Why the Sky Is Blue (and Sunsets Are Orange and Red)

Sunlight looks white or pale yellow, but it’s actually a mix of every color of visible light blended together — the same colors that appear separated in a rainbow. When sunlight enters Earth’s atmosphere, it collides with the tiny gas molecules that make up air, and this is where the sky’s color actually comes from.

Different colors of light have different wavelengths — blue and violet light have shorter, "tighter" wavelengths, while red and orange light have longer ones. When sunlight hits the small molecules in the atmosphere, shorter wavelengths (blue and violet) get scattered around in every direction far more than the longer wavelengths (red, orange, yellow) do. This effect is called Rayleigh scattering, and it means blue light gets bounced all over the sky in every direction, reaching human eyes from every part of the sky at once — which is why the whole sky appears blue rather than just the direction of the sun.

(Violet light actually scatters even more than blue, but the sky doesn’t look violet mainly because the human eye is less sensitive to violet, and some violet light gets absorbed higher in the atmosphere before it reaches the ground — so blue dominates what people perceive.)

Sunsets flip this effect around: at sunset, sunlight has to travel through much more atmosphere at a low angle to reach an observer’s eyes, which scatters away almost all of the blue and violet light along the way, leaving mostly the longer-wavelength reds, oranges, and yellows to come through directly — which is exactly why sunsets glow warm colors instead of blue.