The surprising truth about why the sky is blue.

Look up on a clear day and the sky is blue – so reliably blue that most people never ask why. The answer is not that the sky reflects the ocean, and it is not that air itself is blue. The colour is sunlight, sorted by the air itself on its way down to your eyes.

Sunlight is a mixture, air is a filter

Sunlight looks white, but it is a mixture of colours, each travelling at a different wavelength. The shortest wavelengths in visible light are at the blue and violet end; the longest are red. As sunlight passes through the atmosphere, it collides with gas molecules that are far smaller than those wavelengths.

Rayleigh scattering favours the short end

Those collisions scatter short wavelengths far more strongly than long ones – an effect called Rayleigh scattering. Blue light is scattered in all directions across the sky, so wherever you look, some of that scattered blue reaches your eyes. Red and yellow light, scattered much less, tends to travel straighter through.

So why not violet?

Violet light is actually scattered even more than blue. The sky does not look violet for two combined reasons: some violet is absorbed higher in the atmosphere, and human eyes are simply less sensitive to violet than to blue. The colour you see is the strongest scattered colour your eyes are good at detecting – blue wins on both counts.

Why sunsets turn red

At sunrise and sunset, sunlight crosses a much longer path through the air. Along that longer path, so much of the blue is scattered away before the light reaches you that what remains is dominated by the reds, oranges and yellows that were scattered least. The same physics paints the noon sky blue and the evening sky red; only the length of the journey changes.

Sources

• Encyclopaedia Britannica: Why Is the Sky Blue?
• NASA SciJinks: Why Is the Sky Blue?