Sunset colors

[jkmccrann]

The colours of a spectacular sunset are caused most frequently by the presence in the atmosphere of a large amount of ash, as witnessed during the many spectacular sunsets of the mid-1990s due to the eruption of Mt. Pinatubo in the Philippines.

Is that because of some of the light reflecting back off the ash into outer space and the fact only certain types of light make it through?

[grant hutchison]

The enhanced sunset colours are because ash in the high atmosphere scatters the red light of the setting sun. So the sky looks red over a wide area, and for some time after sunset at ground level.

Grant Hutchison

[George]

The enhanced sunset colours are because ash in the high atmosphere scatters the red light of the setting sun. So the sky looks red over a wide area, and for some time after sunset at ground level.

Grant Hutchison

I believe this is partially true. This site points out the sulfate aerosols are about the same size as the wavelength (500nm). These particles will not help Rayleigh scattering. However, I understand these, and larger, particles would fall out and leave the smaller ones to allow the scattering you are stating.

The paper I sited mentions the red is caused by reflection of the red sun off the high altitude ash.

[added: Here is another site which states the ash acts as a neutral density filter. ]

[grant hutchison]

George, I think you may have misunderstood my post. I wasn't saying anything about Rayleigh scattering ... just pointing out that volcanic aerosols are efficient scatterers of light, and when there is red light around (at sunset), that's what they scatter.
So I'm saying exactly the same thing as this sentence from your first link:

The reflection of the setting Sun from the bottom of stratospheric volcanic aerosol layers produces the characteristic red sunsets that are used as one means of detecting past eruptions.

Grant Hutchison

[George]

Yes. I see now. This topic intrigued me so I did my normal googling and found a variety of answers, including Rayleigh scattering. [I think my brain on scattering is producing a scattered brain. ]

We have a somewhat similar situation west of San Antonio due to wind and dust.

[Ken G]

One way to put Rayleigh scattering in perspective is that it can explain why the Sun itself looks red, but it needs help to explain why you get that red band that goes all the way around (even in the antisolar direction, sometimes). There you are looking at light that was scattered by something other than tiny Rayleigh particles, since it's not blue. Then you get it to actually be red by having additional Rayleigh scattering remove the blue on its way to your eye, just as it did to the direct sunlight.

[George]

One way to put Rayleigh scattering in perspective is that it can explain why the Sun itself looks red, but it needs help to explain why you get that red band that goes all the way around (even in the antisolar direction, sometimes). There you are looking at light that was scattered by something other than tiny Rayleigh particles, since it's not blue. Then you get it to actually be red by having additional Rayleigh scattering remove the blue on its way to your eye, just as it did to the direct sunlight.

Is the antisolar directed band the the region adjacent to the sun along the horizon? If so, what you say makes sense. [Bands on the sun's disk and parallel with the ground would be a result of density variations in the atmosphere of scattering particles.]

Is there much chance Raman scattering plays a small roll by redshifting light?

While were on this subject, I don't recall ever getting a clear answer as to why the martian sunset has a blue halo. I had suggested the atmospheric particles were just large enough to scatter red but not blue, opposite to what we see here. Wouldn't that be cool?

[Ken G]

Is the antisolar directed band the the region adjacent to the sun along the horizon?

Actually, I meant all the way around, even opposite to the direction of the Sun.

Is there much chance Raman scattering plays a small roll by redshifting light?

I don't know, but I don't think so. I don't expect a lot of intensity in Raman scattering, other forms would dominate I should think.

While were on this subject, I don't recall ever getting a clear answer as to why the martian sunset has a blue halo. I had suggested the atmospheric particles were just large enough to scatter red but not blue, opposite to what we see here. Wouldn't that be cool?

I think the issue may be that Martian dust is larger than the Rayleigh-scattering particles we have. This means it could be called Mie scattering on Mars, though I don't know how much is that, and how much is simply the red hue of iron oxides. But the blue halo sounds like it could be a Mie effect, as you suggest.

[grant hutchison]

While were on this subject, I don't recall ever getting a clear answer as to why the martian sunset has a blue halo.

The pinkish colour of the Martian sky is caused by red dust particles reflecting red light towards your eye. But when you look towards the sun you'll see only the shadowed side of these particles, so they'll serve simply to dim the light rather than to colour it. Instead you'll see a bright pale patch around the solar disc because of Mie scattering of white light from smaller particulates, which becomes brighter the closer to the solar disc you look. I'd guess the bluish tinge might be a colour contrast effect, an artefact of the camera, a bit of Rayleigh scattering from Mars' thin atmosphere, or some combination of these.
I don't know if the effect occurs only when the sun is close to the horizon ... I suspect we have no picture of the sun at high elevations because of concerns about camera damage. However, it might be plausible that a Mie+Rayleigh pale blue halo occurred only at low solar elevations, when sunlight was passing through a greater path-length of atmosphere.
I've seen the same thing while look at the solar disc through smoke from a bonfire - the view was generally dark grey, but a region around the solar disc was pale blue. The solar disc itself was comfortable to look at, because of the attenuation caused by the shadowed sides of all the smoke particles in my line of sight.

I had suggested the atmospheric particles were just large enough to scatter red but not blue, opposite to what we see here

But Rayleigh scattering always predominates at short wavelengths, because of the power-law relationship with wavelength. Are you suggesting a particle size that Rayleigh-scatters red light but is too big to Rayleigh-scatter blue light?

Grant Hutchison

[George]

I'd guess the bluish tinge might be a colour contrast effect, an artefact of the camera, a bit of Rayleigh scattering from Mars' thin atmosphere, or some combination of these.

I went to JPL and I didn't find, with my limited time, the entire rover image gallery. I wanted to show you the sunset image which shows the halo. It was not an imaging arteface, IIRC.

I don't know if the effect occurs only when the sun is close to the horizon ... I suspect we have no picture of the sun at high elevations because of concerns about camera damage. However, it might be plausible that a Mie+Rayleigh pale blue halo occurred only at low solar elevations, when sunlight was passing through a greater path-length of atmosphere.

I believe this is correct. IIRC, the blue halo is a low altitude effect.

But Rayleigh scattering always predominates at short wavelengths, because of the power-law relationship with wavelength.

Yes. But is it not hightly dependent also on particle size? Oxygen and nitrogen are givens for our atmosphere, but what about the scattering changes due to larger particulate atmospheric matter (ie carbon dioxide).

Are you suggesting a particle size that Rayleigh-scatters red light but is too big to Rayleigh-scatter blue light?

It is more of a question than a likelyhood, but....yes. If we could control particle sizes in a lab, could we see more red scattering than blue?

[grant hutchison]

I went to JPL and I didn't find, with my limited time, the entire rover image gallery. I wanted to show you the sunset image which shows the halo. It was not an imaging arteface, IIRC.

S'okay: I know the picture. JPL and NASA have such a range of options for what they're prepared to call "true colour" or "natural colour", I always feel obliged to insert "artefact" as a cautionary consideration in matters of colour. Just to show I've thought of it, if nothing else ...

It is more of a question than a likelyhood, but....yes. If we could control particle sizes in a lab, could we see more red scattering than blue?

I don't know the answer. My concern was that, once the particles are big enough, they won't just stop scattering blue light: they'll begin to reflect it in a different way. But I can see how it might be that red was scattered diffusely (in a Rayleigh distribution) while blue was primarily forward-scattered (in a Mie distribution). Interesting idea.

Grant Hutchison

[George]

I would really like to become competent in understanding just how each variable plays a roll in elastic scattering. Heliochromology has been a nice stepping stone in this regard, yet I am still the amateur. Hopefully, I will have time this winter to dive into a little.

There is an interesting, and brief, treatment here . Notice how the particle size has dominance (6th power) over wavelength (4th power) dependance. However, this is only a piece of the scattering story, obviously.

[grant hutchison]

There is an interesting, and brief, treatment here.

Oooo you rascal. Tricked me into clicking on a Wikipedia link. Shame on you.

Grant Hutchison

[George]

Oooo you rascal. Tricked me into clicking on a Wikipedia link. Shame on you.

I finally got you, but, shucks, it was a fortuitous accident. Is Wikipedia too simplistic? [It just happened to be better than the other three I googled and I was lookng for the d^6 term].

[quickly added...I know I printed several papers on scattering and one demonstrates that scattering increases as the 6th power over a specific small range of particle size (at least I think this is so). If you're interested, I'll have time this weekend to mess with it a little.]

[Kaptain K]

We have a somewhat similar situation west of San Antonio due to wind and dust.

Not to mention the annual torching of the fields in Mexico, at least when the wind is in the right direction!

[George]

Not to mention the annual torching of the fields in Mexico, at least when the wind is in the right direction!

Yes. I had forgotten about that.

I'll bet McDonalds Observatory, near Alpine, has some great sunset shots.

[grant hutchison]

I finally got you, but, shucks, it was a fortuitous accident. Is Wikipedia too simplistic? [It just happened to be better than the other three I googled and I was lookng for the d^6 term].

It was just a pretty obscure joke. I've recently been taken to task for expressing dissenting opinions about the usefulness and desirability of Wikipedia on another thread.

Grant Hutchison

[George]

It was just a pretty obscure joke. I've recently been taken to task for expressing dissenting opinions about the usefulness and desirability of Wikipedia on another thread.

Grant Hutchison

Darn, I wish I'd known about it a week ago. I could've been a little hero. But I don't go there very often. [If I confined my babbling to that forum, what would I have to say in the others. ]

[Wiki doesn't include heliochromology, so how good could it be. ]

[George]

I recently saw a nice Mars presentation in the virtual world of 2nd Life (wow, what a place). Patio Plasma was the speaker - I missed her 1st Life name (shoot) - and when the blue "corona" image came up - it's not actually called a halo, so I learned - I bet her an ice cream it was due to selective scattering. I was right, and darned if they didn't give me an ice cream, too.... but freeze dried.

Anyway, she was kind to clarify the blue corona issue. It is due to Mie Scattering because the dust particles are about 1 micron in size. Perhaps there is also some Rayleigh Scattering, too, or reflection off larger red particles, but I did not press for details, naturally, since I am so shy.

Grant was right from the start since it is due to scattering of the red colors, and Ken correctly named it as it is primarily due to, apparently, Mie Scattering.

[mugaliens]

When I look at a sunset, preferrably with a hefeweisen in hand, at an upper slope of a decent mountain, I expect to see several things.

The first is the wonderful red produced by the sun's interaction with our atmosphere. Depending upon what's presenct in our atmosphere at the time, that may take on many different shades.

I hope to feel a warm afternoon's breath upon my face, to hear songbirds preparing the continuance of their progeny, while overhearing an old man recounting years past just a table away.

But what I really enjoy seeing the most is how the light of that sunset plays off the hair and the eyes of the beautiful girl across from me, how it warms her smile, softens her skin, and warms our hearts towards one another.

Raw physics is interesting enough in it's own right, but it pales in comparison to the true nature of our existence, which is to connect with the right woman, and ensure future generations carry the genes of whatever gifts we've been given throughout successive generations.

Personally, I can think of no finer way to accomplish that other than beginning on the uppermost slope of a mountain in Europe overlooking a village below, seated across from a dame of striking beauty and intelligence, and admiring how her features change as our Sun slowly slides beneath the horizon throughout the course of our conversation.

If all my days exist to culminate in this even, then they will have been worth it. If I ever find someone who's willing to repeat the event, or similar, ancilliary events, including fireside cuddles, cooking together in the kitchen, raising babies (very tiresome, but very rewarding), baking bread (tiresome, but occasionally rewarding, depending upon how good one is...), I'll consider myself a very lucky man to have found such a woman that would allow a man like myself to be the man God meant me to be.

Despite the baggage of knowledge on board, courtesy of a long life of education and military service, I am, at heart, a simply man, mostly interested in those things which matter most - God, family, friends, neighbors, and other issues which might adversely affect myself and my family.

Life is a gift, and when life gives you other people in your life to share that life with you, you're blessed.

Strangely enough, this message board supplants, in part, the relationships each of us would find elsewhere if this board didn't exist. It's a part of who we are, who we've become, and where we're going. We've all seen our share of issues, we've all worked through our share of problems. None of us are through with this process - for some of us it's more public than the others, but for all of us it's a part of life itself.

The good news is that we're in this together. We can PM one another for help and support (as I've done from time to time), or we can share with our church, our extended families - whoever we feel comfortable with sharing.

Sunset colors.

It seems the end of one's life.

I, however, see it as the beginning of a life. I may not be the one you're familiary with, but it's the one where you belong.

Enough said - enjoy the sunset.

[Gillianren]

Raw physics is interesting enough in it's own right, but it pales in comparison to the true nature of our existence, which is to connect with the right woman, and ensure future generations carry the genes of whatever gifts we've been given throughout successive generations.

Ahem. Not for all of us, it's not!

. . . baking bread (tiresome, but occasionally rewarding, depending upon how good one is...)

Actually, I find baking bread quite pleasant. It's especially helpful for working out one's frustrations on the dough.

[mugaliens]

The enhanced sunset colours are because ash in the high atmosphere scatters the red light of the setting sun. So the sky looks red over a wide area, and for some time after sunset at ground level.

Grant Hutchison

And yet I witnessed a glorious red sunset yesterday.

Where's my Mount Pinatubo?

[grant hutchison]

And yet I witnessed a glorious red sunset yesterday.

Where's my Mount Pinatubo?

The OP asked specifically about the global enhancement of sunsets by major volcanic eruptions.
Other local, tropospheric particulates will do the same job: water droplets, dust, smog, ash, smoke.

A complementary consideration might be that you have no rip-roaring volcanic sunset for comparison. There is a weird crop (880kb pdf) of paintings by observant artists (perhaps most famously, Turner) that can be dated to "volcano years", and which show quite amazing sunset shades.

Grant Hutchison

[Gillianren]

The sunsets back home in LA are often lovely; it's the smog.

[George]

The sunsets back home in LA are often lovely; it's the smog.

I assume you would see a red sun, at least if one is east of LA? It is still an issue of particle size but I'd guess smog particles are small enough.

[Gillianren]

I was to the north, actually (by maybe ten or fifteen miles), but there is a well-known particulate aspect to the sunsets there. Of course, it's more dramatic during fire season!

[George]

It is known that the Moon can appear blue due to the selective scattering properties of certain sized particles, but this is rare. I want to say fires can provide these particles, and that not every fire will produce them. However, maybe it is a volcano thing, instead. Apparently, particles about 1 micron in size will allow Mie Scattering to scatter red light much more than blue, thus a blue Moon would be seen.