[katieHaylor (OP)]

Ralph asks:

Why do clouds change colour to red, orange and yellow at sunrise and sunset? Why couldn't they also be green or some other colour like in a rainbow?

What do you think?

[chiralSPO]

They could go green if the light came from a different star (with different spectrum), or if the composition of the atmosphere were different, or if the eye observing the phenomenon had different sensitivity to our own. However, with our own sun and the Earth's atmosphere, a (typical) human eye will observe the red light.

The sun emits light across the nearly the whole spectrum, but its maximum power is in our visual region. Molecules and aerosols in our atmosphere scatter the light, and do so more effectively for shorter wavelengths than longer (blue is scattered more green, which is scattered more than red). This causes not only the apparently blue sky (diffuse light is enriched in blue light), but also the yellow sun at noon-time—with no atmosphere the sun would appear essentially white, but with the direct rays having more of the blue removed than green, yellow or red, it appears more yellow. As the Earth rotates the sun appears to move lower in the sky, and the rays have to travel a longer distance through the atmosphere, resulting in more scattering. The direct rays become even redder because blue, green and even yellow light are effectively scattered by the atmosphere at this angle (compared to red). We can even see some of this scattered light somewhat near the sun during sunset.

[opportunity]

Ralph asks:

Why do clouds change colour to red, orange and yellow at sunrise and sunset? Why couldn't they also be green or some other colour like in a rainbow?

What do you think?

Not being comical, but "who's Ralph"?

"You" see what "you" see. You ask a question in very non-descript tapestries, alias "Ralph".

Is Ralph trying to understand what you want him to see...or are "you" blind? No offence intended.

Light "refracts" through "mediums", like gases, atmospheres.

Are there green gases in our atmosphere.....that's a good question...right?

Getting through all that, why does a rainbow have seven colours right? Thats's the next question, right?

Why do we see light this way? Lets ask Ralph how he should see this. Its a big question.

[alancalverd]

Ralph: Chiral's explanation is a good start. You might care to look up Rayleigh scattering for more detail.

Opportunity: there is an infinite number of colors in a rainbow, which is essentially a continuum. Just as we divide animals into species for convenience, and humans into polite geniuses and rude idiots ( a 2-dimensional spectrum), so it is convenient to attach a few labels to parts of the visible spectrum. The human eye being most sensitive around 550 nm wavelength, we label that as green and put it in the middle with 3 colors either side. Other species have quite different spectral responses.

[opportunity]

Ralph: Chiral's explanation is a good start. You might care to look up Rayleigh scattering for more detail.

Opportunity: there is an infinite number of colors in a rainbow, which is essentially a continuum. Just as we divide animals into species for convenience, and humans into polite geniuses and rude idiots ( a 2-dimensional spectrum), so it is convenient to attach a few labels to parts of the visible spectrum. The human eye being most sensitive around 550 nm wavelength, we label that as green and put it in the middle with 3 colors either side. Other species have quite different spectral responses.

If we can't define why a rainbow is a 7-spectrum, maybe we should accept it is a part of how we can only perceive what we do perceive? Then we can talk about spectral analysis of sunsets and sun sunrises?

[Colin2B]

Not being comical, but "who's Ralph"?

"You" see what "you" see. You ask a question in very non-descript tapestries, alias "Ralph".

Is Ralph trying to understand what you want him to see...or are "you" blind? No offence intended.

Katie is one of the Producers of the Radio/podcast shows. They often get email questions from the general public who are not forum members eg Ralph, some of these questions are selected to be asked here.

If we can't define why a rainbow is a 7-spectrum, maybe we should accept it is a part of how we can only perceive what we do perceive? Then we can talk about spectral analysis of sunsets and sun sunrises?

As Alan says there are an infinite number of colours in the rainbow, however Newton had the honour of naming the bands and he chose 7. Why? Because as well as being a great scientist he was into mysticism and numerology where 7 is a significant number. As are many other numbers and ratios.

[chris]

@opportunity

Not being comical, but "who's Ralph"?

Ralph listens to the programme and got in touch over email. He's listening in Australia, he tells us. We've posted his question on his behalf and emailed him the link to this thread; perhaps he'll see that you are interested and get in touch?

[evan_au]

As @chiralSPO said, reddish hues are the dominant light left after a long passage through the atmosphere at sunset & sunrise.

But sometimes, other colors do appear at Sunrise or Sunset - like the fleeting "green flash".
Warning: Do not stare at the Sun - you can easily damage your eyes. If it's uncomfortable, it can do damage.
A fleeting glance after the disk of the Sun has gone should be ok.

See: https://en.wikipedia.org/wiki/Green_flash

why does a rainbow have seven colours right? That's the next question, right?

As all our US readers will know, it doesn't. It has 6.
- Only British rainbows have 7 colours (plus a few of their non-US colonies, like New Zealand).
- Colour is partly a linguistic construct - I understand Russian has separate words for Light Blue and Dark Blue, while Vietnamese does not distinguish Green and Blue.
- And these linguistic constructs mean that some cultures can consistently see different colours where other cultures see only one. It's bizarre when you see colour swatches selected to have these characteristics!

But I can explain why humans would see the rainbow has around 6-8 broad bands of colour:
- The human eye has 3 sensors in cone cells, with a broad range of sensitivity peaking in the Red, Green and Blue parts of the spectrum.
- The Red and Green ranges overlap strongly (fine gradation of colours), but the blue doesn't overlap much at all (coarse gradation of colours).
- A rainbow breaks sunlight into a continuous range of pure colours (somewhat muddied by the Sun's apparent 0.5° width in the sky)
- At the Red end of the spectrum, it is mostly the Red cones that are triggered
- In the Orange zone, both Red and Green cones are triggered strongly, but almost no Blue
- In the Yellow zone, Green cones are triggered strongly, and Red cones weakly
- In the Green zone, Green cones are triggered strongly, and Red & Blue cones weakly
- In the Blue Zone, the Blue cones are triggered strongly, and Green weakly
- In the Purple/Violet/Indigo zone, the Blue cones are triggered strongly, but Red and Green hardly at all
See: https://en.wikipedia.org/wiki/Color_vision#Physiology_of_color_perception

If you want to see all the colours of Sunlight clearly, you need to pass it through a narrow slit before passing it through a prism. The narrow slot avoids the blurring effect of the Sun's apparent 0.5° width.
See: https://en.wikipedia.org/wiki/Fraunhofer_lines

Some women have two Red sensors with different spectral sensitivity, and can see more colours at the red end of the spectrum than us poor males (but see linguistic construct, above).
Other species have up to 16 color sensors (some of them farther into the infra-red and ultraviolet than ours), so these creatures would see far more bands of colour than humans.
See: https://en.wikipedia.org/wiki/Mantis_shrimp#Eyes

[opportunity]

I ask about blindness as, in terms of colour, I am 33%  blind (100% red-colour blind). I have no red pigment sensors in my eye. As my cardiologist tutor once said, "why study medicine if you cant see red"? He raised a good question in my penultimate year of study.

I think this question is a tough one, yet it relies on knowing the facts and why they exist, the fundamentals of light. It makes it difficult as a subjective concept if some of us rely on pure theory.

The funny thing is I've never seen what has been described here. The effect I see comes from the green part of the spectrum.  Playing cricket with a red cricket ball was a nightmare against the backdrop of a green pitch.

I've actually seen the green flash a few times in getting up early of a morning. "But", I've never heard of or experienced a reddish hue, which understandably puts me off guard. In fact I've never heard of it until this post. So it's a real phenomena? Why again is red the dominant light left after sunset? I'm thinking it could be from lead or something in the atmosphere? I think though chiralSPO is on the money. That was going to be my first response, the idea of light travelling throuugh a longer part of the atmosphere as it sets. So, this would be true of a sunrise also, as an "initial" event?

Have to say again that this post caught me off guard, and I forget that I only see 66% of the vivid variety of light people see, no red though. Yet my last point/question I still hold and would appreciate an asnwer there, namely is there a red-hue effect with sunrise, "initially"?...good to know what I'm missing out on.......and to be honest, as I said being caught off guard having seen many sunrises and sunsets in my life, I thought the initial post was an attempt of humor......I understand now, thanks Chris for clearing the dynamic of the post.

[evan_au]

I have no red pigment sensors in my eye

This is a frequent problem for us males, since the red/green genes are on the X chromosome, and we don't have a backup copy like women.

For interest, how many distinct bands of color do you see when you look at a rainbow?

See: https://en.wikipedia.org/wiki/Color_blindness#Red%E2%80%93green_color_blindness

[opportunity]

I have no red pigment sensors in my eye

This is a frequent problem for us males, since the red/green genes are on the X chromosome, and we don't have a backup copy like women.

For interest, how many distinct bands of color do you see when you look at a rainbow?

See: https://en.wikipedia.org/wiki/Color_blindness#Red%E2%80%93green_color_blindness

When I was in med school, the tests confirmed I'm a complete protanopia. I requested a test because I kept failing all my physical dissection and histology classes.....I couldn't see the difference between an artery and a vein, I couldn't even tell if someone was blushing, cyanotic....you get the idea.... "despite" having aced all my other subjects. That's what I meant when I said my Cardiology tutor questioned my value to the trade of Cardiology if I can't see what Cardiologists need to see in the absence of tech.....its about seeing red.

How many bands do I see of the rainbow? You know, I don't see 7 I know that...."in real life", "but" in textbooks when it is all coloured in, red in the book "appears" as a mix between violet and green, "very strange" to me, as violet is at the other end of the spectrum.

The important thing to consider is that "naturally", as I don't pick up on the red wavelength, everything else takes precedence, blue and green. Blue sky for me though, its pretty powerful, as my brain picks up on that like its 50% of what I see. Your next question is perhaps how I know that, "why its powerful".

Just to give you an idea, back in 1993 when I went to the cinema to watch Schindlers List.....we're walking out of the movie and we're talking about how deeply effecting it was, and someone says, "what about the scene where the girl is shot and they use colour, "red", to portray the symbolic effect of her bleeding".....silly me said, "I didn't notice that". Big mistake. You know, those colours don't appear to protanopes.

So, when this post comes up, "why do clouds go red at sunset".....I'm thinking someone's having a joke. I completely forget that I'm a complete minority with what I perceive.

[Colin2B]

Why again is red the dominant light left after sunset? I'm thinking it could be from lead or something in the atmosphere?

One of the objectives of this part of the forum is educational, so we try to stick to known mechanisms or reasonable (based on evidence) speculation.
Do you have any special reason for singling out lead? Does the particle size or molecule physics make it more likely to scatter or absorb shorter wavelengths? Is its concentration high compared to other atmospheric components eg dust or water vapour? How much lead was present in Biblical times (eg from ice core or archaeological evidence) when red sunsets were mentioned? All of these would help to give an informed answer.

I think this question is a tough one, yet it relies on knowing the facts and why they exist, the fundamentals of light.

As you can see from @chiralSPO  and @evan_au  replies the answers are very straightforward. They rely less on facts as an understanding of basic principles and mechanisms. Most people who study physics or chemistry will have started in secondary school (as with other specialist subjects) and have spent at least 10/11 years study by the time they have completed their basic education. This will include not just the theory, but practical as they will have performed most of the classic experiments of Newton, Faraday, Huygens, etc and many of the newer ones eg Michelson and Morley.

It makes it difficult as a subjective concept if some of us rely on pure theory.

Physics and chemistry are not subjective subjects and relying on pure theory is not enough. Many laypeople, including well educated ones, make the mistake of believing that ideas like big bang are just one academic’s opinion hence any opinion is valid. What they don’t see is the detailed experimental work in places like CERN to understand how atoms behave at extreme energies and then calculating the detail of projecting that behaviour to an early universe in such a way as to be consistent with all current observations. Not something for the faint hearted or those lacking a deep understanding of the mechanisms involved.

[opportunity]

Why again is red the dominant light left after sunset? I'm thinking it could be from lead or something in the atmosphere?

One of the objectives of this part of the forum is educational, so we try to stick to known mechanisms or reasonable (based on evidence) speculation.
Do you have any special reason for singling out lead? Does the particle size or molecule physics make it more likely to scatter or absorb shorter wavelengths? Is its concentration high compared to other atmospheric components eg dust or water vapour? How much lead was present in Biblical times (eg from ice core or archaeological evidence) when red sunsets were mentioned? All of these would help to give an informed answer.

I think this question is a tough one, yet it relies on knowing the facts and why they exist, the fundamentals of light.

As you can see from @chiralSPO  and @evan_au  replies the answers are very straightforward. They rely less on facts as an understanding of basic principles and mechanisms. Most people who study physics or chemistry will have started in secondary school (as with other specialist subjects) and have spent at least 10/11 years study by the time they have completed their basic education. This will include not just the theory, but practical as they will have performed most of the classic experiments of Newton, Faraday, Huygens, etc and many of the newer ones eg Michelson and Morley.

It makes it difficult as a subjective concept if some of us rely on pure theory.

Physics and chemistry are not subjective subjects and relying on pure theory is not enough. Many laypeople, including well educated ones, make the mistake of believing that ideas like big bang are just one academic’s opinion hence any opinion is valid. What they don’t see is the detailed experimental work in places like CERN to understand how atoms behave at extreme energies and then calculating the detail of projecting that behaviour to an early universe in such a way as to be consistent with all current observations. Not something for the faint hearted or those lacking a deep understanding of the mechanisms involved.

I see.

Well, I mentioned "lead" because back in the 80's it was reported there was a type of brown-red haze owing to the lead in fuel at the time, back in the day. Thought I'd throw that one in.

How about focussing on what I said here:

I've actually seen the green flash a few times in getting up early of a morning. "But", I've never heard of or experienced a reddish hue, which understandably puts me off guard. In fact I've never heard of it until this post. So it's a real phenomena? Why again is red the dominant light left after sunset? I'm thinking it could be from lead or something in the atmosphere? I think though chiralSPO is on the money. That was going to be my first response, the idea of light travelling through a longer part of the atmosphere as it sets. So, this would be true of a sunrise also, as an "initial" event?

On the idea of lead though, if you've ever travelled to the Australian outback, Northern Territory, Uluru to be specific, and there's a wind blowing kicking up the red dust, you're going to have a spectacular sunset. Everyone says that. I can't see it, everyone says it though. It's apparetly so many versions of red.....ok you get my point. The italics above is being more scientific though I hope.

I'm trying to be scientific, as you understandbly request; I'm interested to know in the "science" of this post. Sure, an explanation has been offered for sunsets regarding the red-wavelength of light, yet I asked if that applies to an "initial event" of sunrise.

Colin, I'm a big supporter of your answers, don't get me wrong. I know exactly what being scientific is all about. I just think my reply in this post started off on the wrong foot, please understand how that could be possible.

[alancalverd]

It could be that Newton had exceptional discrimination at the blue-violet end of the spectrum, or was influenced by the then-current and commercially important development of indigo dye cultivation in  the West Indies, to name this region of the spectrum. It is not used in the international color code for electronic components (black, brown, red, orange, yellow, green, blue, violet, white, silver, gold) nor in American physics textbooks, which assign only 6 primary colors to the rainbow.

Apropos travelling in northern Australia, I found the desert very tiring on the eyes. Finally put it down to the fact that the rock and dust are red and blue, with strong spectral peaks and no green. This makes it impossible to focus sharply (especially if you are short-sighted like me) because the two colors do not focus at the same point (differential refraction / chromatic aberration, not Rayleigh scatter). But I gained a lot of respect for termites!

[opportunity]

It could be that Newton had exceptional discrimination at the blue-violet end of the spectrum, or was influenced by the then-current and commercially important development of indigo dye cultivation in  the West Indies, to name this region of the spectrum. It is not used in the international color code for electronic components (black, brown, red, orange, yellow, green, blue, violet, white, silver, gold) nor in American physics textbooks, which assign only 6 primary colors to the rainbow.

Apropos travelling in northern Australia, I found the desert very tiring on the eyes. Finally put it down to the fact that the rock and dust are red and blue, with strong spectral peaks and no green. This makes it impossible to focus sharply (especially if you are short-sighted like me) because the two colors do not focus at the same point (differential refraction / chromatic aberration, not Rayleigh scatter). But I gained a lot of respect for termites!

Quite right.

Is it possible that the visible light spectrum is "regarded" as a "spectrum" of colours owing to the peaks of colour-recognition some type of scientific study has achieved over the past few centuries. For instance, if we played out the visible spectrum, projected it to an audience, a type of Pink Floyd meets prism affair, dark side of the moon album cover type thing, yet as an actual and not painted thing, did a survey with people on how many colours strike them as most obvious, is that science? Physiology is a science. Ophthalmology is a science. So what about the colour-blind folk?

My point is we talk about "colours", but what is the "physics" of sunset events shaping what we perceive as "colours", and does that equally translate to sunrise, and if not why not?

[Colin2B]

I just think my reply in this post started off on the wrong foot, please understand how that could be possible.

That's understandable, but it is a common confusion which we've seen before.

Well, I mentioned "lead" because back in the 80's it was reported there was a type of brown-red haze owing to the lead in fuel at the time, back in the day. Thought I'd throw that one in.

Excellent, context is always useful.
Although lead oxide is red I’m not sure that the tetraethyllead used in fuel produces a particulate which would reflect red, perhaps @chiralSPO  could help here. I suspect that the haze is from other pollutants and lead is an indirect cause. As you know particulates are removed from car exhaust by the catalytic converter, unfortunately lead deposits stop the catalyst working so they can’t be used in cars using leaded fuel, hence greater pollution.

if you've ever travelled to the Australian outback, Northern Territory, Uluru to be specific, and there's a wind blowing kicking up the red dust, you're going to have a spectacular sunset.

Most of the particles involved in red sunset/sunrise are aerosols, that is solid or liquid particles small enough not to settle out rapidly. As @chiralSPO said, If the particle is small compared with the wavelengths of light, it will scatter short wavelengths - blues and violets - more than long wavelengths, such as red. Many man-made aerosols are small enough to do this so they enhance to the sunsets of Los Angeles and other polluted cities.
Natural aerosols come from forest fires, mineral dust kicked up by sandstorms, sea spray and volcanic eruptions, etc. Volcanoes can inject sulfuric acid droplets into the atmosphere and burning fossil fuels releases sulfur dioxide gas into the air, which then turns into sulfuric acid aerosols.
If air pollution is very bad the sky is so saturated that you don't even see the sun clearly and the sunset can appear bright but washed out. This is because aerosols that are close in size or larger than the wavelengths of visible light tend to scatter all colours indiscriminately, increasing the overall brightness of the sky but reducing the colour contrast.
Another big factor in a spectacular sunset is a projection screen. Just like your Pink Floyd eg you need something to project onto, clouds are good so is dust.
However, you don’t want blanket cloud. Here in the northern hemisphere weather systems come to the uk from the west. The leading edge of a warm front has stratus (blanket) cloud which doesn't allow the sun through, when the warm sector has passed the cloud break into cumulus (fluffy piles) which act as a nice projection screen. This is why “red sky at night” works, because the bad weather has passed, high pressure (which tends to trap particles) is on the way and it’s likely to be at least 24hrs before another front is on its way, or if it is on the way it will block the clear path of the sun.

does that equally translate to sunrise, and if not why not?

Yes, the scattering of light works just the same for sunrise. The only difference is the good weather eg high pressure has moved east, so possibility of poor weather is increased.

[Bill S]

Great response, Colin.  Thanks for that.

[chiralSPO]

Excellent, context is always useful.
Although lead oxide is red I’m not sure that the tetraethyllead used in fuel produces a particulate which would reflect red, perhaps @chiralSPO  could help here. I suspect that the haze is from other pollutants and lead is an indirect cause. As you know particulates are removed from car exhaust by the catalytic converter, unfortunately lead deposits stop the catalyst working so they can’t be used in cars using leaded fuel, hence greater pollution.

I would bet that the red color from old cars is probaby due to NO_x emissions. NO₂ has a brownish-red hue, and it also reacts with volatile organics to form photochemical smog https://en.wikipedia.org/wiki/Smog#Photochemical_smog

[evan_au]

"But", I've never heard of or experienced a reddish hue, which understandably puts me off guard. In fact I've never heard of it until this post. So it's a real phenomena?

With no red cones, you would not be fully sensitive to the dominant color at sunrise & sunset.

Just to understand the impact, How bright do red traffic lights appear to you?
- Are they the same intensity and color as green (just up the top), or  is the red light almost black?
- Do the newer LED-based traffic lights appear different than the older incandescent traffic lights?

I understand that for dichromats, there is a pure color around light blue which is indistinguishable from white.
For trichromats (most of us), all pure (rainbow) colors are clearly different from white.

[Colin2B]

Excellent, context is always useful.
Although lead oxide is red I’m not sure that the tetraethyllead used in fuel produces a particulate which would reflect red, perhaps @chiralSPO  could help here. I suspect that the haze is from other pollutants and lead is an indirect cause. As you know particulates are removed from car exhaust by the catalytic converter, unfortunately lead deposits stop the catalyst working so they can’t be used in cars using leaded fuel, hence greater pollution.

I would bet that the red color from old cars is probaby due to NO_x emissions. NO₂ has a brownish-red hue, and it also reacts with volatile organics to form photochemical smog https://en.wikipedia.org/wiki/Smog#Photochemical_smog

Thanks @chiralSPO . We poor diesel users are now in trouble because of NO_x, you couldn’t invent a scrubber/remover could you - please.