Why is the sky blue?
Why is the sky blue?
Sunlight is white. Air is clear. So where does the blue come from? 🌤️
Saturday. Ava lies on the grass in a London park.
“Why is the sky blue?” she asks. “It's the sea, reflected,” says her big brother. 🌊
Is the sky blue because of the sea?
No.
The sky is blue over deserts too. 🏜️ The sea is blue partly because water soaks up red.
Sunlight looks white. But it is every colour, mixed. 🌈
Newton split it with a glass prism.
scattering
[skat·er·ing]
noun
light hitting something tiny and bouncing off in a new direction.
tiny things scatter short, blue waves more than long, red ones.
From NASA Space Place and 気象庁
1,000×
smaller than a wave of light
That is how small the bits of air are. 🫧 Mostly molecules of nitrogen and oxygen.
From NOAA
Send a sunbeam through the air.
Blue and violet bounce off first, all over the sky. 💙 The rest of the beam goes on.
sun
you
× air
How much air the sunbeam crosses
A white sunbeam crosses a band of air. Coloured dots bounce off it, mostly blue and violet, and the sky behind turns blue.
A drawing. The colours follow Rayleigh's rule.
From NASA Space Place and Rayleigh
about 6×
more blue scattered than red
Short waves bounce much more. Halve a wave, and it bounces 16 times more. 📏
From Rayleigh
Blue comes at you from every direction.
So the whole dome looks blue. The sun, missing a little blue, looks a bit yellow. ☀️
From NASA Space Place and JAXA
But violet waves are even shorter. Why isn't the sky purple? 💜
Three reasons.
Turn the page. 👇
From NOAA
Why not violet?
Less to start
Sunlight has less violet than blue.
Lost up high
Some violet scatters away high above us.
Your eyes
Our eyes feel blue much more than violet.
From NOAA and Royal Observatory Greenwich and 気象庁
The spectral irradiance of skylight is shown to be a metameric match to unsaturated blue light.
Glenn S. Smith, American Journal of Physics, 2005
The sky sends us lots of violet too. Our eyes add it all up to a pale blue. 👁️
From Smith
Who worked it out? 🕵️
About 1508 · Leonardo da Vinci: sunlit vapour in front of the dark. A good guess.
1869 · John Tyndall makes a blue glow in a glass tube.
1871 · Lord Rayleigh writes the rule: short waves scatter most.
1910 · Albert Einstein: air on its own is enough. No dust needed.
From Museo Galileo and Tyndall and Rayleigh and Einstein
1787. Saussure climbs Mont Blanc with a ruler for blue. 🏔️
Paper squares, pale to deep. At the top, the sky was almost too blue for his card.
Geneva
Mont Blanc
A wheel of 16 blue paper squares, pale to deep. The pointer is on square 7, for the sky over Geneva.
The pointer swings past square 15, the darkest, for the sky on top of Mont Blanc.
From Linda Hall Library and Nautilus
Evening. The sun sinks behind the rooftops. 🌇
The sky glows orange. “Where did the blue go?” asks Ava.
At sunset, the light crosses much more air.
Up to 38 times more. The blue is scattered out on the way. Red and orange are what is left. 🧡
sun
you
× air
How low the sun is: from noon to sunset
The sunbeam crosses more and more air. Its far end turns from white to orange to red, and the sky turns gold.
From Kasten & Young and NOAA
Sunsets are red because the sky is blue.
It is the same light. The blue was taken out on the way to you.
From NOAA
⅔
of the blue above you at sunset
comes from ozone, not scattering. 🔬 Found in 1953, checked again in 2023.
From Hulburt and Lange et al.
What makes the best sunsets?
Clean air
Haze makes colours dull, not brighter.
High clouds
They catch the last light and glow.
Big volcanoes
After Pinatubo in 1991, glowing skies lasted about 18 months.
From NOAA
16 October 2017. The sun over England turns red at lunchtime.
Storm Ophelia had carried desert dust and wildfire smoke north. 🌪️ The dust took out the blue.
From Wyche et al. and CBS News
Then why are clouds white? ☁️
Big drops.
Cloud drops are much bigger than light waves. They scatter every colour the same. All colours mixed = white.
From Met Office and 気象庁
No air, no blue.
On the Moon there is no air. The sky is black, even at noon. 🌑
From NASA StarChild
Mars has a backwards sky. 🔴
Earth
blue by day, orange at sunset
Mars
orange by day, blue at sunset
Noon
Sunset
Mars air is thin, less than 1% of ours. 🌬️ Fine dust paints the day orange, and lets blue through near the sun.
The colors come from the fact that the very fine dust is the right size so that blue light penetrates the atmosphere slightly more efficiently.
Mark Lemmon, NASA Curiosity team, 2015
Mars dust is just the right size to let blue through, so Mars sunsets are blue. 🌅
From NASA/JPL
Bees can read the sky. 🐝
Sky light is polarised, in a pattern we can't see. Bees use it like a compass.
From Kraft et al. and Tyndall
On the way home, her brother tries again.
“So the sky is just sunlight, bounced?” “Yes,” says Ava. “And our eyes help.” 👁️
🫧 Tiny bits of air bounce blue light all over the sky.
🌇 Sunsets are the leftovers: the red that wasn't bounced.
👁️ Your eyes turn the sky's mix into blue.
Tomorrow, look up at noon. Then again at sunset.
Same sun. Same air. A longer path. So: why is the sky blue? 🌤️
Words from the story
- Scatter
- When light hits something tiny and bounces off in a new direction.
- Wavelength
- The length of one wave of light. Blue waves are short, red waves are long.
- Molecule
- The tiniest bit of a gas, like nitrogen or oxygen.
- Ozone
- A gas high up in the air. It soaks up some orange and red light.
- Atmosphere
- The layer of air around a planet.
- Polarised
- Light whose waves all wiggle the same way. Our eyes can't tell, but bees can.
Where this comes from
- Why Is the Sky Blue?. NASA Space Place, 2022
- The Colors of Sunset and Twilight. Stephen F. Corfidi, NOAA/NWS Storm Prediction Center, 2014
- Why are clouds white?. Met Office
- Why is the sky blue?. Royal Museums Greenwich
- 雲・大気現象・大気光象について（よくある質問）. Japan Meteorological Agency
- 空が青いのはどうしてですか？. JAXA Institute of Space and Astronautical Science
- 푸른 행성 지구. Korea Astronomy and Space Science Institute
- On the blue colour of the sky, the polarization of skylight, and on the polarization of light by cloudy matter generally. Proceedings of the Royal Society, 1869
- On the light from the sky, its polarization and colour. Philosophical Magazine, 1871
- Theorie der Opaleszenz von homogenen Flüssigkeiten und Flüssigkeitsgemischen in der Nähe des kritischen Zustandes. Annalen der Physik, 1910
- Human color vision and the unsaturated blue color of the daytime sky. American Journal of Physics, 2005
- Explanation of the Brightness and Color of the Sky, Particularly the Twilight Sky. Journal of the Optical Society of America, 1953
- Revisiting the question “Why is the sky blue?”. Atmospheric Chemistry and Physics, 2023
- Revised optical air mass tables and approximation formula. Applied Optics, 1989
- PIA00917: Color Variations in the Sky at Sunset. NASA/JPL, Mars Pathfinder, 1997
- NASA's Curiosity Rover Views Serene Sundown on Mars. NASA/JPL, 8 May 2015
- Mars Fact Sheet. NASA Space Science Data Coordinated Archive
- Why is space black?. NASA Goddard
- Scientist of the Day: Horace Bénédict de Saussure. Linda Hall Library, 17 February 2016
- The Mountain Man Who Measured the Sky's Brilliance. Nautilus, 17 February 2026
- Painting informed by science (Codex Leicester). Museo Galileo, Florence
- Honeybee navigation: following routes using polarized-light cues. Philosophical Transactions of the Royal Society B, 2011
- The red sky: investigating the hurricane Ophelia Saharan dust and biomass burning aerosol event. Environmental Science: Atmospheres, 2022
- Why is the sun red? Hurricane Ophelia causes rare weather phenomenon. CBS News, 16 October 2017
- 黄砂に関する基礎知識. Japan Meteorological Agency
- 황사업무. Korea Meteorological Administration
- 道路交通法施行令 第2条. e-Gov 法令検索 (Digital Agency, Japan)
- 「青信号？緑だよね…」実は法令上も「緑」だったのに「青」へ. 乗りものニュース, 31 December 2023
- 표준국어대사전: 푸르다. National Institute of Korean Language
- Colours of Clouds. Hong Kong Observatory
Ava is made up. The sunbeam, the blue ruler and the two skies are drawings that show the idea; their colours come from Rayleigh's rule, not from photos. “About 6 times” is our sum from that rule, for blue at 450 nm and red at 700 nm. The ozone share is a model result.
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