Why is the sky blue?

The short version

Sunlight looks white, but it is every colour of the rainbow mixed together: a rainbow is sunlight split up so that you can see them. The air is made of tiny pieces, far too small to see, and as sunlight passes through, they knock some of its light off sideways. They do that to blue light much more than to red, about five times as much, so blue light is sent bouncing all over the sky.

Wherever you look up, some of that scattered blue is coming towards you. That is the blue of the sky.

Why isn’t the sky purple?

Violet light is scattered even more than blue, but sunlight has less violet in it to begin with, and our eyes are much better at seeing blue than violet. Put together, the sky looks blue to us.

Why are sunsets orange and red?

When the sun is low, its light comes to you sideways, through far more air than it crosses at midday. On that long way most of the blue is scattered off before it arrives, and what is left is the yellow, orange and red. Clouds catch that light and glow pink and gold, and the sky above them can turn lilac.

Try it: a sunset in a glass

In a dark room, stir a few drops of milk into a clear glass of water and shine a torch through it from one side. Look at the glass from the side and the water has a faint blue tinge. Look through the glass straight at the torch and the light looks orange, like a sunset. The tiny specks of milk scatter the blue the way the air does.

If the sky is blue, why are clouds white?

A cloud’s droplets are far bigger than the pieces of air, big enough to scatter every colour alike. So a cloud sends back all of the sunlight, and all of the sunlight mixed together is white (what are clouds made of?).

What colour is the sky high up?

The higher you climb, the less air there is above you to scatter the light, so the sky turns a deeper, darker blue. From an airliner far above the clouds it is darker than any sky you see from the ground, and in space, where there is no air at all, the sky is black even with the sun shining.

In the story

Everything above is true of every sky. This part is Little Blue’s world.

“Little Blue is a very small blue bird with a very big dream,” and the book begins with the sky he dreams of: “Every morning he sits near the fence and watches the sky.” Higher Than the Clouds climbs right through the grey into the blue.

In chapter twelve he comes home at sunset: “The sun is setting now. The old hangar glows with warm light.” And the next morning, in the book’s own words, “the sky is still enormous.” The theme, The Little Blue Pilot (Sunshine Squad Remix), sings it outright: Little blue pilot, big blue sky!

Three words to keep

• Scatter: to send light bouncing off in every direction.
• Spectrum: all the colours sunlight is made of, in order, the way a rainbow shows them.
• Atmosphere: the blanket of air around the Earth, thinner the higher you go.