Thinking of Blue Skys

A blue sky behind some blue flowers

It’s rainy season on my island in the Pacific. The days are grey and not particularly inviting. We’ve been staying inside a lot, which can drive me a little bit stir crazy. So last weekend, when the sun came out we crammed in some time outdoors. With the mid-winter browns and greens that are typical around here (my island is not tropical), the blue sky was the most vibrant colour around.

Although we perceive blue blanketing the sky, in reality, the sky has no colour. Instead, the hue is created from the interaction between our atmosphere and the incoming sunlight. Our atmosphere is made up a bunch of different stuff --  nitrogen (78%) and oxygen (21%) with bits of dust, water vapour and some inert argon, among other things (some of which we’ve put there).

Water vapour and dust are the physically biggest components of the atmosphere, and are relatively large compared to the wavelengths of light. When light hits the water vapour and dust, it’s reflected in different directions, but the light remains white (an example is the clouds). So why does the sky appear blue?

Over time, all sorts of theories have surfaced to explain the blueness, which started heading the right direction with Goethe’s 1810 explanation: “If the darkness of infinite space is seen through atmospheric vapours illuminated by the daylight, the blue colour appears.” His theory said the sky’s colour comes from something within the atmosphere during the light of day -- which is true, but vague.

About the same time a more scientific inquiry was being made into the nature of scattering light. John Tyndall showed in an 1869 lab experiment that the blue hues of the sky could be created when white light was scattered by tiny particles. A few years later in 1871, John William Strutt, aka Lord Rayleigh, was the first to describe the actual mechanism that makes the sky appear blue was a result of light interacting with gas molecules in the atmosphere.

These gas molecules are tiny compared to the wavelengths of light – several thousand times smaller. When light strikes one of these molecules, that molecule absorbs a specific wavelength (or colour) of the light's energy and later re-emits the same colour in all directions; a process called Rayleigh scattering.

Most of the longer wavelengths of light pass through our atmosphere unaffected, resulting in the full spectrum of sunlight with a higher ratio of blue wavelengths from the scattering. For this extra blue light to make the sky appear a brilliant blue, a dark background is required. Fortunately, beyond our atmosphere is the blackness of outer space, which makes an ideal dark background. The combined effect of the extra blue light and the black of outer space results in a sky that appears blue.

If you shift your gaze towards the horizon, the brilliant blues give way to paler colours and perhaps even white. The light reaching you from near the horizon passes through much more atmosphere, so the scattered blue light is scattered again and again, reducing its intensity. Preferential scattering of blue light by our atmosphere occurs everywhere, not just above us. For example, light reflected from your hand to your eye is affected by this scattering, but the effect is so minuscule we can't detect it. Over a longer distance, like to a range of distant mountains, there is enough atmosphere to superimpose a blueish haze on our view of the mountains.

Since the creation of a blue sky overhead is entirely depended on the preferred wavelengths the molecules in the atmosphere absorb -- in our case, molecules in Earth’s atmosphere absorb energetic light (blues) at a much greater rate than less energetic light (reds). So the blue we see above us is an Earth thing, on another planet, the sky could look dramatically different. Check out the possibilities here.