Why the Sky Is Blue

On a clear day the sky overhead is a deep and even blue, a colour so familiar that few people stop to ask where it comes from. The answer is not obvious, because the air itself is colourless and the sunlight that fills it appears white. To understand why the sky is blue, one has to look closely at what light is and at what happens when it passes through the gases that surround the Earth. The explanation brings together the nature of light, the behaviour of tiny particles and the way the human eye responds to colour.

Although sunlight looks white, it is in fact a mixture of all the colours of the rainbow. When a beam of sunlight passes through a glass prism, it spreads out into a band of colours running from red at one end to violet at the other. Each colour corresponds to light of a different wavelength. Red light has the longest waves, while blue and violet light have the shortest. This range of wavelengths, invisible as separate colours until the light is spread out, is present in every ray of ordinary daylight.

As sunlight travels down through the atmosphere, it meets countless molecules of gas, chiefly nitrogen and oxygen. These molecules are far smaller than the wavelength of visible light, and when light strikes them it is sent off in new directions in a process known as scattering. The crucial point is that the amount of scattering depends strongly on wavelength. Short waves are scattered far more powerfully than long ones, so the blue part of sunlight is thrown about the sky many times more than the red part. This uneven scattering is the direct cause of the colour we see.

Because blue light is scattered in every direction by the air, it reaches our eyes from all parts of the sky rather than only from the direction of the Sun. When we look up at a patch of sky well away from the Sun, the light arriving from that patch is mostly the blue that has been scattered towards us by the air along the way. The rest of the sunlight, richer in reds and yellows, largely continues straight on and does not reach our eyes from that direction. The sky therefore glows a scattered blue, painted across the whole dome above us.

A careful reader might ask why the sky is not violet, since violet light has an even shorter wavelength than blue and is scattered still more strongly. There are two reasons. The Sun sends out somewhat less violet light than blue to begin with, and, more importantly, the human eye is far less sensitive to violet than to blue. Our eyes combine the light they receive and report the mixture as the blue we recognise. The physics alone would favour violet, but the response of our vision shifts the result towards blue.

The same scattering explains the fiery colours of sunrise and sunset. When the Sun is low on the horizon, its light has to pass through a much greater thickness of atmosphere before reaching us. Along this long path, almost all of the blue light is scattered away sideways and lost, leaving mainly the reds and oranges to travel straight through to our eyes. This is why the setting Sun and the clouds around it can blaze with warm colour, while at the same moment the sky directly overhead may still appear pale blue.

It follows that the blue of the sky depends entirely on the presence of air. An astronaut standing on the airless Moon sees a black sky even in full sunlight, because there are no molecules to scatter the Sun's rays, and the stars remain visible in daytime. On Earth, the atmosphere acts as a vast screen that catches and spreads the shorter wavelengths, turning the empty blackness of space into the bright blue vault we take for granted. The colour of the sky is thus not a property of light or of air by itself, but of the meeting between the two.

What seems at first a simple question, then, opens onto a good deal of physics. The whiteness of sunlight conceals a spectrum of colours; the clear air scatters those colours unequally; and the eye interprets the scattered light in its own particular way. Put together, these facts account not only for the everyday blue overhead but also for the red of evening and the blackness of the sky beyond the air. Once the mechanism is grasped, the ordinary sky becomes a daily demonstration of how light and matter interact.