Why the Sky Is Blue

A Rainbow Inside White Light

Sunlight looks white, but it is a mix of every color — red, orange, yellow, green, blue and violet. Raindrops can pull them apart into a rainbow.

Light travels as waves. Red light has the longest waves; blue and violet have the shortest, less than two-thirds the length of red.

A meadow and the sky above it · sky colors computed from scattering

Bumping Into Air

Air is made of nitrogen and oxygen molecules, a thousand times smaller than a wave of light. As sunlight passes through, a little of it is bounced off in new directions by those molecules. This is called scattering.

The shorter the wave, the more it scatters: blue light is scattered nearly six times as much as red. So the light thrown in every direction is mostly blue.

Blue From Everywhere

When you look anywhere but at the Sun, you are seeing that scattered light. It reaches your eyes from every part of the sky, mostly blue, so the whole sky looks blue.

The blue is deepest high in the sky, away from the Sun. Near the horizon you look through much more air. Blue light heading toward you gets scattered away again on the way, while more red and green is scattered in, so the colors even out and the sky turns pale, almost white.

Why Not Violet?

Violet light has even shorter waves than blue and scatters even more. So why isn’t the sky violet?

Sunlight contains less violet than blue to begin with, and our eyes are not very sensitive to violet. Mixed together, the scattered light looks sky blue to us.

Red at Sunset

When the Sun is about to set, its light comes in sideways through a thick slab of air — about 38 times as much as when the Sun is straight overhead. Along the way almost all the blue is scattered out of the beam, leaving orange and red.

That is why the setting Sun is red, and why the clouds it lights glow red. Yet the sky overhead is still blue.

The same meadow, with the Sun near the horizon

One Sunbeam, Two Paths

Put noon and sunset side by side. In the top half the Sun stands 60 degrees high, and its light crosses only about 15 percent more air than when the Sun is straight overhead; about three-quarters of the blue gets through. In the bottom half the Sun is 1.5 degrees above the horizon, and its light slants through about 22 times as much air. Almost all the blue is scattered out of the beam, and what reaches your eyes is mostly red and orange.

The scattered blue is not lost. It lights up the sky somewhere else. That is why at sunset only the Sun’s side of the sky turns red, while the sky overhead stays blue.

Top: Sun 60° high · bottom: 1.5° · the same meadow

A Black Sky in Space

Above the atmosphere there is no air to scatter sunlight. Even in full sunshine the sky is black. The stars are still there, though the Sun’s glare makes them hard to see.

From the International Space Station, the atmosphere is just a thin blue line along the Earth’s edge. All of our blue sky fits inside that line.

Atmosphere drawn about 6 times thicker than it is