun to check, because it can damage your eyes.
Light travels in waves
Light moves as waves, a bit like waves rolling across the sea. Light waves are far too small to see, and they come in different sizes.
The distance from the top of one wave to the top of the next is called the wavelength. Each colour of light has its own wavelength.
Red light travels in long, lazy waves. Blue light travels in shorter, choppier waves. Violet has the shortest waves of all the colours we can see.
Keep that difference in mind. Short waves are the key to the whole answer.
Why the sky is blue: air scatters blue light
The air around Earth is a mix of gases. Those gases are made of molecules, tiny particles that are much smaller than a wave of light.
When sunlight meets these molecules, some of it bounces off in a new direction. Scientists call this scattering. The molecules scatter short waves much more than long ones.
So blue light gets scattered far more than red light. NOAA is the United States weather and ocean agency. Its scientists say pure air scatters violet light three to four times more than it scatters the longer waves.
The scattered blue light bounces all over the sky. Wherever you look, some of it is heading straight towards your eyes. That is why the whole sky glows blue, not just the part near the Sun.
The rest of the sunlight carries straight on to the ground. That is why the Sun still shines brightly, even though some of its blue has been shared out across the sky.
This kind of scattering by tiny air molecules has a name. It is called Rayleigh scattering, after the British scientist Lord Rayleigh.
Why the sky is not violet
Here is a puzzle. Violet light has even shorter waves than blue, so air scatters it even more. So why is the sky not violet?
There are two main reasons. First, sunlight holds less violet light than blue light. Less violet goes into the air, so less violet comes back out of it.
Second, our eyes are much more sensitive to blue than to violet. The NOAA scientist Stephen Corfidi writes that without this, the clear daytime sky would look violet instead of blue.
The light that reaches you from the sky is really a mix of violet, blue and other colours. Your eyes and brain add that mix together. The answer they give is sky blue.
Why the sky is paler near the horizon
Look at a clear sky and you will notice something. It is deepest blue straight above you. Near the horizon, where the land or sea meets the sky, it is much paler.
Light from low in the sky has crossed far more air to reach you than light from overhead. Along the way, its blue light is scattered and scattered again, many times, in many directions.
The ground reflects and scatters some light too. All that extra scattering mixes the colours back together. Mixed colours look white, so the sky near the horizon turns pale blue or almost white.
Overhead, the light takes the shortest path through the air. It is scattered fewer times, so the blue stays strong.
Why sunrises and sunsets are orange and red
At sunrise and sunset, the Sun is low in the sky. Its light has to travel much further through the air to reach you than it does at midday.
On that long journey, most of the blue and violet light is scattered out of the beam before it arrives. What is left to reach your eyes is mostly yellow, orange and red light. Most of the blue never arrives.
That is why the Sun looks orange or red when it is low, and why the sky around it glows gold. The same air that makes the midday sky blue makes the evening sky red.
Clouds can make a sunset better. Stephen Corfidi notes that the most memorable sunsets tend to have at least a few clouds. The clouds catch the last red and orange rays of the Sun and reflect them down to the ground.
When a big volcano erupts, it can paint the sky too. A huge eruption can throw tiny particles 12 to 18 miles (19 to 29 km) up. That is more than twice as high as Mount Everest.
About 15 minutes after sunset, those high particles can still catch the sunlight and glow. These colours are called afterglows. After Mount Pinatubo in the Philippines erupted in June 1991, lilac and red afterglows were seen over the eastern United States that September.
Why clouds are white or grey
Clouds are made of tiny drops of water. Those drops are much bigger than the gas molecules in the air.
Big drops like these scatter all the colours of sunlight almost equally. No colour is picked out, so the light stays a mix of every colour. A mix of every colour looks white.
One cloud holds millions of these droplets. Together they scatter so much light that the cloud shines bright white against the blue sky.
So why are some clouds grey? The droplets send much of the light back upwards or out to the sides. That leaves the top and sides of a cloud whiter than its base, which gets less light. Thick rain clouds let even less light through, so their bases look grey or dark.
A grey cloud is made of the same kind of droplets as a white one. Its underside is simply in its own shade.
Why the sky on the Moon is black
On the Moon, the sky is black even at midday, with the Sun blazing overhead.
The reason is simple. The Moon has almost no air. Its atmosphere is so thin that scientists say it hardly has one at all. With almost no gas molecules, there is nothing to scatter sunlight across the sky.
Sunlight still pours down onto the grey ground, which is why the surface looks bright. But the light travels straight from the Sun to the ground. None of it is bounced sideways into the sky, so the sky stays black.
The same is true out in space. Far from any planet’s air, the sky is black in every direction.
This is the best proof of the whole idea. Take away the air, and the blue sky disappears with it.
Why Mars has a butterscotch sky and blue sunsets
Mars turns Earth’s sky colours upside down. Its dust makes all the difference. By day, its sky is a dusty yellowish colour that astronomers call butterscotch. At sunset, the sky around the Sun turns blue.
Mars has very thin air, less than one per cent as thick as the air on Earth. If Earth’s air were a book of 100 pages, the air on Mars would be less than one page. There are too few gas molecules there to make much of a blue sky.
Instead, fine rusty dust hangs in the Martian air. The bigger dust particles soak up blue light and scatter the other colours. That gives the daytime sky its butterscotch colour.
At sunrise and sunset, the light has to cross much more of this dusty air. Mars’s fine dust scatters light in a different way from Earth’s air, and the sky close to the Sun turns a cool blue-grey. NASA’s robot rovers have photographed these blue sunsets on Mars.
Sky colours compared
| Where and when | Sky colour | Why |
|---|---|---|
| Earth at noon | deep blue overhead, paler near the horizon | air molecules scatter blue light all over the sky |
| Earth at sunset | orange, red and gold near the Sun | the light crosses much more air, so most of the blue is scattered away first |
| Earth under thick cloud | grey | big water droplets scatter every colour and block light from above |
| The Moon in daytime | black | almost no air, so nothing scatters the sunlight |
| Mars in daytime | butterscotch, a dusty yellowish colour | very thin air, and dust that soaks up blue light |
| Mars at sunset | blue-grey around the Sun | fine dust scatters blue light close to the Sun |
Why the sea looks blue
The sea gets its blue mostly from the water itself, not from the sky above it.
NOAA’s ocean scientists say the ocean is blue because water absorbs the red part of sunlight. Absorb means soak up. Like a filter, the water takes out the reds and leaves the blues for us to see. Deep water works like a blue filter.
A glass of water looks clear because it holds far too little water for this to show. It takes a great depth of water, like the open sea, before the blue is easy to see.
The sea is not always blue. NOAA explains that it can look green, red or other colours when light bounces off sediment floating in the water. Sediment means tiny bits of sand, mud and other material.
The surface of the water does reflect some of the sky as well. That is why a sea under heavy cloud can look grey. But on a sunny day, the deep blue comes mainly from water soaking up red light.
How scientists solved the puzzle
The first clear answer came from a laboratory in London.
In the 1860s, the scientist John Tyndall worked at the Royal Institution. He built a simple glass tube to copy the sky, with a white light at one end to stand in for the Sun.
Then he slowly filled the tube with smoke. From the side, the beam of light looked blue. Seen from the far end, it looked red. He had made a tiny daytime sky and a tiny sunset in one tube.
Tyndall thought the light was bouncing off specks of dust or water vapour in the air, like the smoke in his tube. He was partly wrong. Scientists now know that the molecules of the air itself do most of the scattering.
Lord Rayleigh, whose real name was John William Strutt, discovered how tiny air molecules scatter light. He showed that the shorter the wave, the more it is scattered. That is why the effect is named Rayleigh scattering.
You can repeat a version of Tyndall’s test at home, with milk instead of smoke. The steps are in the Try it box below.
Three things people get wrong
1. “The sky is blue because it reflects the sea.” It does not. The sky is just as blue over the middle of a desert, hundreds of kilometres from any ocean. Its colour comes from air scattering blue sunlight. If anything, it works the other way round. A calm sea reflects a little of the sky, though its blue comes mainly from water soaking up red light.
2. “Air is blue.” Air has no colour of its own. A room full of air looks clear, and at night the same air above you looks black. The blue only appears when sunlight shines through many kilometres of air and gets scattered on the way. The Moon proves it: with almost no air, it has no blue sky at all.
3. “Red sunsets are caused by pollution.” They are not. Even perfectly clean air turns the setting Sun orange and red, because its light crosses so much air that the blue is scattered away. Stephen Corfidi goes further. He says clean air is the main ingredient in brightly coloured sunsets. Haze and smog near the ground usually make the colours duller, not brighter, and they turn a blue daytime sky greyish or white.
Scientists do not all describe this the same way. NASA says that dust and pollution can add to a red sky, because they also scatter blue light. Both agree on the main point. The long path through the air does most of the work.
Did you know? In the 1860s, John Tyndall made a tiny sky inside a glass tube at the Royal Institution in London. When he filled the tube with smoke, its beam of light looked blue from the side and red from the far end.
Try it
Make a sunset in a glass. Fill a clear glass or a clear plastic bottle with water and stir in a few drops of milk. In a dark room, shine a torch or a phone light into one side of the glass. Look at the glass from the side, at right angles to the beam: the milky water has a faint blue glow. Now look through the glass from the opposite side, straight towards the light: the light looks yellow or orange, like a setting Sun. The tiny bits of milk scatter blue light sideways, just as air does. If the water just looks white, start again with less milk. Never shine the light into anyone’s eyes.
Knowledge check
Why does blue light get scattered more than red light?
Show answer
Blue light travels in shorter waves, and the tiny molecules in air scatter short waves much more than long ones.
Why is the sky on the Moon black, even in the daytime?
Show answer
The Moon has almost no air, so there is nothing to scatter sunlight across its sky.
Frequently asked questions
Why is the sky blue and not violet?
Violet light is scattered even more than blue, but sunlight contains less violet than blue, and human eyes are much more sensitive to blue. The mix of scattered colours therefore looks sky blue to us.
Why is the sky red or orange at sunset?
At sunset, sunlight crosses far more air to reach you than at noon. Along the way most of the blue light is scattered out of the beam, so the light that is left looks orange and red.
Is the sky blue because it reflects the sea?
No. The sky is just as blue over deserts far from any ocean, because air scatters blue sunlight. The sea is blue mainly because water absorbs red light.
What colour is the sky on Mars?
The daytime sky on Mars is a dusty butterscotch colour, because fine dust hangs in its very thin air. Around the rising or setting Sun, the Martian sky turns blue-grey.
Why is the sky black on the Moon?
The Moon has almost no air, so there are no gas molecules to scatter sunlight. Even in bright daylight, the sky above the Moon stays black.
Why are clouds white?
Cloud droplets are much bigger than air molecules and scatter all colours of sunlight almost equally. The colours stay mixed, and mixed sunlight looks white. Thick clouds look grey underneath because less light reaches their base.
Who discovered why the sky is blue?
In the 1860s, John Tyndall showed at the Royal Institution in London that light in a smoky tube looks blue from the side. Lord Rayleigh discovered how tiny air molecules scatter light, and the effect is named after him.
Sources
- NASA Space Place: why is the sky blue? — sunlight as a mix of colours, prisms, light waves, blue light scattered by air molecules, why the horizon is paler, red sunsets, and the colours of the Martian sky
- Royal Museums Greenwich (Royal Observatory): why is the sky blue? — Newton’s prism, the white noon Sun, Rayleigh scattering and Lord Rayleigh, why the sky is not violet, the Moon’s black sky, and Mars’s thin air, butterscotch sky and bluer sunsets
- NOAA Storm Prediction Center: the colours of twilight and sunset (Stephen Corfidi) — violet scattered three to four times more than longer waves, eye sensitivity, why clean air gives the brightest sunsets, clouds at sunset, and volcanic afterglows after Mount Pinatubo
- Met Office: why are clouds white? — why cloud droplets scatter all colours almost equally, and why cloud bases look grey
- NOAA National Ocean Service: why is the ocean blue? — water absorbing red light, and sediment and particles giving the sea other colours
- Royal Institution: John Tyndall’s blue sky apparatus — Tyndall’s 1860s smoke-filled tube, blue from the side and red from the end, and the air molecules that do the scattering
