The Sky Is Only Sometimes Blue

If asked what colour Earth’s sky is, you wouldn’t be unforgivably wrong to answer that it’s blue. A more correct answer, however, would be “it’s blue, sometimes”.

Earth’s sky is black at night and grey in overcast weather. It’s brilliant crimson, orange and yellow at sunset, and a sultry blend of indigo, violet and pink at dawn. Around noontime on clear days, it’s white at the horizons and on brooding, stormy days, when there is a promise of severe thunderstorms and hail, it can be slate grey with a slight tinge of green.

The sky is many colours. It’s only sometimes blue. Ever wonder why? Doesn’t matter, I’m going to tell you anyway and what better place to start than by shedding some light on… light!

What Is Light?

What we know as light really only represents a fraction of the full spectrum of energy radiated by the sun and the other stars in our Universe (and other possible Universes). Visible light is the narrow range of electromagnetic energy that can be seen by humans and is responsible for illuminating our world in a cacophony of beautiful colour. It’s made up of teensy particles called photons (think photography, meaning “light”), which, unlike gas molecules, don’t float about arbitrarily bumping into the sides of objects like pong balls. Rather, photons travel in waves, just like nausea after some bad Chinese.

Waves are awesome for more than just surfing. They have all sorts of physical properties that, once understood, give us the key to understanding the behaviour of sound and light and our perceptions thereof… such as the colour of the sky!

Like, Wave Properties, Man

Any (serious) surfer will tell you that waves have many properties, including height, amplitude, energy, frequency and wavelength. These are all measurable quantities that can be applied to ALL kinds of waves, including energy and sound waves. For this particular topic, however, we shall be focusing on a property called frequency.

The frequency refers to the number of waves that occur in a given time period. So, imagine you’re sitting on a cliff that faces out to sea. In a period of one minute, you count every wave crest that passes your direct line of sight. The number of crests you count per minute is the frequency. Sounds pretty simple doesn’t it? Now try counting the light waves that are bouncing off your dad’s horrible Hawaiian shirt. Obviously you can’t. We can’t see light waves, or sound waves for that matter, but we CAN perceive the differences that arise as a result of differences in their frequency.

Sound waves with a high frequency (refer to the above diagram with the squiggly lines) are perceived by our ears to be high-pitched. Like the sound your wife makes when she gets mad at you for leaving your cheesy socks next to the bathroom sink. Sound waves with a low frequency are perceived by our ears to be low-pitched, like Barry White’s crooning. Similarly, light that travels at a high frequency is perceived by our eyes to be blue or violet and light with a low frequency, as red or orange. In between, you’ll find green and yellow. Together, they all make up the gay flag!

The above spectrum shows the variation of colour as determined by the frequency of the visible light emitted by the sun (or any star). Violet and blue lie at the extreme high-energy end of the colour spectrum, while red and orange lie at the low-energy end.

As it was initially explained, visible light represents a mere fraction of the full range of energy produced by our star. The “electromagnetic spectrum” may sound like a horribly complex term, but you’ve actually met most of the members of the family! Let’s take a look… Take a deep breath. It’s not complicated. I believe in you!

The Electromagnetic Spectrum

The squiggly line in the middle represents the size of the wavelengths of the various “kinds” of electromagnetic energy, from the low energy radio and microwaves (that you use to heat up your TV dinners) to the high energy X-ray and Gamma rays (that you definitely don’t use to heat up your TV dinners).

Slap bang in the middle of this diagram, you will see the blue box titled “visible”. This is visible light and it refers to a range of energy frequencies that account for all the colours we see and, in general, the light that illuminates our world.

Now, as we move to the right of the spectrum, the waves become more energetic and the frequency increases. Electromagnetic radiation becomes ultraviolet and then X-ray, as is used in medical diagnostic technology to reveal your bony insides. Finally, at the high-frequency end of the electromagnetic spectrum, we get gamma radiation, which is so ridiculously energetic that a minute’s exposure would either incinerate you, or cause such terrible mutation of your cells that you’d turn into Joan Rivers.

Thankfully, the gamma radiation produced by the unending nuclear fusion reactions in the heart of the Sun doesn’t quite make it to the Sun’s surface and so, our little planet is safe. Earth’s ozone layer also manages to deflect much of any high-energy radiation that heads our way from other locations in the universe, except for small amounts of UV light, which can cause sunburn and melanoma, amongst other kinds of skin cancers.

But, how on EARTH does this all link back to the colour of the sky?

By understanding how the frequency of visible light determines its position on the colour spectrum, we are given the key to understanding the colour of the sky!

Why Is The Sky (Sometimes) Blue?

When visible light reaches our planet, it encounters all the trillions of molecules of gas, water and other particulates that are so abundant in the atmosphere. While the majority of the spectrum can travel through this veritable obstacle course unscathed, blue light is unlucky enough to be of the perfect wavelength or “size” and so can’t help but collide with all these molecules and particles.

It’s like trying to roll a marble (blue light) tennis ball (green light), skateboard (yellow light), bicycle (orange light) and car (red light) through a car park FULL of marbles. Which one do you think it going to have the greatest difficulty getting from A to B without being deflected off its path? Blue light obviously and as a result, it gets scattered off its original course, which is what we see when we look up at a blue sky. This effect is known as Rayleigh scattering and is named after the obnoxiously titled English physicist, John William Strutt, 3^rd Baron Rayleigh Peacock Eminent La-di-da.

In reality, more than just blue light is scattered. A little bit of violet and green and even red light is scattered, too. But it’s predominantly blue that has fender benders across the daytime sky. If you throw a teaspoon of violent, green and red into a bucket of blue paint, the resultant colour will still be blue. This all changes, however, as the sun carves its path across the sky, drawing inexorably closer to the horizon…

Red, Orange and Yellow Sunsets

Sunset over the Satara Rest Camp, Kruger National Park.

From our perspective, the atmosphere at the horizons is thicker owing to the oblique angle at which we are looking at it. The following two diagrams illustrate this point beautifully, saving me a fair amount of wind…

In the first image, the length of the path the sunlight travels to reach the little sunbathing dude, as denoted by the black arrow, is much shorter than in the second image, when the sun sits on the horizon. This longer distance means that by the time the light finally does arrive at the dude’s eyeballs, all the blue light has been scattered out, leaving only the low-energy frequency light: reds, oranges and yellows. This is why sunsets look like sex-on-the-beach cocktails.

It’s also why they inspire cocktails… and sex on the beach.

Interestingly, at midday, the light travelling to us from the horizon still needs to claw its way through a thicker layer of atmosphere. While this light IS scattered red light, its mixture with all the blue scattered light from the rest of the sky causes the one extreme end of the colour spectrum to meet the other, effectively cancelling each other out. The resulting colour is white. In other words, at the horizons, all members of the visible colour spectrum are reunited, leaving you with *drumroll* white light.

Why Are Some Sunsets More Spectacular Than Others?

Discounting the sunsets you watched while totally baked on that good shit your cousin somehow smuggled in from Canada, the more spectacularly hued sunsets can be attributed to the composition of the atmosphere.

The more particles there are in the sky, be it dust, pollution, smoke, water vapour or the workings of a local volcano with indigestion, the more aggressive the scattering and the more enhanced these effects will be. This explains why there is nothing more beautiful – implications aside – than a sunset over a horribly polluted sky.

Cloudy With A Chance Of Green

There is a strange greenish tinge to the sky that can sometimes develop just before a severe thunderstorm drops its load. It’s especially noted with powerful storms that are able to form large hail and tornadoes. I’ve heard two theories explaining why this happens, but it would seem that the jury is still out on which one is more correct:

1. Severe thunderstorms typically occur during the latter half of the day and especially towards sunset. These kinds of thunderstorms also form very high cumulonimbus towers and the abundant water vapour within these clouds sends blue light scattering like skittles on a waxed floor. With the sunset throwing red scattered light on the blue underside of the clouds, the resultant visual effect can be a greenish tinge, as you can see in the picture above.

2. The other explanation is that the presence of large hailstones within a thundercloud can actually scatter light whose frequency is slightly lower than the standard blue. What colour comes next after blue? Green of course, hence the greenish otherworldly tinge. I prefer this explanation since it’s more awesome.

Having said all this, a greenish sky is not a sure-fire indicator that a tornado is on the way, as is a popular myth amongst the residents of Tornado Alley. But it does indicate the presence of a very tall convective storm, which you can pretty much bank on ruffling a few leaves. Maybe even relocating a cow.

Class Dismissed: Your Take-Home Message

Sunrise, somewhere in the middle of the beautiful South African nowhere

The sky appears to us in a myriad of colours throughout the day and it all comes down to the fact that visible light has multiple personality disorder. Whichever colour you do see is a result of that particular frequency of light being scattered more effectively than the others. But our foray into the physics of light has explained more to us than just the hue of the sky… it has also revealed just how many fascinating things wave properties account for, from the pitch of your irate wife’s voice to Indian Ocean tsunamis.

I intend to explore both of these in good time, but in the meanwhile…

What personality is your sky right now?