The Sky Is Only Sometimes Blue

Beautiful sunrise

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?

Rays of sunshine breaks through the clouds

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

Cool surfing goat

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.

High and low frequency waves

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!

Visible light spectrum

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

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.

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, 3rd 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

Beautiful sunset

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…

Light entering Earth's atmosphere

Light entering Earth's atmosphere 2

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?

Beautiful red sunset

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

Green thunderstorm clouds

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.
  1. 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

Beautiful yellow sunset

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?



46 thoughts on “The Sky Is Only Sometimes Blue

      • Truly breath taking.Lots of comments but no mention of haarp technologies or the obvious military influence on the weather . I’m certain a smart girl like you must accept that this must be at least considered , how about the geo engineering that’s been happening since the early 90s. Surely the colours are effected by this,a few of your photos are obvious chemtrailed atmosheres ,which in todays world is an everyday occurrence. Blue indeed,what ever the reason for blue , its not suppose to be white.

      • I said the horizons at noon are white. Not the sky. And I can never include absolutely everything on a subject, which is why I’m grateful to my readers for their contributions and additions. What I find very interesting is how people try to influence the weather by pouring fine particles of silver nitrate from aeroplanes at height. These act as condensation nuclei for water vapour and can encourage cloud formation and perhaps even light rain. I’m not familiar with harp technologies or the influence of the military on the weather aside from possible nuclear activity. I’m sure it does deserve consideration, but this blog was more about the physics of light and how this influences the colour of the sky. So please do elaborate…

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  2. Sorry madam but it is “unending nuclear fusion” in the sun not fission. I feel kind of dirty about that but couldn’t resist.
    When that volcano in Iceland with the name I’m not going to even attempt repeating here erupted a couple of years back I thought that all that dust that was stopping planes would make the skies in Europe a nice red but I’m sorry to say that I couldn’t see any difference.
    On another note the dust effect puts another layer of truth to the phrase “Red sky at night, barn on fire” which is nice.

  3. I like the post. Anyway, could you explain, in terms of weather, why the medieval period was called the ‘Dark Ages’? I expect that sunrises/sets went unseen for many years due to cloudy gray skies in the day time as well as darkness at night.

    • The “Dark Ages” were thusly named to describe the slump western Europe fell into after the Roman Empire bonked and ate itself out of existence. The lack of technology, cultural richness and economic affluence that hit the region lead to it falling into a period of squalor, poverty, hunger and disease. Those were metaphorically “dark times”… it wasn’t literally dark.

      I’m ASSUMING you’re not pulling my leg with this question. I am terribly gullible 😉

  4. Rayleigh scattering is similarly true on Mars, which is why the robot rovers enjoy pink sky and blue sunsets. Same physics, but the place just lacks the right atmosphere. Also Jupiter, though the atmosphere’s mostly methane, hydrogen and various sulphides (just occurred to me that this composition equates to ‘fart’, which is a bit scary given that Jupiter’s 142,984 km across).

    • Hahahahaha! Well done! You have joined the ranks of people who have made me snort coffee from surprise laughing. Jupiter’s atmosphere is essentially one big fart. I may VERY WELL use that in a future blog and if I do, I’ll be sure to give you the credit 🙂

  5. Great article, informative and entertaining, can’t beat that! You really seem to know your science. I didn’t realize that the sky displayed warmer colors of the spectrum at sunset because the sun is further away and the blue light gets scattered. I think I understood that correctly anyway!

    • Well I did study science for six years so one can only hope that I learned SOMETHING 😉 Just to clarify, the sun isn’t technically “further away”, it’s rays just have to travel through a thicker layer of atmosphere, which effectively scatters out all the blue light before it reaches our eyes.

      And you’re a fossil lady? What does that mean? We have a veritable treasure trove of fossil sites here in South Africa and going digging for them was one of my favourite things as a kid. I still have a couple of beautifully preserved trilobites and bivalves and other invertebrates lying around the house.

      • Good to know because I see many sunsets over Lake Michigan and display photos of them on a couple of art websites! And, yes, I’m the fossillady, meaning I write about fossils. I have some that were given to me, some I discovered and some that I inherited. It’s only a fun hobby for me and I do wish there was a treasure trove of fossil sites a little closer where I live! PS I’m glad you went after the reblogger below . . . gotta keep em honest! Had to do it once myself . . .Take care, Kathi

      • We have some wonderful fossil sites here in South Africa. My parents used to take us kids fossil hunting on occasion and my dad (alas, not me) dug up some particularly beautiful trilobite specimens.

        My blog is my most precious possession and passion. I love people to get involved and, of course, spread the science by reblogging, but plagiarism is about the most offensive thing to me. Thanks for being a dedicated reader, fossil lady. I shall have to take a look at some of your posts and maybe add my own 2 cents in the near future!

  6. Hey, awesome blog. I did a talk on this topic in university a couple of times and found it incredibly interesting. Once we had to do a speech using only the thousand most common words (we used this link and it was a unique challenge to explain particles, scattering and the light wave/particle duality.
    I think my favourite thing about light is that for everything we look at, the colour we perceive is the colour that isn’t absorbed. So when you look at a red apple under a green light, it appears black or grey. I found it incredibly strange to realise colour isn’t an intrinsic quality, but something that relies totally on another factor.

    Thanks for the blog, best of luck with your writing!

    • I totally agree with you. When we covered the electromagnetic spectrum and colour scattering in physics, I was in absolute awe that the colour of the world around us is a matter of perception and, as you say, not an intrinsic quality. It’s odd to think that a blue table scatters blue light and a red table scatters red light when they seem to be made from exactly the same material. Fascinating stuff 🙂 Thank you for your comment!

  7. Great, succinct explanation of color. Did you ever do any posts on the particle nature of light?

    Even knowing about color absorption and diffusion, it’s still a mind blowing concept.
    And a fun one, as you showed.

    • Thank you! It’s such an interesting topic and quite mind-blowing when you consider that the colour we see around us are really a product of reflected and refraction. I haven’t yet done a post on the particle nature of light, although it was touched on in this blog. I wanted to tackle sound waves next, so stay tuned 🙂

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    • I see you have reblogged my post without actually referencing “Why? Because Science” or providing any credit. In fact, underneath the heading, it says that it was written by Amego Santo who is NOT the author of this blog. I know this because I AM the author of “Tsunami”. For your sake and mine, I hope this is a boo boo, because what you have done is taken my intellectual property and passed it on as your own. If it is a mistake, correct it promptly. Otherwise, I will show you the verity behind that expression “Hell hath no fury like a woman scorned.”

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  12. Hi,

    I wonder, can you explain to me why the sky in my city is grey? Even at night they sky is not black but grey. It seems like there is a smoke. 😦

    Nice article 🙂

    • Well, let’s start off with where you live…? Which city?

      The sky is grey because clouds block the passage of incoming light from the sun. Water vapour reflects all of the 7 colours that make up light, so actually, clouds appear white. However, from the underside, this light gets attenuated to a darker shade of white, which is grey. That’s why a cloudy day looks grey. At night, with illumination from the city lights for example, the sky can still appear slaty grey. Pollution and fog can have the same influences.

  13. Pingback: Somewhere Over the Rainbow, Photons Fly! | Why? Because Science.

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