Why The Sky Is Blue

Sunlight, to your eyes, looks white. But white light is a mix of every color, each a different wavelength. Blue is short and choppy. Red is long and lazy.

The question

So where does the sky get its color?

The reveal

The sky doesn't have a color. It gives color — by stealing it.

When sunlight enters the atmosphere, it slams into gas molecules, mostly nitrogen and oxygen. Short wavelengths (blue) scatter far more easily than long ones (red). This is Rayleigh scattering, and it's why blue light bounces around the entire sky in every direction. Some of that scattered blue reaches your eyes from everywhere overhead.

Red light, with its long wavelength, mostly sails straight through, past you and into the ground.

The bonus question

Then why are sunsets red?

At sunset, sunlight skims through a much thicker slice of atmosphere to reach you. By the time it arrives, almost all the blue has already been scattered out sideways and lost. What remains is the leftover: warm reds, oranges, and golds. A sunset is just the day's exhausted remnant light — the colors that survived.

A strange detail

Violet light scatters even more than blue. So why isn't the sky violet? Partly because sunlight emits less violet to begin with, partly because our eyes are less sensitive to violet, and partly because our blue-sensitive cones pick up the dominant signal. Your eye edits the sky toward blue.

The blue above you isn't painted on. It's a constant, real-time scatter concert happening in every cubic meter of air.