Why is the sky blue,
but sunsets red?
Blue by day, red in the evening. Same sky, same sun — yet the colours swap around. In fact these two aren't separate phenomena at all — they're two sides of exactly the same cause. Air "scatters blue light around", and that's why the sky looks blue by day and red in the evening.
Look up at a clear daytime sky and it's blue all over. Yet nothing up there is actually giving off blue light. Air is transparent, and the sun itself looks whitish.
And yet, even looking away from the sun, the sky is still blue. Blue light is reaching your eyes from a direction where there's supposedly nothing at all.
Then, in the evening, that same sun turns red. The sun's temperature hasn't dropped, and its colour hasn't changed.
What's changed isn't the sun — it's the amount of air its light has travelled through to reach us.
Air molecules don't just let light through — they bounce it off in all directions. Blue light gets scattered more than four times as much as red light. That's why blue rains down from the whole sky.
Evening light travels through far more air than daytime light does. The blue gets scattered away and used up en route, leaving only red to reach your eyes.
In other words, the blue sky and the red sunset are the same phenomenon, seen either from the "scattered" side or the "surviving" side. Let's take them in order.
What does "scatter" actually mean?
Sunlight is white light made of many colours mixed together. Pass it through a prism and it splits into the colours of the rainbow because that mixture gets separated out.
As this light travels through air, it hits air molecules (nitrogen and oxygen). Some of the light then stops travelling in a straight line and flies off in all sorts of directions. That's "scattering".
What matters is that how easily light scatters depends heavily on colour. Light behaves like a wave, and different colours have different wave sizes (wavelengths). Red is a long, lazy wave; blue is a short, tight one.
The shorter the wave, the more easily it catches on small particles. Air molecules are much smaller than the wavelength of light, so this effect is extreme. As a rough figure, blue scatters more than four times as much as red.
Scattered blue light flies off in every direction. Some of it lands in your eyes. That's why, even looking away from the sun, blue light still reaches you from that direction.
Put another way, the sky looks blue because the air itself appears to glow. On the airless Moon, the sky stays pitch black even in daytime. With nothing to scatter light, nothing arrives from any direction but the sun's.
So why isn't the sky purple?
Here's a sharp question. There's a colour with an even shorter wavelength than blue: violet. If shorter wavelengths scatter more, shouldn't the sky look violet?
It's a great question, and there isn't just one reason. At least three things combine here.
- Sunlight itself carries less violet to begin with. The sun emits weaker light in the violet range than in blue. Even if it scatters easily, less material means less quantity.
- Violet gets thinned out high in the atmosphere. High up, some violet and ultraviolet light gets absorbed. Less of it reaches the ground.
- Human eyes barely respond to violet. The colour-sensing cells in our eyes respond strongly around blue but grow far less sensitive at violet. And when violet mixes with blue, the brain reads it as "bluish white".
In short, the sky's blue is the result of three things combined: physics, the sun's properties, and the properties of the human eye. If a creature with different eyes looked up at the same sky, it would see a different colour.
The sun, the air, and the eye — all three work together.
Why sunsets are red
In the evening, the sun sits near the horizon. Its light then reaches us at a much steeper slant, through a far longer stretch of air than when the sun is overhead. Near the horizon, light is said to pass through dozens of times more air than it does straight overhead.
Over that long stretch, blue light keeps getting scattered away. Scattered blue veers off sideways, and is lost from the light travelling straight toward your eyes.
What survives to the end is the colour that scatters least — red and orange. That's why the setting sun looks red, and the sunset sky glows orange.
Here's the interesting part. The blue sky and the red sunset both come from one single fact: that blue scatters easily. Watch the scattered blue from the side, and you get a blue sky; watch the surviving red head-on, and you get a sunset.
- Why clouds are white: cloud droplets are far bigger than air molecules — roughly as big as, or bigger than, the wavelength of light. At that size, the difference between colours nearly vanishes, and every colour scatters about equally. Mix all colours together and you get white. The same reason shaved ice and foam look white.
- Why sunsets turn unusually vivid after a volcanic eruption: fine ash particles linger in the sky for a long time and change the way scattering works. Unusually vivid sunsets have been recorded worldwide after major historical eruptions (see how eruptions themselves work).
- Why distant mountains look bluish: the air between you and the mountain scatters blue light toward you. The further away, the bluer it looks. Painters shading distant scenery blue are capturing exactly this effect.
- The saying "red sky at night, sailor's delight": near Japan, weather tends to move from west to east, so seeing a red sunset is evidence of clear skies to the west. The old saying has some real grounding.
Something you can check in your kitchen
- Fill a clear glass or plastic bottle with water
- Add just a few drops of milk and stir gently (the trick is not too much — just a faint cloudiness)
- In a dark room, shine a phone light on it from the side and view the water from the side → it looks bluish-white
- Now put the light on the far side, and view it from the direction the light has passed through the water → it looks orange
- The longer the container, the deeper the orange becomes
The fine milk particles are playing the role of air molecules. Viewed from the side, it's a blue sky; viewed through the light that has passed all the way through, it's a sunset. The very same water shows two different colours, just by changing which direction you look from. What happens in the real sky can be reproduced right in a glass.
Summary
The sky is blue because air scatters blue light especially well. Sunsets are red because that very same property means blue gets used up over a long path through the air. It's one single fact, seen from different angles.
The blue sky and the red sunset aren't separate phenomena.
They're two sides of the same thing.
The same phenomenon — "light travelling a long distance turns red" — is also behind why the moon looks red during a total lunar eclipse. See this article for details. If you'd like to read about another kind of "blue" that works by a completely different mechanism, try our article on the ocean's blue. And ultraviolet light, with an even shorter wavelength than blue, scatters even more fiercely and rains down from the whole sky. That's why you can get sunburnt even on a cloudy day — explained in this article.
Want to go deeper? — terms, formulas, and how this fits the textbooksFrom middle-school science to open research questions, each level is labelled
- MSCovered in middle-school science
- HSCovered in high-school "Physics Basics"
- HS+Covered in high-school "Physics", or treated as advanced/sidebar material in textbooks
- UnivNot covered in high school — university-level specialist material (electromagnetism, atmospheric optics)
- ResearchNot yet settled even at university level — an active research question
MSTerms: words for talking about light and colour
- Wavelength: how fine the wave of light is. Visible light runs roughly from 380 nm (violet) to 780 nm (red). nm means nanometre (one millionth of a millimetre).
- Dispersion: white light splitting into rainbow colours through a prism (how rainbows work). It happens because each colour bends by a different angle.
- Scattering: light hitting particles and changing direction every which way. This is the "scatter" used throughout this article.
- Rayleigh scattering: scattering when the particle is much smaller than the light's wavelength. Scattering by air molecules is this kind. This is the true cause of the sky's blue.
- Mie scattering: scattering when the particle is about the same size as the wavelength, or bigger. Clouds, fog, and milk droplets fall into this category, where the difference between colours shrinks.
HSHS+Working it out with a formula: how many times more does blue scatter than red?
You can actually calculate this. Only one relationship is needed, and it involves nothing more than division and powers.
I ∝ 1 / λ⁴
| I strength of scattering | how much light is deflected |
| λ wavelength of light | in nm (nanometres) |
| ∝ proportional to | not "equals", but "proportional to" |
Read it like this: the shorter the wavelength, the more it scatters. And because it's a fourth power, even a small difference in wavelength makes a big difference in scattering.
To compare two colours, just divide. Once the numerator and denominator are tidied up, you're left with just the ratio of λ raised to the fourth power.
| Wavelength of blue | λ ≈ 450 nm |
| Wavelength of red | λ ≈ 650 nm |
| Set up the comparison | blue ÷ red = (650 ÷ 450)⁴ |
| First, work out the bracket | 650 ÷ 450 ≈ 1.44 |
| Raise it to the fourth power | 1.44⁴ ≈ 4.3 |
| Answer | Blue scatters about 4.3 times as much as red |
The wavelengths only differ by a factor of 1.44, yet the result comes out at 4.3 times. That's the fourth power making its strength visible. This is the substance of "the sky is blue".
Evening redness happens because the length of air the light passes through changes. The lower the sun, the longer the light's path through the air. That length can be calculated too.
| Length when directly overhead | taken as 1 |
| At 30° elevation | 1 ÷ 0.50 = 2.0× |
| At 10° elevation | 1 ÷ 0.17 ≈ 5.9× |
The longer the path, the more blue — which scatters so easily — veers off sideways and gets used up along the way. Only red, which scatters least, keeps travelling straight to your eye. Blue is strong, so the sky is blue; that same blue leaves first, so the setting sun is red. The two are two sides of the same formula.
※ 0.50 and 0.17 are the sine of each elevation angle. The real atmosphere thins out with height, so near the horizon the actual path is longer than this simple calculation gives.
A natural question arises here. Violet has a shorter wavelength than blue, so shouldn't the sky look violet? Let's calculate it.
| Wavelength of violet | λ ≈ 400 nm |
| Ratio relative to red | (650 ÷ 400)⁴ ≈ 7.0× |
By the numbers, violet scatters even more than blue. Yet the sky doesn't look violet. The reason lies outside the formula: sunlight carries little violet to begin with, and the human eye is poor at sensing violet.
I think this is an important example. The formula tells you what's physically happening, but not how it will look. Only by combining the light source's properties with the receiver's properties do you get the actual answer.
The key point is that strong fourth-power effect. A mere 1.4-fold difference in wavelength turns into more than a fourfold difference in scattering. This "fourth power" is what decides the sky's colour.
HS+Incidentally, scattered light is polarised. Sky light at 90 degrees from the sun is especially strongly polarised. That's why, looking at the sky through polarised sunglasses and tilting your head, the brightness of the sky changes. Bees are thought to read this polarisation pattern to find direction.
UnivWhy the fourth power?
Light is an electromagnetic wave. When it hits an air molecule, the electrons inside the molecule get shaken by the light's electric field and start to oscillate. An oscillating charge itself radiates a new electromagnetic wave (dipole radiation). This is what scattered light really is.
Here, the electron's acceleration is proportional to the square of the frequency, and the power it radiates is proportional to the square of the acceleration. Combine the two and you get a proportionality to the fourth power of frequency, i.e. wavelength to the power of −4. That's where Rayleigh's law comes from.
More generally, once a particle's size is comparable to or larger than the wavelength, you need Mie theory, which solves Maxwell's equations exactly for a sphere. As particles get bigger, the wavelength-dependence weakens, and you get something white, like clouds. The sky's blue (molecules) and clouds' white (water droplets) are two ends of the same phenomenon — scattering — divided by particle size.
ResearchWhat's still under debate
- The deep blue of twilight can't be explained by Rayleigh scattering alone. After sunset, there's a period when the sky turns a deep, saturated blue. It's become clear that absorption by ozone plays a large role in this colour. Ozone weakly absorbs the orange-to-red part of visible light, and that effect builds up as light travels a long path through the atmosphere. The textbook explanation "the sky's blue is explained by Rayleigh scattering" falls short for this particular time of day. Exactly how to weigh the contributions of ozone versus Rayleigh scattering remains an active research question.
- Predicting the sky's exact colour as a number is still not easy, even today. The actual colour of the sky depends on many factors: the amount and type of aerosols (dust and fine particles), water vapour, ozone, and reflection from the ground. Radiative-transfer calculations are highly advanced, but reproducing the exact sky colour of a given day and place requires atmospheric input data that isn't known with sufficient precision.
- There are attempts to read past atmospheric conditions from old paintings. Some research tries to estimate historical aerosol levels from the colour tones of sunsets painted after major volcanic eruptions. But there are difficulties in how to handle a painter's artistic choices and pigment discolouration over time, so conclusions vary.
- Perception of direction via polarisation is also not fully understood. It's well established that insects use the sky's polarisation pattern, but the details of how that information gets processed, and the mechanism behind the faint polarisation-sensing phenomenon in humans known as Haidinger's brush, are still being researched.
Even the colour of the sky we see every day is still not easy to pin down numerically, in terms of "why exactly that colour".
How this connects to the textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| MS | Science — properties of light / prisms and colour | White light as a mixture of colours; the idea of scattering |
| HS | Physics Basics — waves / properties of light | Correspondence between wavelength and colour; range of visible light |
| HS+ | Physics — wave motion / polarisation (often an advanced topic) | The fourth-power wavelength law; polarisation of scattered light |
| Univ | Electromagnetism / optics / atmospheric physics | Dipole radiation, Mie theory, radiative transfer |
| Univ | Physiology / colour science | Cone-cell sensitivity, and why the sky isn't violet |
| Research | Atmospheric optics (unresolved) | Ozone's contribution, quantitative prediction of sky colour, perception of polarisation |
- Rayleigh, Lord (J. W. Strutt), On the light from the sky, its polarization and colour, Philosophical Magazine, 1871 (the original paper on Rayleigh scattering).
- Bohren, C. F. & Huffman, D. R., Absorption and Scattering of Light by Small Particles (a standard textbook on scattering theory).
- Hulburt, E. O., Explanation of the brightness and color of the sky, particularly the twilight sky, Journal of the Optical Society of America 43, 113–118, 1953 (on twilight sky blue and ozone's contribution).
- Lynch, D. K. & Livingston, W., Color and Light in Nature (a guide to colour in the natural world).
- Japan Meteorological Agency (気象庁), explanatory material on atmospheric optical phenomena.
※ Wavelength and scattering-ratio figures vary somewhat depending on which value is taken to represent each colour. This article uses commonly cited approximate values.
※ This article is a general-audience science explainer. The figures given are approximate, meant to aid understanding of the underlying mechanism, and can vary with conditions. Note also that looking directly at the sun can damage your eyes — please limit any observation to brief moments around sunrise or sunset, when the light is weak.