Why is the sky blue?

It’s one of the first science questions children ask, and the answer is a lovely piece of physics. The sky is blue because the air scatters blue sunlight much more strongly than red sunlight. That scattered blue light reaches your eyes from every direction, so the whole sky glows. The same process explains red sunsets, white clouds and why the sky on Mars looks so different.

Sunlight is a mix of colours

Sunlight looks white, but it contains every colour of the rainbow. Isaac Newton showed this in the 1660s by splitting sunlight with a glass prism. Each colour corresponds to a different wavelength: violet and blue light have short wavelengths (around 400–450 nanometres), while red light has long wavelengths (around 650–700 nanometres).

Rayleigh scattering

Earth’s atmosphere is mostly nitrogen and oxygen molecules, which are far smaller than the wavelength of visible light. When light hits particles that small, it is scattered in all directions. The British physicist Lord Rayleigh worked out in the 1870s that the amount of scattering depends very strongly on wavelength: it is proportional to one divided by the wavelength to the fourth power.

That fourth power is what makes the effect so dramatic. Compare blue light at about 450 nm with red light at about 700 nm:

Colour	Wavelength	Scattering compared with red
Red	~700 nm	1×
Green	~530 nm	about 3×
Blue	~450 nm	about 5–6×
Violet	~400 nm	about 9×

As sunlight travels through the air, a large share of its blue light is bounced around the sky. When you look at any patch of sky away from the Sun, you are seeing that scattered light.

So why isn’t the sky violet?

Violet is scattered even more than blue, so this is a fair question. There are three reasons. First, sunlight contains less violet than blue to begin with. Second, some violet is absorbed high in the atmosphere. Third, and most importantly, our eyes are much more sensitive to blue than to violet. The colour receptors in our eyes combine the scattered light into the familiar sky blue.

Why sunsets are red and orange

At midday, sunlight takes a fairly short path through the atmosphere. At sunrise and sunset, it travels sideways through many times more air before reaching you. Along that long path, most of the blue light is scattered out of the beam. What survives is the longer-wavelength light: yellow, orange and red. Dust, smoke and pollution can add extra scattering, which is why sunsets after wildfires or volcanic eruptions can be especially vivid.

Why clouds are white

Cloud droplets are much larger than air molecules, about the same size as or larger than the wavelength of light. Larger particles scatter all colours roughly equally (called Mie scattering), so the light that comes out of a cloud is a mix of everything: white. Thick clouds look grey underneath simply because less light gets through them.

The sky on other worlds

• The Moon: almost no atmosphere, so almost no scattering. The sky is black even in daylight.
• Mars: fine reddish dust makes the daytime sky look butterscotch or pinkish. NASA rovers have photographed Martian sunsets that look blue around the Sun, the reverse of Earth.

Try it at home

Fill a clear glass with water and stir in a few drops of milk. Shine a torch through the side in a dark room. From the side, the water looks faintly blue; looking through it toward the torch, the light looks orange. The tiny milk particles scatter blue light sideways, just like air does.

In one sentence: air scatters short-wavelength blue light much more than red, so blue reaches us from all over the sky. The atmosphere shapes the ocean’s behaviour too; read how tides work.