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It’s one of the first questions children ask, and many adults never quite get a clear answer. The good news is that one simple idea explains the blue sky, red sunsets and white clouds.
Sunlight is every colour at once
Sunlight looks white, but it’s really a mix of all the colours of the rainbow. You can see this when sunlight passes through a prism or raindrops and splits into a spectrum.
Each colour is light of a different wavelength. Red light has long waves; blue and violet light have short waves.
Air scatters blue light the most
The air is made mostly of nitrogen and oxygen molecules, which are far smaller than the wavelength of visible light. When sunlight hits them, some of the light bounces off in all directions. This is called scattering.
These tiny molecules scatter short wavelengths much more than long ones. Blue light is scattered several times more strongly than red. So as sunlight crosses the sky, blue light is bounced around in every direction, while most of the red, orange and yellow light carries straight on.
When you look at any part of the sky away from the Sun, you’re seeing that scattered blue light coming towards you. That’s why the whole sky glows blue. Scientists call this Rayleigh scattering, after the British physicist Lord Rayleigh, who explained it in the 19th century.
The sky is blue for the same reason sunsets are red: air scatters blue light much more than red.
So why isn’t the sky violet?
Violet light has an even shorter wavelength than blue, so it’s scattered even more. But the sky doesn’t look violet, for two main reasons:
- There’s less violet in sunlight to begin with.
- Our eyes are more sensitive to blue than to violet.
Put together, the mix of scattered colours looks blue to us.
You may also notice the sky is paler near the horizon. Light from low in the sky travels through much more air, so it’s scattered again and again, and the colours mix towards white.
Why sunsets are red and orange
At midday, sunlight takes a fairly short path through the atmosphere to reach you. At sunrise and sunset, the Sun is low, and its light has to pass through a much thicker layer of air.
Along that long path, most of the blue light is scattered away to the side. What’s left to reach your eyes is mainly the longer wavelengths: yellow, orange and red.
Dust, smoke, sea salt and pollution in the air can scatter even more of the blue, which is why sunsets after a dusty day or near a wildfire can be especially vivid.
Why clouds are white
Cloud droplets are much larger than air molecules, larger even than the wavelengths of visible light. Particles that big scatter all colours roughly equally, so the light coming out of a cloud is still a mix of every colour, which looks white.
Thick clouds look grey underneath because less light makes it all the way through them.
Why space is black
In space there’s almost no air, so there’s nothing to scatter sunlight across the sky. Astronauts see a bright Sun against a black background, even in daytime.
The sky on Mars
Mars has a very thin atmosphere, mostly carbon dioxide, filled with fine reddish dust. That dust scatters light differently from Earth’s air.
According to NASA, the Martian daytime sky looks orange or reddish. At sunset, it’s reversed: the sky around the Sun takes on a blue-grey tone. Earth and Mars have opposite colour schemes.
Frequently asked questions
Is the sky blue because it reflects the ocean?
No. That’s a common myth. The sky is blue over deserts and inland areas too. The colour comes from sunlight being scattered by the air itself. If anything, the sea often looks blue partly because it reflects the sky.
Why is the sky a deeper blue on some days?
On clear, dry days with little dust or haze, you mostly see the pure blue of scattering by air molecules. Humidity, dust and pollution add larger particles that scatter all colours, making the sky look paler and milkier.
Why does the sky look darker blue from a plane or a mountain?
Higher up, there’s less air above you to scatter light, so the sky looks a deeper, darker blue, a step on the way to the black of space.
