Why is the sky blue?

The short version: sunlight is white — a mix of every visible wavelength — but the atmosphere scatters short-wavelength light (blue) much more strongly than long-wavelength light (red). So when you look up at the sky (in a direction that isn't the sun), most of what reaches your eye is scattered blue.

That's the tweet-length version. Every part of it hides something interesting.

What "scattering" actually means

A photon of sunlight enters the atmosphere. It's tiny compared to any air molecule. Nitrogen and oxygen molecules are electrically polarizable — meaning they can oscillate when an electromagnetic wave passes them. When the wave (light) hits, the molecule wiggles briefly and re-emits the light in a random direction.

That re-emission is scattering. Rather than pass through, some of the light bounces off in all directions.

Why blue more than red

The key insight is that scattering strength depends dramatically on wavelength. Lord Rayleigh worked out the math in 1871: scattering by molecules much smaller than the wavelength scales as 1/λ⁴, where λ is the wavelength.

Since violet light has a wavelength around 400 nanometers and red is around 700 nanometers, violet scatters about (700/400)⁴ ≈ 9.4 times more than red. Blue is a bit longer than violet, so it scatters about 5-6 times more than red.

That's why when you look at a patch of sky that isn't the sun, mostly blue light reaches your eye — the blues have bounced around and eventually come toward you, while the reds have mostly gone straight through.

So why isn't the sky violet?

This is the interesting bit that people miss. Violet scatters even more than blue. So if scattering alone determined sky color, the sky should be violet, not blue.

Two things save us:

1. Human eyes are less sensitive to violet. Our color receptors — cone cells — peak at green, then blue, then red. Violet triggers only a weak response.
2. The Sun emits less violet than blue. The solar spectrum peaks in the green-yellow range and drops off toward violet.

Multiply those two facts by the scattering coefficient and blue wins. Our brains interpret the mix of scattered light as "blue sky."

Why sunsets are red

At sunset, sunlight is traveling almost sideways through the atmosphere to reach you. Instead of passing through 100 km of air (looking straight up at noon), it's slicing through several hundred kilometers.

Over that long path, essentially all the blue and green have been scattered out. Only the red and orange survive to reach your eye directly. The clouds catch that direct red light and glow. The parts of the sky not aimed at the sun still look blue because they're getting Rayleigh-scattered light from above.

Why the sky is a different color on Mars

Mars has a thinner atmosphere, mostly carbon dioxide, plus a lot of suspended dust. Rayleigh scattering is much weaker there, so the sky doesn't blue up the way Earth's does. But the dust scatters longer wavelengths preferentially, giving the daytime Mars sky a butterscotch-tan color. And at Martian sunsets, the geometry reverses: the sun's disk appears bluish against a tan sky.

Why the sea is blue (and it's a different reason)

Deep clear water actually absorbs red and orange wavelengths chemically. Blue light penetrates further and is either scattered back or reflected off deeper particles. So the ocean's blue is partly reflected sky, but it would still look bluish on a cloudy day because of the water itself. This is why deep ocean is a different, richer blue than a puddle that just mirrors the sky.

What about clouds?

Clouds are made of much bigger water droplets — big enough that they scatter every visible wavelength almost equally. This kind of scattering is called Mie scattering, and because all colors bounce, we see them as white. Very dense clouds look grey because most of the light gets absorbed / scattered so many times that little reaches the bottom.

Learn more

• Guess the cloud game
• Clouds & sky trivia quiz
• Rainbows & optics trivia quiz
• Weather glossary A-Z