Why The Sky Is Blue

Wikipedia tells us that the sky is blue because air scatters short-wavelength light more than longer wavelengths.

The sunlit sky is blue because air scatters short-wavelength light more than longer wavelengths. Since blue light is at the short wavelength end of the visible spectrum, it is more strongly scattered in the atmosphere than long wavelength red light. The result is that the human eye perceives blue when looking toward parts of the sky other than the sun.


Evidently, Wikipedia has forgotten that Indigo and Violet have shorter wavelengths that blue light.

Visible Spectrum

Evidently, Wikipedia has also forgotten that Indigo and Violet are in the visible spectrum.


Therefore, if there was any validity in the mainstream hypothesis then the sky should be Violet in colour because Violet has the shortest wavelength in the visible spectrum.

Wikipedia also informs us that we see blue when we look towards the heavens [except when we look directly at the Sun].

The result is that the human eye perceives blue when looking toward parts of the sky other than the sun.


Evidently, Wikipedia has forgotten we can see white clouds in the sky without a hint of a blue tint.


Evidently, Wikipedia has never wondered where all the blue comes from in the spectrum of direct sunlight if the blue light is scattered [in all directions] more than any other colour.

Spectrum of Sunlight

Wikipedia finally tells us that sunrises and sunsets are red because too much blue and green light has been scattered by the atmosphere.

Near sunrise and sunset, most of the light we see comes in nearly tangent to the Earth’s surface, so that the light’s path through the atmosphere is so long that much of the blue and even green light is scattered out, leaving the sun rays and the clouds it illuminates red.


We have already recognised that Wikipedia has a mental block regarding Indigo and Violet so perhaps we will let that pass this time. We should also recognise that when Wikipedia says Red they really mean Red, Orange and Yellow. However, we must also recognise that the Wikipedia folks don’t get out much as night and have never looked at the stars.

However, if the folks from Wikipedia care to step outside on a clear night they should be able to see some stars and [with luck] they might see some stars just above the horizon. Now, if they look carefully they should observe the stars as small points of white light. Not red light. Not orange light. Not yellow light. The stars appear as white light because the Earth’s atmosphere is not magically scattering the blue [and even the green] light. Wikipedia is completely wrong again. The light from stars is not scattered by the Earth’s atmosphere when the stars are close to the horizon [or otherwise] and the same applies to our local star [the sun].


The key to understanding our blue skies was articulated by the late [and great] Ralph Rene when he realised that the Earth’s atmosphere fluoresces.

Ralph Rene - Our atmosphere fluoresces
The Last Skeptic of Science – Ralph Rene – 1998


Though astronauts and cosmonauts often encounter striking scenes of Earth’s limb, this unique image, part of a series over Earth’s colorful horizon, has the added feature of a silhouette of the space shuttle Endeavour.

The image was photographed by an Expedition 22 crew member prior to STS-130 rendezvous and docking operations with the International Space Station.
Docking occurred at 11:06 p.m. (CST) on Feb. 9, 2010. The orbital outpost was at 46.9 south latitude and 80.5 west longitude, over the South Pacific Ocean off the coast of southern Chile, with an altitude of 183 nautical miles when the image was recorded.

The orange layer is the troposphere, where all of the weather and clouds which we typically watch and experience are generated and contained. This orange layer gives way to the whitish Stratosphere and then into the Mesosphere.


Ralph Rene’s eureka moment arrived in 1990 whilst he was driving in Florida.
Looking at the sky he realised that the light sky blue at the horizon slowly deepened with elevation until it became a deep electric blue.

Ralph Rene - Sky Blue and Electric Blue
The Last Skeptic of Science – Ralph Rene – 1998

Ralph Rene also realised that a longer atmospheric path [over the horizon] should mean the sky produces more photons compare to the shorter path when looking vertically upwards into the sky.

Ralph Rene - The longer atmospheric path creates  more photons
The Last Skeptic of Science – Ralph Rene – 1998

Ralph Rene then used a light meter to test his hypothesis and found [without any shadow of a doubt] that the sky above the horizon is brighter that the sky vertically above the Earth’s surface.

Ralph Rene - Sky Brightness Experiment
The Last Skeptic of Science – Ralph Rene – 1998

The Blue florescence of the Earth’s atmosphere is clearly highlighted when the spectrum of solar irradiance [received on Earth] is compared to the theoretical blackbody irradiance of the Sun.

Evidently, the Emission Gains in the visible spectrum are driven by the absorption of Ultraviolet light in the Earth’s atmosphere.

Extraterrestrial Total Solar Irradiance

Interestingly, the level of Ultraviolet light received on Earth varies during the solar cycle.

UV and EUV Emissions
This figure shows the emission intensity of ultraviolet (UV) during cycle 22 and 23.

These variations in Ultraviolet light changes the florescence of the atmosphere and these changes are incorporated into the observations of the Earth’s Albedo and the Earthshine [blue line – below].

Earth Reflectance

Variations of the earth’s reflectance, a key climate parameter, can be monitored by measuring the brightness ratio of the dark and bright side of the Moon.
Image credit: Earthshine Project, BBSO.


There are many gases in the Earth’s atmosphere that are florescent but in the upper atmosphere there are two gases that dominate: Hydrogen and Helium.

International quiet solar year daytime ionospheric and atmospheric composition

Interestingly, Hydrogen and Helium emit the main colours seen in the sky: Blue, Yellow and Red.
Also, these two gases do not emit Green light [which explains why Green is never seen in the sky].

Hydrogen and Helium


The early morning “charge-up” of the atmosphere [as the sun rises] energises Helium and Hydrogen so that they emit Red and Yellow [during sunrise] before switching to Blue emissions when the atmosphere is fully “charged”. A reverse “run-down” process happens in the atmosphere when the sun sets and the Helium and Hydrogen drop to lower energy levels and emit Red and Yellow before “switching-off” for the night.


Handbook of Geophysics and the Space Environment, 1985

Solar spectral irradiance for different air mass values

Figure 2.10 Solar spectral irradiance for different air mass values assuming the U.S. Standard atmosphere, 20 millimeters of perceptible water vapor, 3.4 millimeters of ozone, and very clear air (Thekaekara, 1976).

The Sun’s Energy – William Stine and Michael Geyer

The variability of colours during sunsets and sunrises is probably determined by the levels of water vapour, hydroxyl and other molecules in the atmosphere because these molecules must be photo-disassociated [by ultraviolet light] before the atmospheric ionization process [which drives the florescence of atmosphere through electron collisions] becomes fully “charged”.

Below is an animation of how the blue line of hydrogen is made.

The electron is excited into the fourth state by a collision with another electron, then it goes back down to the second state and emits the turquoise photon. Then it quickly returns to the ground state, emitting a UV photon which our eyes are not sensitive to. Other lines come from other transitions. Each element has a different set of energy levels, so it absorbs and emits different photons and thus has different lines.

how the blue line of hydrogen is made

Below there are two animations. One shows hydrogen’s electron gaining energy from a collision. The other shows hydrogen’s electron gaining energy from a photon that was emitted from another hydrogen atom.

hydrogen electron gaining energy


The Balmer series or Balmer lines in atomic physics, is the designation of one of a set of six different named series describing the spectral line emissions of the hydrogen atom. The Balmer series is calculated using the Balmer formula, an empirical equation discovered by Johann Balmer in 1885.

The visible spectrum of light from hydrogen displays four wavelengths, 410 nm, 434 nm, 486 nm, and 656 nm, that correspond to emissions of photons by electrons in excited states transitioning to the quantum level described by the principal quantum number n equals 2.

There are also a number of ultraviolet Balmer lines with wavelengths shorter than 400 nm.

Visible spectrum of hydrogen


Hydrogen compounds at mid-latitudes

Aeronomy of the Middle Atmosphere:
Chemistry and Physics of the Stratosphere and Mesosphere

Physics of the earth in space - the role of ground-based research
Physics of the earth in space: the role of ground-based research – 1969

The Fluorescing Sky

However, if you are not convinced about Hydrogen and Helium then there are other gases in the atmosphere that glow and ionised-air glow provides an electric blue emission.

The ionized-air glow is the emission of characteristic blue–purple–violet light, of color called electric blue, by air subjected to an energy flux.

In dry air, the color of produced light (e.g. by lightning) is dominated by the emission lines of nitrogen, yielding the spectrum with primarily blue emission lines. The lines of neutral nitrogen (NI), neutral oxygen (OI), singly ionized nitrogen (NII) and singly ionized oxygen (OII) are the most prominent features of a lightning emission spectrum.

Neutral nitrogen radiates primarily at one line in red part of the spectrum. Ionized nitrogen radiates primarily as a set of lines in blue part of the spectrum. The strongest signals are the 443.3, 444.7, and 463.0 nm lines of singly ionized nitrogen.

Violet hue can occur when the spectrum contains emission lines of atomic hydrogen. This may happen when the air contains high amount of water, e.g. with lightnings in low altitudes passing through rain thunderstorms. Water vapor and small water droplets ionize and dissociate easier than large droplets, therefore have higher impact on color.

The hydrogen emission lines at 656.3 nm (the strong H-alpha line) and at 486.1 nm (H-beta) are characteristic for lightnings.

Rydberg atoms, generated by low-frequency lightnings, emit at red to orange color and can give the lightning a yellowish to greenish tint.

Generally, the radiant species present in atmospheric plasma are N2, N2+, O2, NO (in dry air) and OH (in humid air). The temperature, electron density, and electron temperature of the plasma can be inferred from the distribution of rotational lines of these species. At higher temperatures, atomic emission lines of N and O, and (in presence of water) H, are present. Other molecular lines, e.g. CO and CN, mark presence of contaminants in the air.


Nitrogen glow


Spectral lines of nitrogen

Noble gas discharge tubes
Noble gas discharge tubes: helium, neon, argon, krypton, xenon

gas discharge tubes
Other gas discharge tubes: hydrogen, deuterium, nitrogen, oxygen, mercury

How to recognise a Fluorescing Sky

Related posts:

Cosmic Electromagnetic Shower

The Fluorescing Sky of Earth

The Mystery of the Mariner 2 Photometry

M57 - Fluorescing Skies

Gallery | This entry was posted in Cosmic Rays, Earth, Science. Bookmark the permalink.

9 Responses to Why The Sky Is Blue

  1. Michael J. Dunn says:

    Begin at the beginning. Rayleigh scattering is inversely proportional to the fourth power of wavelength, so it will be stronger for colors at shorter wavelengths than blue. But how strong does the scattering have to be, before it is essentially complete diffusion? There is good reason to expect that by the time we get to the blue region of the spectrum, further differences in scattering are unobservable. And, because the solar spectrum is nearly flat in the visible range of the spectrum, the relative available intensity of blue, indigo, and violet light is about equal (with violet being a bit lower).

    So, why isn’t the sky violet? First of all, we can only expect that the sky would be a blend of blue, indigo, and violet, for the reasons given above. Secondly, the photopic luminosity response curve for human vision (found at http://en.wikipedia.org/wiki/Luminosity_function) peaks near a wavelength of 570 nm (green) and drops drastically approaching violet (near 450 nm). Integrate the Rayleigh scattering spectrum against the human visual response and our peak response will be at blue light. Eyes are not spectrometers, so we only respond to the integrated spectrum.

    • malagabay says:

      Thank you for repeating the textbook answer.

      Unfortunately, observing the spectrum of direct sunlight clearly indicates that the “shorter wavelengths” in direct sunlight are NOT being scattered more than the “longer wavelengths” as the sunlight travels through the Earth’s atmosphere.

      the spectrum of direct sunlight

      I personally recommend the critique of the textbook theory written by Miles Mathis.

      Regarding Rayleigh scattering:

      This leads us to ask, “Are the short wavelengths supposed to be scattered more often, or at a greater angle?”

      If we study the Rayleigh equation carefully, we find the answer is the angle, not the probability of being scattered.

      The Rayleigh equation gives us an intensity of the scattered light, given an incoming wavelength.

      In other words, the Rayleigh equation requires the incoming light to be of one wavelength.

      We can only insert one wavelength into that equation at a time, as you see.

      We let red light hit a molecule, for example, and the equation tells us how intense the scattered white light will be. This is because the scattering turns the red incoming light into all colors (or, rather, no colors).

      That is what scattering means.
      That is why the sky is bright instead of dark.

      Why is the Sky Blue?
      by Miles Mathis

      Regarding “not violet”:

      His own figure, from eyeball physiology, shows in technicolor detail that the green receptors are also stimulated at violet wavelengths, so why talk about the red and ignore the green? He isn’t making any sense.

      Even if the red were stimulated while the green were not, you still can’t build blue from violet plus red, at least not according to the current model.

      Since the retina is not white, you can’t stack these red and violet responses to get blue. That would require red canceling the red in violet. But if the eye is seeing both red and violet, that will make the violet more red, not less red.

      You can’t make violet into blue by adding red in paint mixing, CMY, or RGB.

      The current model is an RGB model, and when crossing violet light and red light on a white background, you don’t get blue. In RGB, you cross red and blue to get magenta. Just the opposite of his implication.

      But all this is moot, since his figure shows a green response, too.

      We need to mix red, green, and violet, to address his logic.

      In RGB, that should give us something quite near white.
      In CMY, it would give us something nearer black.

      His whole explanation of “why not violet” is a wash, and he would have been better to come up with a different dodge.

      Why is the Sky Blue?
      by Miles Mathis

  2. malagabay says:

    Miles Mathis provides a wonderful insight into the excess luminosity of the sky and implicitly states there is too much blue in the sky because “the sky is a mixture of white plus blue” and “the whiteness matches the Rayleigh equations.”

    The mainstream doesn’t even really have a current theory on this one.

    They use scattering in the atmosphere to explain the whiteness as well as the blue, but when it comes to explaining the brightness, we get next to nothing.

    We currently have neither proof nor disproof that the existing theories can explain the brightness. Basically, it is just assumed that the current mechanisms can match the luminosity we see at noon.

    When they say that the brightness matches the Rayleigh equations, they are not being honest.

    What they should say is that the whiteness matches the Rayleigh equations.

    We see that the sky is a mixture of white plus blue, and scattering does indeed explain the white. Scattering ensures that your eye will be receiving a mixture of wavelengths, and that mixture is seen as white.

    But scattering in no way explains the measured luminosity.

    The Brightness of the Sky or, the spectacular fudge that is Rayleigh scattering
    Miles Mathis

    Another extremely important point made by Miles Mathis is that both Newton and Rayleigh wrote their equations “specifically to match data”.

    In other words:

    Newton and Rayleigh provide heuristic equations – not mechanical explanations.

    Even the Rayleigh equation is back-engineered to match the known brightness.

    What do I mean by that?

    Well, a lot of physical equations—including Newton’s gravity equation—are written specifically to match data. And in a lot of cases, no harm done.

    If we want engineering equations to launch rockets or something, those are the kind of equations we need.

    But if we want theoretical equations that explain how fields work mechanically, those equations can really get in the way—as I have shown over and over and over.

    Newton’s gravity equation, a thing of great beauty as a matter of heuristics, has turned out to be the greatest impediment to a unified field equation. It has been an impediment because it hides the real fields under a mathematical field. Because the equation was compressed as a matter of real fields, no one has ever seen that it contains the sum of two fields , not just one.

    Well, the same thing applies here.

    The Rayleigh equation was written to match data.

    The Brightness of the Sky or, the spectacular fudge that is Rayleigh scattering
    Miles Mathis

    • George says:

      Now, I’m no standard physics interpretation guy, however I’m disappointed. I’m all for new ideas, but he seems to make quite a few missteps here!

      Hmm. I don’t get it. In what sense is “the light in the sky a mixture of white and blue”? The “light in the sky” consists of light of all different wavelengths (i.e. colours). None of those wavelengths = “white”. There is no such thing as a “white photon”, for instance. White is just the name used for a light source that emits all visible wavelengths (which looks “white” to the human eye).

      How the sky appears to us depends on:

      1. The original spectrum of sunlight (how much light of each colour came from the sun).
      2. How the atmosphere redistributes wavelengths (colours) across the sky (how much each colour gets ‘spread out’ on its way through the atmosphere).
      3. The response curves associated with our vision (how intense each colour seem to us subjectively, even if they’re really the same intensity).

      If you read his paper, he doesn’t seem to understand that Raleigh scattering just means that a particular photon gets its direction changed, and that the likelihood of that depends on its wavelength (colour). The photon’s wavelength (colour) is unchanged. He seems to think that the colour gets changed into white, and that a new photon is created! That’s not what it says at all.

      He also only looks at half the process: He criticises the intensity equation (which describes how intense light would be in a particular direction for a particular wavelength) but doesn’t take the next step to the Rayleigh scattering cross-section (which takes all the individual results from that equation and builds it into a full picture). This is vital!

      Really, the model is simple and makes sense:

      1. The light from the sun, before hitting the atmosphere, has a strong green-blue bias but falls off in the violet.
      2. When it hits the atmosphere, it’s like the photons are pinballs hitting a pinball machine that bounces blue pinballs more than other pinballs. The bluer photons get diffused throughout the sky; the redder photons much less so.
      3. The response curve of our eyes is strongly blue/green-biased (depending on light levels) further increasing the effect.

      Rayleigh just says: The more blue a photon is, the more likely it is to get bounced about on the way through the atmosphere – nothing more. The sky looks more blue than it does violet because of the sun’s spectrum and our eye’s response curve, both of which have much less violet in them than blue.

      It’s the three things multiplied together that make the effect so strong.

      Meanwhile, saying that equations are structured to match the data is obvious – in other words, you create your theory to match the observations. That’s science! That’s not to say that equations are “just math” though; they represent the model or picture that’s been arrived at. Surely? In Newton’s case, the idea of a ‘gravitational field’ with a certain ‘pull’ between objects.

      Now, if he’s proposing an alternative model with a different field structure and a different theory, that’s fine. There are potentially many ways to formulate an explanation of what we see, and new one’s might make for more useful pictures! Great! But to say that Newton’s equations describe just a “mathematical field” is false: it describes a field associated with a particular model/theory. It is perfectly “mechanical” in that it says: in a particular situation, putting an object in ‘this location’ will mean that ‘this happens’.

      I’m super-keen for new ideas, but I think sometimes his enthusiasm has meant he has not properly understood where the old theory comes from / how it was put together. That’s essential if you’re going to replace it!

    • George says:

      Oh, and on the brightness thing, he’s just off: He suggests that scattering implies a dimming as we progress through the atmosphere, but this isn’t true. It just says that blue light will get spread out – not absorbed, emitted, increased, or reduced, just spread out in the sky. That rather than a white sun with a dark black background, we will have a yellow sun with a bright blue background.

      He seems to misunderstand that the intensity equation is just a stepping stone to the scattering cross-section equation, and that just describes how likely each colour is to become ‘spread out’.

      That’s how it seems anyway.

  3. George says:

    Science is supposed to be fueled by proof. However, since World War Two heavily credentialed academic gas bags have run the show.

    Both Ralph Rene and Miles Mathis seems to have a real chip on their shoulders. I was interested initially, but they seem to be conspiracy theorists for its own sake. Not to say there aren’t interesting ideas to be had, but their articles are spoilt by this attitude – and it blinds them to properly understanding the standard models. They are too keen to find “the problem”, and so make misinterpret and make mistakes, spoiling their own cases.

    “The blue sky” one is particularly bad, since they seem to almost wilfully misunderstand the ideas of spectrum, response, energy, and scattering. If you really work through it, the current model works pretty well. That doesn’t stop you coming up with an alternative model, which can have its own merits – but if you’re going to point out ‘flaws’ in the existing model, you have to understand it and present it properly.

    And things like this are just embarrassing:

    What is happening mechanically? No one knows, because they will not define the light as either a wave or a particle. We aren’t told if the wave is being scattered or if photons are being scattered. All the math and sentences imply it is the wave, but none of the specifics are ever addressed.

    It’s long been established that light has both wave-like and particle-like properties, sometimes described as light being particles guided by waves (as in, a particle’s trajectory corresponds to that of pilot wave). This is completely covered by quantum mechanics. So the wave-part describes where we expect to find photons – hence scattering deals with how the wave-part is affected, and that dictates where the photons will go. “None of the specifics are ever addressed” because there’s no need: it is already understood by anyone who has studied physics, rather than half-read wikipedia.

    The problem is, it takes about 2-4 years of full-time study to really understand this stuff from the ground up, including what the maths represents (and the maths comes from the models, it isn’t “just equations). Popular science books and Wikipedia obviously summarise, take shortcuts, and simplify to give a general picture to a general audience – using these sources as the basis of “finding flaws” is foolish.

    That’s not to say that one should just “trust the experts” blindly, but nobody’s out to fool anyone. It’s true that people do get attached to established theories, because they come to think in terms of them – but there’s no conspiracy. Generally, the replacements people propose are far less detailed than the original theory – but are simpler and provide a more intuitive picture to the general person. But sometimes things are counter-intuitive until you dig into the details. That’s the case here.

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