Why Is the Sky Blue?

Look up on a clear, sunny day and one colour seems to stretch almost everywhere: blue.

But the sky isn't actually a giant blue ceiling above Earth. There is no blue surface up there at all. In fact, if you travelled high enough above Earth's atmosphere and into space, the sky around you would appear black.

So where does all that blue come from?

The answer begins about 150 million kilometres away, with the Sun.

Sunlight Is More Colourful Than It Looks

Sunlight usually looks white, but white sunlight is actually made from many different colours.

You may have seen these colours in a rainbow: red, orange, yellow, green, blue and violet. A glass prism can also separate white light into a colourful spectrum.

Each colour of visible light travels as waves with a different wavelength. Red light has a relatively long wavelength, while blue and violet light have much shorter wavelengths.

That difference turns out to be very important when sunlight reaches Earth.

What Happens When Sunlight Reaches the Atmosphere?

Earth is surrounded by an atmosphere made mostly from gases such as nitrogen and oxygen.

When sunlight enters the atmosphere, it encounters the tiny molecules that make up these gases. Light doesn't simply travel past all of them unchanged. Some of it gets scattered in different directions.

Different colours aren't scattered equally.

The shorter wavelengths of visible light, particularly blue and violet, are scattered much more strongly by the tiny molecules in our atmosphere than colours with longer wavelengths, such as red.

This process is called Rayleigh scattering.

So Why Does the Whole Sky Look Blue?

Imagine sunlight travelling down towards you.

Some of the blue light is scattered by molecules in the atmosphere. Instead of continuing along its original path, that scattered light travels in many different directions.

Then it can be scattered again.

The result is that blue light reaches your eyes from all across the sky. Look north, south, east or west on a clear day and scattered blue light is arriving from those directions.

Your brain interprets all that incoming blue light as a blue sky.

The sky isn't producing blue light of its own. You're seeing sunlight that has been scattered through Earth's atmosphere.

Imagine a Playground Full of Bouncing Balls

Here's one way to picture what is happening.

Imagine rolling different-sized balls across a crowded playground filled with children.

The larger balls are a little like red and orange light. They are more likely to keep travelling in roughly the same direction.

Now imagine lots of tiny, lightweight blue balls. As they move through the playground, they are much easier to knock and send bouncing off in different directions.

Soon, those little blue balls seem to be coming from everywhere: from the left, the right, ahead of you and behind you.

Sunlight doesn't really bounce between air molecules exactly like balls in a playground, but the comparison helps us picture the important idea: blue light is scattered around the atmosphere much more strongly than red light.

That's why, when you look almost anywhere in the daytime sky, plenty of blue light is travelling towards your eyes.

But What About Violet?

Here's where things get particularly interesting.

Violet light has an even shorter wavelength than blue light, so it is actually scattered very strongly too.

Shouldn't that make the sky violet?

There are several reasons it doesn't.

The Sun sends us different amounts of light at different wavelengths, and our eyes don't respond equally to every colour. Human vision is much less sensitive to violet than it is to colours nearer the middle of the visible spectrum. Some shorter-wavelength light is also absorbed in the upper atmosphere.

Put everything together, including the way our eyes perceive mixtures of scattered light, and the daytime sky generally appears blue rather than violet.

Why Are Sunsets Red and Orange?

If blue light is scattered during the day, something interesting should happen when sunlight has to travel through even more atmosphere.

And it does.

When the Sun is high overhead, its light takes a relatively short route through the atmosphere before reaching you.

At sunrise and sunset, however, the Sun sits low near the horizon. Its light has to travel through a much longer stretch of atmosphere.

Along that longer journey, much of the shorter-wavelength blue light is scattered away from the direct path towards your eyes.

The longer wavelengths, including reds and oranges, are scattered less strongly. More of them remain in the direct sunlight that reaches you.

That's why sunsets can glow with spectacular shades of yellow, orange, red and pink.

Dust, smoke, pollution and other tiny particles in the atmosphere can affect those colours too, which is one reason no two sunsets look exactly alike.

Is the Sky Always the Same Shade of Blue?

Not at all.

The colour you see can change depending on the atmosphere, the position of the Sun and even where you are looking.

Near the horizon, sunlight travels through more atmosphere before reaching you, and larger particles and tiny droplets can scatter light in different ways. This often makes the horizon look paler or whiter than the deep blue sky directly overhead.

Humidity, haze, smoke and dust can all change how the sky appears.

Even an apparently simple blue sky is constantly changing.

Are Skies Blue on Other Worlds?

A blue sky isn't something every world gets.

A planet or moon's sky depends on whether it has an atmosphere and what that atmosphere contains.

The Moon has almost no atmosphere, so there aren't enough gas molecules to scatter sunlight across the sky. Even when the Sun is shining, an astronaut standing on the Moon sees a mostly black sky.

Mars has an atmosphere, but it is very different from Earth's and contains lots of fine dust. During the Martian daytime, the sky often appears yellowish, orange or reddish. Interestingly, sunsets on Mars can show a bluish glow near the Sun.

Different atmosphere, different sky.

The Blue Above Us Is Scattered Sunshine

The next time you look at a clear blue sky, you're actually watching physics happening on a planetary scale.

Sunlight has travelled millions of kilometres from the Sun, entered Earth's atmosphere and encountered countless tiny molecules. Those molecules scatter the shorter wavelengths of visible light particularly strongly, sending blue light towards your eyes from across the sky.

So why is the sky blue?

Because Earth's atmosphere takes white sunlight and scatters its blue wavelengths in every direction.

Something we see almost every day turns out to be an enormous natural light show happening right above our heads.

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