The air is full of colorless gas. So what makes a sunny sky so blue?
On a beautiful summer day, puffy white clouds drift in a sky that’s very blue. It looks like an ocean you could dive into.
But why blue? The air we breathe is mostly nitrogen and oxygen, two gases that have no color. So how can the sky look so blue?
Here Comes the Sun
Let’s start with one clue: the sky is blue on a sunny day but black at night. Aha! So the Sun might have something to do with it.
True, sunlight doesn’t usually have much color. Sunlight does contain all the colors of the rainbow: red, orange, yellow, green, blue, and violet. But when all these colors mix together, we see little color. That’s why sunlight normally looks pale.
All these different colors mix together in a beam of sunlight. But what makes blue special? What pulls the blue out of a sunbeam so that this color fills the whole sky?
We can find a clue by visiting the Moon. For anyone standing on the Moon, the sky is always black, even when the Sun is up.
So it must take more than just sunshine to make a sky blue. What does Earth have that the Moon doesn’t? Take a deep breath. That’s right: air! Earth has air, but the Moon doesn’t.
Out of Thin Air
Earth’s air is made mostly of nitrogen and oxygen molecules, particles that are much too tiny to see. In 1871, Lord Rayleigh, a scientist in England, realized that when light strikes particles that are this small, some of the light scatters off in all directions.
So not all sunlight comes straight toward us. Instead, air molecules scatter some of the sunlight away from the Sun, shooting sunlight all over the sky. That’s why the sky is bright even in parts of the sky that are far from the Sun.
But why blue? Rayleigh realized that nitrogen and oxygen molecules are the right size for scattering blue light. This color is scattered more than any other color except violet. Blue light is scattered about four times more than red light, the least scattered of all the colors.
As sunlight hits the atmosphere, the nitrogen and oxygen molecules scatter some of the blue light across the sky and down into our eyes. And that is why the sky looks blue.
Why No Violet Sky?
One color is scattered even more than blue light, and that’s violet light. So why isn’t the sky violet?
The answer has two parts. First, the Sun makes more blue light than violet light. Second, our eyes see blue light better than violet light. For these reasons, a sunny sky looks blue rather than violet.
The Sun is a star of many colors. But it takes more than just sunlight to color the sky blue. The sky is blue because of the same air that keeps you alive and able to see the beauty of a sunny day.