Unlocking the mystery: why is the sky blue? A deep dive

3 Mar 2025

You might think that explaining why the sky is blue would be kind of simple. But even a brief explanation of it requires a lot of science. The colours of everything you see are produced in different ways. Some of those colours are explained with physics, others by my own field of chemistry.

The nitrogen and oxygen that you are breathing right now are made up of very small particles called molecules. A molecule of nitrogen or oxygen is really, really small. Each molecule is only about 0.4 nanometers, or 16 billionths of an inch.

It would take 250,000 nitrogen molecules to equal the width of one strand of your hair. You can think of the molecules as behaving like very tiny balls that constantly bounce around.

When sunlight travels through the atmosphere, it passes between lots of those teensy nitrogen and oxygen molecules. Sometimes the light runs right into one of them.

In short, the sky looks blue because the blue portion of sunlight is much more likely to bounce off the molecules in the atmosphere than the other colours of light.

Tennis balls and marbles

Now, picture the nitrogen and oxygen molecules as tennis balls and the light as heaps of marbles.

When one of those light marbles hits a nitrogen or oxygen tennis ball, the tennis ball “eats” the marble and then very quickly spits it back out again, but in a random direction. That process is what physicists call scattering.

It was around 1870 when the British physicist John William Strutt, better known as Lord Rayleigh, first found an explanation for why the sky is blue: blue light from the sun is scattered the most when it passes through the atmosphere. His discovery is why the scientific term for this effect is called Rayleigh scattering.

The other gases in the atmosphere can be really important too, such as the effects of carbon dioxide or methane on the global climate. But they have only a very small effect on the colour of the sky.

If there were no scattering, the sky would be dark like it is on the moon, which does not have an atmosphere.

A rainbow represents all the different components that make up sunlight. As that light passes through the water droplets suspended in the air, it is broken up into the component colours called the visible spectrum – red, orange, yellow, green, blue, indigo and violet, more easily remembered as ROY G BIV.

Light at the blue end of the rainbow is scattered more efficiently than the other colours. It is as if the tennis balls are very selective in terms of which marbles they eat, and they prefer the blue ones over the other colours.

The result is that the blue light is scattered across the sky so you see blue everywhere on sunny days.

The rest of the colours mainly travel straight through the atmosphere.

Redder when the sun sets and rises

Of course, the sky is not always blue. And Rayleigh scattering also explains why the sky tends to be reddish when the sun is close to the horizon – at sunrise and sunset.

When the sun is near the horizon, its light passes through a lot more of the atmosphere to reach the Earth’s surface than when it is directly overhead. The blue and green light is scattered so well that you can hardly see it. The sky is coloured, instead, with red and orange light.

Colours mean a lot to us in so many different ways. Understanding the science behind colours and expressing ourselves through art with colours have been important for humans for our entire recorded history. That’s something to keep in mind as you decide what colour shirt to wear tomorrow morning. DM

First published by The Conversation.

Daniel Freedman is dean of the College of Science, Technology, Engineering, Mathematics & Management at the University of Wisconsin-Stout.

This story first appeared in our weekly Daily Maverick 168 newspaper, which is available countrywide for R35.

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Tennis balls and marbles

Redder when the sun sets and rises