Sunlight looks white. So why is the sky blue?

Start hereThe colour is not in the air, and not in the light alone

Two things have to be true at once for a blue sky: sunlight must contain many colours, and the air must treat those colours differently. Neither on its own is enough. Air is not blue — a jar of it is colourless — and sunlight is not blue either.

What makes the sky blue is what happens between them.

How it worksFour steps from the Sun to your eyes

Follow one beam of sunlight from space down to where you are standing.

1. White light arrives. It contains the whole range of colours, from long red wavelengths to short blue and violet ones.
2. It meets air molecules. Nitrogen and oxygen molecules are far smaller than the wavelength of visible light.
3. Blue scatters hardest. Shorter wavelengths get redirected much more strongly than longer ones, so blue light bounces around the sky and reaches you from every direction.
4. At sunset the path gets longer. Low sun means light crosses far more air, most of the blue is scattered out of the beam before it arrives, and what is left is red and orange.

Often misunderstoodThree explanations that sound right but are not

Each of these gets repeated a lot, and each one falls apart with one question.

The sky is blue because it reflects the sea.

The sky is blue over deserts and over the middle of continents, thousands of kilometres from any sea. The sea often looks blue partly because it reflects the sky, not the other way round.

A red sunset means tomorrow will be fine.

Sunset colour depends on the dust and moisture the light passes through, which is loosely related to weather systems in some regions and not in others. It is a hint at best, not a forecast.

The sky is blue when you look at Earth from space.

From orbit the sky above you is black, because there is almost no air left to scatter light. The blue you see in photographs is the thin layer of atmosphere seen from outside.

Try itWatch the same patch of sky three times in one day

You do not need equipment. You need the same window and three moments.

1. Look straight up in the morning, at midday and near sunset, from the same spot.
2. Each time, write down the colour directly overhead and the colour near the horizon.
3. On the third look, ask which direction the light has travelled furthest to reach you.

Talk about itWhy is the setting Sun redder than the noon Sun?

A good question for two people, because the answer is the same mechanism seen from the other end. If blue is being taken out of the beam, what is left over — and where did the blue go?

SourcesWhere we checked this.

• Why Is the Sky Blue?NASA Space Place — Used for the colour composition of sunlight, scattering in the atmosphere, and why sunsets appear red.
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