Your turn
Each lab is one part of the video. Change things and watch the sky change.
Each lab isolates one step. Sky colours on Earth come from a simple scattering model; not to scale.
Tap the buttons and slide the sliders.
Tap Air off. What colour is the sky with no air to scatter sunlight?
Remove the atmosphere and nothing scatters sunlight toward you: the sky goes black, as on the Moon.
Tap the button. What colour is the sky with no air?
The Sun stays; only the air goes.
Slide from red to violet. Do the waves get longer or shorter?
Sweep 700 → 400 nm and watch the crest spacing shrink.
Slide from red to blue. Do the waves get wigglier?
Wave spacing drawn to scale with the wavelength (both hugely magnified).
Pick a colour. Which one scatters the most? Watch the sky behind take on the scattered colour.
Compare the scattered fraction for red, green, blue and violet. It follows 1/λ⁴.
Tap a colour. Which one bounces around the most?
Molecules drawn huge; scattering odds follow the Rayleigh 1/λ⁴ law.
Grow the particles. When does the scattered light (and the sky) turn white?
As particle size grows past the wavelength, scattering becomes nearly colour-neutral.
Make the drops bigger. Does the sky turn white?
A smooth blend between the two limits, not a full Mie calculation.
Slide the Sun down, or drag it in the sky. How much more air does the light cross?
The sky colours are computed: 5778 K sunlight × Rayleigh extinction over the Kasten-Young air mass.
Slide the Sun down. What colour is the sky now?
Sky colours come from a simple single-scattering model.
Pick a world, then move the Sun. Which world has blue sunsets?
Earth is modelled; the Moon and Mars skies are illustrative.
Tap Moon or Mars. What colour is the sky there?
Earth: computed. Moon and Mars: illustrative.
Look up on a sunny day. The sky is blue! Here is the secret.
Sunlight looks white. But it has all the colours of the rainbow mixed together. A glass prism can split them apart: red, orange, yellow, green, blue and violet.
Air is made of tiny, tiny bits, much too small to see. When sunlight hits them, some light bounces off, like a pinball. Blue light bounces the most. So blue light comes at you from all over the sky.
Clouds are made of little water drops. They are much bigger than the bits of air. Big drops bounce all the colours the same, and all the colours together look white.
In the evening the Sun is low. Its light takes a long trip through lots of air. Most of the blue bounces away on the trip, so mostly orange and red light get through.
Sunlight looks white, but it is a mix of every colour of the rainbow.12 When sunlight passes through the air, the tiny nitrogen and oxygen molecules scatter some of it off in new directions. They scatter short-wavelength blue light far more strongly than long-wavelength red light.45 So blue light reaches your eyes from every part of the sky, and the whole sky glows blue. Scientists call this Rayleigh scattering.3
In one sentence: the sky is blue because air scatters the blue part of sunlight in every direction, straight into your eyes.
Shine a beam of sunlight through a glass prism and it fans out into a rainbow, from red to violet. Isaac Newton showed this with a prism.45 Raindrops do the same thing when they make a rainbow.2
Light travels as waves, and each colour has its own wavelength: the distance from one wave crest to the next.3 Red light has long waves and violet light has short ones:
| Colour | Rough wavelength | How much air scatters it (compared with red) |
|---|---|---|
| Red | about 700 nm | 1× |
| Yellow-green | about 550 nm | about 2.6× |
| Blue | about 450 nm | about 6× |
| Violet | about 400 nm | about 9–10× |
A nanometre (nm) is a billionth of a metre, so these waves are tiny. A single human hair is about 50,000 nanometres thick.2 The scattering column comes from the Rayleigh law below.5
Light travels in a straight line unless something reflects it, bends it or scatters it.1 Air is mostly nitrogen (about 78%) and oxygen (about 21%).9 Those gas molecules are much smaller than a wavelength of light.34 When light meets one, a little of it is sent off in a new direction. That's scattering.
For particles this small, the amount of scattering depends very strongly on wavelength. Lord Rayleigh (John William Strutt) worked out the rule:45
scattering ∝ 1 / wavelength4
That "to the fourth power" matters a lot. Make a wave half as long and it scatters 2 × 2 × 2 × 2 = 16 times more. Blue-violet light at about 400 nm is scattered roughly (700 ÷ 400)4 ≈ 10 times more than red light at about 700 nm.5
So as sunlight streams through the air, most of the red, yellow and green goes straight through, while much more of the blue gets bounced around the sky.12 Look at any patch of sky away from the Sun and you're seeing light that was scattered towards you. Most of it is blue.
For a long time scientists thought dust or water droplets did the scattering. In 1911 Albert Einstein calculated that the air molecules themselves are enough, and experiments agreed.5
In 1859 John Tyndall showed the same effect with a tank of water and a few drops of milk.5 You can repeat it. Fill a clear glass with water, stir in a few drops of milk, and shine a torch (flashlight) through it in a dark room. From the side the water looks faintly bluish. Look through the glass straight at the torch and the light looks yellow-orange. The blue has been scattered out sideways, just like at sunset. (Milk particles are bigger than air molecules, so the effect is weaker and whiter than the real sky, but you can see the idea.)
If short waves scatter most, and violet is shorter than blue, why isn't the sky violet? There are three reasons:234
Clouds are made of tiny water droplets or ice crystals.11 These are still small, but they're far bigger than a wavelength of light. Particles that big scatter all colours about equally (this is called Mie scattering), and all colours mixed together look white.56 Mist and fog look white for the same reason.6
| What scatters the light | Size compared with a light wave | Which colours scatter | Looks |
|---|---|---|---|
| Air molecules | Much smaller | Blue far more than red | Blue sky |
| Haze, smoke, fine dust | Similar or a bit bigger | Depends on the particles | Pale, milky or reddish |
| Cloud droplets | Much bigger | All about equally | White clouds |
At noon, sunlight takes a fairly short path down through the air. When the Sun is low, its light has to slice through a much longer stretch of atmosphere to reach you.17 Scientists measure this with the air mass: 1 when the Sun is straight overhead, about 2 when it's 30° up, about 5.6 at 10°, and close to 38 at the horizon.10
Along that long path, most of the blue and violet light gets scattered away to the sides. What's left and reaches your eyes is mostly orange and red, so the Sun and the sky around it glow warm.234 Extra particles in the air, like dust, smoke, pollution or ash from a volcanic eruption, can make sunsets even redder and more dramatic.23 The same long trip through the air is what turns the Moon red: see why the Moon turns red.
The same long path explains why the sky near the horizon is paler, almost white, even at midday. That light has crossed so much air that the blue has been scattered and re-scattered many times, and the colours mix back together.12
The sky is only blue because there is air to scatter the light. Climb a high mountain and there's less air above you, so the sky overhead turns a darker, deeper blue. Go higher still and it fades to black. Space is black because it has almost no molecules to scatter sunlight.3
The Moon has almost no atmosphere, so if you stood on it, the sky would look black even in full daylight.4
Mars has a very thin atmosphere, less than 1% as dense as Earth's, and it's full of fine dust.14 The dust gives the Martian daytime sky a butterscotch, orange-red colour. At sunset the opposite of Earth happens: the sky around the Sun turns blue-grey.14
Sunlight contains every colour. Tiny nitrogen and oxygen molecules in the air scatter short-wavelength blue light much more strongly than long-wavelength red light, so blue light reaches your eyes from every direction in the sky. This is called Rayleigh scattering.
Sunlight is a mix of all the rainbow's colours. When it hits the air, the blue light bounces around the most, like a pinball, and ends up coming at you from all over the sky. The other colours mostly go straight through.
The air itself has almost no colour; a glass of air looks clear. The sky looks blue because of the light it scatters. Without air, as on the Moon, the sky is black even in the daytime.
Violet scatters even more than blue, but the Sun sends out less violet than blue, some violet is absorbed high in the atmosphere, and our eyes are less sensitive to violet. The mix we see comes out pale blue.
No. The sky is blue because air scatters sunlight. Water looks blue mainly because it absorbs red light; on calm days it also reflects some of the sky's colour.
When the Sun is low, its light crosses much more air to reach you, close to 40 times as much at the horizon as when the Sun is straight overhead. Most of the blue is scattered away along the way, leaving mostly orange and red light.
Clouds are made of water droplets that are much bigger than the wavelength of light. Big droplets scatter all colours about equally, and all colours mixed together look white.