Why Is the Sky Blue?

White light is secretly every colour

The starting point for a blue sky is something that seems to have nothing to do with blue: the Sun's light is white. But white light is not a colour of its own — it is all colours mixed together. When sunlight passes through a prism or a raindrop, it fans out into the familiar spectrum from red through orange, yellow, green and blue to violet, revealing the hidden ingredients that were there all along.

Each of those colours is light of a different wavelength. Red light has the longest waves in the visible range, violet and blue the shortest, with the others in between. That difference in wavelength is the single most important fact for understanding the sky, because it turns out that light of different wavelengths behaves very differently when it meets the air.

Air molecules scatter blue the most

The atmosphere is full of gas molecules, mainly nitrogen and oxygen, far too small to see. When sunlight streams through, it constantly bumps into these molecules, and each collision sends a little of the light ricocheting off in a new direction. This redirecting of light is called scattering, and it is the key to the blue sky.

The amount of scattering depends sharply on wavelength. Small molecules scatter short wavelengths much more powerfully than long ones — the effect is so steep that blue light is scattered roughly ten times more strongly than red. This is known as Rayleigh scattering, after the physicist who explained it. Red and yellow light largely pass straight through the air, while blue light is caught and flung repeatedly in all directions.

That scattered blue light is what fills the sky. When you look up at any patch of sky away from the Sun, the light reaching your eye is sunlight that has been bounced toward you by the air itself. Because blue is scattered so much more than the rest, that light is predominantly blue, and so the whole sky glows blue.

1. Sunlight enters the air

   White light from the Sun streams into the atmosphere full of tiny gas molecules.

2. Molecules scatter the light

   Each molecule redirects some light in all directions — this is Rayleigh scattering.

3. Blue scatters far more

   Short blue wavelengths are scattered roughly ten times more than long red ones.

4. Blue fills the sky

   Scattered blue light reaches your eyes from every direction, so the sky looks blue.

Why not violet?

If shorter wavelengths scatter more, a sharp question follows: violet light has an even shorter wavelength than blue, so why isn't the sky violet? The answer comes down to a combination of the Sun and our own eyes. The Sun sends out less violet light than blue to begin with, and some of the violet is absorbed high in the atmosphere, so there is simply less violet in play.

On top of that, human eyes are far more sensitive to blue than to violet. Our colour vision relies on three types of receptors, and the way they respond to the scattered mix of short-wavelength light adds up to a signal the brain reads as blue rather than violet. The sky is really scattering a blend of blue and violet, but between the Sun's output and our eyes' sensitivity, we perceive it as a clean, familiar blue.

The same physics paints red sunsets

One of the nicest things about the blue-sky explanation is that it also explains sunsets, using the very same scattering. The difference is the path the light takes. When the Sun is overhead, its light passes through a relatively thin slice of atmosphere to reach you. When the Sun is low at sunrise or sunset, its light skims in at a shallow angle and travels through a far greater thickness of air before it arrives.

Over that long path, almost all the blue light is scattered away sideways long before it reaches your eye, leaving mainly the longer wavelengths — the reds and oranges — to continue straight through to you. That is why the setting Sun and the sky around it glow warm red and orange. Dust, smoke and haze in the air can scatter and filter the light further, which is why some sunsets are far more vivid than others.

So blue midday skies and red sunsets are not two separate mysteries. They are the same molecules scattering the same sunlight; only the amount of air the light must cross has changed.

Clearing up a common mix-up

A widespread belief is that the sky is blue because it reflects the blue of the oceans. It is an understandable guess, but it gets the direction of cause and effect backwards. The sky is blue over vast deserts, over ice caps and over landlocked continents thousands of kilometres from any sea, and it would be blue on a world with no oceans at all, as long as it had a similar atmosphere.

If anything, the influence runs the other way: the open ocean often looks blue partly because it reflects the blue sky above it, as well as because water itself absorbs reds. The blue of the sky is made in the air by scattering, entirely independent of what lies on the ground below.

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