Why Is the Sky Blue?

Sunlight looks white. So where does all that blue actually come from?

Sunlight looks basically white or pale yellow to your eyes. So why, when you look up at a clear day, is the whole sky a completely different color?

Sunlight is secretly a mix of colors

Sunlight isn't actually one single color — it's a mix of every color you can see, blended together to look white. You can see this for yourself with a prism, or when sunlight passes through raindrops and splits into a rainbow.

Different colors of light travel differently

Light travels in waves, and different colors have different wave sizes. Blue light has short, small waves. Red light has longer waves. When sunlight hits Earth's atmosphere, it runs into countless tiny gas molecules floating in the air.

Blue light's small waves bump into those molecules and scatter — bounce off in random directions — much more easily than red light's longer waves do. That scattered blue light bounces all around the sky, coming at your eyes from every direction, which is why the whole sky looks blue instead of just the spot where the sun is.

Why sunsets aren't blue

At sunset, sunlight has to travel through much more atmosphere to reach your eyes, because the sun is low on the horizon instead of overhead. By the time the light gets to you, most of the blue has already scattered away in other directions. What's left is mostly the longer red and orange wavelengths — which is why sunsets glow orange and red instead of blue.

A question to think about

If you were on a planet with a much thicker or thinner atmosphere than Earth's, what do you think the sky color might look like there?