We all enjoy beautiful, multicolored sunsets. But what causes the brilliant oranges, pinks and purples that we see, and why does it change from day to day and season to season?
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Female Speaker: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the world’s longest-running online astronomy degree program. Visit astronomy.swin.edu.au for more information.
Fraser: Astronomy Cast, episode 411. The Science of Sunset Colors. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos where we help you understand not only what we now but how we know what we know. My name is Fraser Cain. I’m the publisher the universe today. With me is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville, and the Director of Cosmo Quest. Hey Pamela. How you doing?
Pamela: I’m going well. How are you doing Fraser?
Fraser: Good. And you’re gonna be traveling for a bit?
Pamela: I am. So we’re recording this on a special weird day, because I’m about to go and spend about 12 days in the Czech Republic, which is not part of the EU, so there is no state department warning on traveling there. And I’m going to be attending the AFO Academic Film Festival at Alamoke, which is run by a group of really wonderful young people, and the festival is 51 years old, which means the festival has been going on, showing documentary science films since before we landed men on the moon, which is just all levels of awesome. I love doing this. It’s a real privilege to get to go and be a guest.
Fraser: Yeah, I know. This is one of the ones that you always look forward to every year.
Fraser: And I know this is great. So, cool. Well, have a good, safe trip, and we’ll see you when you get back.
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Fraser: Hello Astronomy Cast friends. Fraser here. So once again I wanted to give a shout out to Casper.com, who is our generous sponsor for Astronomy Cast. Now, we’ve talked about this in the past, but I thought I would just sort of give you all the story again, for those who missed it. So Casper gave us a mattress to try out. I had to leave mine in the states at my wife’s house. But I really liked the mattress, and so I ordered one for myself, and then I ordered another one for the spare bedroom. So this is the mattress that I’m sleeping on, and I think you’re gonna wanna sleep on it as well. It’s a high-quality mattress. It’s got this cool kind of latex foam with a memory foam.
And when you get the thing, it’s in this box that really defies the laws of space and time. And then you cut the box open, and the thing sort of inflates in front of you, and it’s really cool. I really like it. Casper mattresses cost $500 for a twin size, $750 for full size, $950 for a king, which is less than what you pay for a traditional mattress. I mean, it can be over $1,500. It’s totally online. You just order it. It shows up. If you don’t like it, you return it. It’s super simple, and I really, really like the mattress. So if you want a mattress, and Casper’s got a great offer for you. So if you go to casper.com/astro and use the promo code astro, they’re gonna give you $50 off a mattress. So all of that plus $50 off. You should totally do it today. So once again, go to casper.com/astro, use the promo code astro, and you will enjoy a good night’s sleep, just like me. Thanks.
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Fraser: So we all enjoy beautiful, multi-colored sunsets, but what causes the brilliant oranges, pinks, and purples that we see, and why does it change from day to day, and season to season? Okay. You threw this one into the docket.
Pamela: I did.
Fraser: And are you getting good sunsets right now? This is actually sunset season here on the west coast. In March, April, and into May a bit, we get the grey clouds that just hang over our area. They dissipate and we get these really great big kind of fluffy clouds, stuff that sort of changes the view of the sunsets and the sunrises every day, and it’s a really great time to see the skies. And then come summer, then, it’s pretty much clear skies, and we don’t get a lot of great sunsets. So what’s going on?
Pamela: Well, so personally, right now, we still have daylight, and judging by the absolute lack of clouds in the sky, we’re probably gonna have a really boring sunset k., in the St. Louis area. But the reason that I put this show in the list is I’m pretty sure we’ve talked about why the sky is blue, but we’ve never talked about why the sunset specifically isn’t always the same red. And there’s a new website that I stumbled across called sunsetwx, and it allows you to predict whether or not – and you have to pay for it, I do admit to that – it will predict whether or not you’re going to get a truly awesome sunset. And the science behind it’s kinda cool. And I’m like, this is science that is true everywhere. And we should talk about this, because one of my favorite images, the one that I use as the background, is a sunset on Mars, which is this fabulous shade of liquid blue, which is actually the color I painted my home office. And all of this is because science.
Fraser: Because science.
Pamela: Because science.
Fraser: Okay. So where do you wanna start here? So should we talk about sort of when you’re seeing a sunset, what are you seeing? And I guess we need to sort of what are you normally seeing when you’re just like seeing the sun in the sky? What are we actually seeing?
Pamela: So if we were able to get rid of our atmosphere –
Fraser: Don’t. Please don’t.
Pamela: Yeah, no. We’re not going to do this.
Fraser: Okay. All right.
Pamela: But if we could.
Fraser: If we could, all right. But we’re not going to.
Pamela: If we could get rid of our atmosphere, we would have a stark shadow exactly where day turns to night, and the sun would just sort of go “Zot, hi, I’m over the horizon. You get no more of me, and now you’re plunged into darkness.” And during daylight when we looked up at the sun, we’d see this white star hanging out, blinding us or radiating us, because there’s no atmosphere to protect us. We’d die, but we’d die while looking at a white star.
Fraser: Right, because as we all know, that is the color of the sun.
Fraser: Yeah, it is white, not yellow. And that’s what you see when you’re on the moon, right? When you’re up on the moon, and then you just see all those photons, they reach your eyeballs with nothing in between, and you just get the raw, pure sunshine effect.
Pamela: Which is why the Apollo astronauts’ head visors that were coated in, among other things, gold to reflect away a great deal of the sun’s light and now we don’t have blind astronauts. And I’m also in favor of that.
Fraser: Okay, so that’s – were there nothing, that’s what we would see. But now we’ve got some atmosphere which you destroyed but now we can bring back for the next step of this. So what are we seeing when we’re seeing the sun through the atmosphere?
Pamela: So during the day, sun high up in the sky, when we look at the sun, we see this yellowy orange ball. The exact color depends on how much water vapor is in the atmosphere, where we are on the surface of the planet, but when the sun’s high in the sky, the reason it’s yellowy orange is because a lot of that blue light that’s coming out of the rainbow distribution of the sun’s black body spectrum – a lot of that blue light is getting scattered by the molecules and atoms in our atmosphere. And as it scatters around, again, this is the blue light, some of it gets scattered down towards the surface of the planet, and that’s what we see giving the sky its blue color. All that is is bounced around scattered light that bounces enough times that it bounces back our eyeballs.
Fraser: And we have, of course, done a whole show on this. Why is the sky blue? So it’s like the blue spectrum of the rainbow of the light, all the blue photons are the ones that are getting scattered the most, and so those are the ones that we’re seeing. While all the other colors generally go in a straight line to where they were intending to go, while the blue ones can kind of come at you from anywhere.
Pamela: Yes, yes.
Fraser: Okay, all right. So we’re looking right up into the sky. We’re seeing the sun. We’re seeing it through the atmosphere. We’re seeing both the color of the sun changed because of the scattering of the atmosphere, and we’re also seeing the blue light from all directions because the blue photons are getting scattered by the atmosphere. So now then what happens when the sun is low to the horizon, and it changes in color?
Pamela: So for those of you who are watching this live on the internet, I’m holding up and trying to get away from reflections, a CD case that has a blue CD in it. Now, the rest of you are just gonna have to imagine this in your head. So imagine that you’re looking at the face of a CD that’s kind of blue. Now, if you’re up at the top of the center part of the CD, the part that you put it in your CD thing and it makes it do the spinny thing. We’re gonna pretend that center of the CD is the planet Earth. Now, if you’re at the top of that and you look straight up, you’re looking through the narrowest possible amount of your atmosphere.
So the amount of light that gets to you – it’s a lot of it, and it hasn’t had to travel very far, so it hasn’t had as many chances to get scattered away. So in the distance, from the top of the atmosphere to the top of the Earth, it scatters the blue – blue sky. Now when the sun is, instead, off to the side, when that light travels to you, it has to travel through a much larger distance as it goes all the way across the horizon, all the way out through the side of the atmosphere, and this much greater distance the light travels. Well, that larger column is able to scatter larger and larger percentage of the other colors in the sun’s black body curve.
So it’s not just scattering the green. It’s starting to scatter the oranges, the yellows, and even the reds. Now the reds are the hardest to scatter. Now this is why we see the sun, for the most part, as red, because all those other colors, they’ve been scattered all over the sky, and some of them get to reflect off of clouds and other things. But what’s really cool is the amount of light that gets scattered, it doesn’t just vary with the distance the light goes through the atmosphere. Otherwise, sunsets would be the same color all the time. It also varies with, well, how much atmosphere, because the moisture actually increases the amount that the light of the sun bends.
It also depends on how much pollution is in the atmosphere, because that increases scattering as well. So if you’re living near an oil refinery, like I do, you get pretty sunsets, especially when it’s humid. If you are living somewhere where a volcano has just finished doing its thing, all of that aerosolized stuff floating around in our atmosphere; it helps to scatter out more and more and more of the sun’s light, getting you these deeper and more vibrant sunsets.
Fraser: And so if there were no clouds, if there was no pollution in the atmosphere, all that kind of stuff – say you’re looking through 20 kilometers of thick atmosphere when you look up, but maybe you’re looking at doing the math, like 40 or more, when you’re looking off to the horizon. It’s giving that – I’m not sure of the exact numbers, so don’t take –
Pamela: Yeah, Fraser is making up numbers.
Fraser: Don’t write that down. I am making up numbers.
Pamela: Do not –
Fraser: But so then you’re getting more – further down the spectrum is getting a chance to scatter away. And that’s just like without any extra stuff. But when we have those aerosols in the atmosphere – around where I live, we get forest fires a lot. And that will cause just insane sunsets. Sometimes, if the forest fires are really thick, you’ll get the sun looks like it’s having a sunset when it’s directly overhead. It turns this red color and you can just barely make out this red disk, even though the sun is directly overhead, and the sky is clear. Other times, it’s not directly around you, and you get these sunsets, because the aerosols are sort of off in some direction, and you’re seeing through that. But why do we get those crazy colors when we have those clouds? Because, I mean, the clouds and sunsets really go hand in hand.
Pamela: And this is where the amount that the light gets bent, the refraction quality of the atmosphere, also starts to matter. So you have two different things that are going on. One, you have the light that’s trying to come straight towards you is failing, because it’s scattering all over kingdom come. And eventually it’s going to get bounced off of a different part of the atmosphere, and if you’re lucky, it makes it back to your eyeball or your camera lens, and onto your film or CMOS chip.
Now some other parts of that light, they’re trying to shoot off in some different direction. But as they pass through the atmosphere – and all of the light is bending at a different point, so even the light that we’re calling coming straight at your eyeball, it really got bent by the atmosphere, but we’re using that as the test case. The light that you perceive as having come straight from the sun.
Fraser: But that’s why – I mean, sorry to interrupt, but that’s why when the sun is close to the horizon, it looks all wibbly wobbly –
Fraser: and out of focus, because even though – there may not be any clouds around it at all, but it’s just the sun itself is having to go through so much atmosphere, the light is getting distorted and refracted around that it’s not able to take that nice clean path to your eyeball.
Pamela: Now, some of the light that was never going to make it straight to your eyeball, it’s going up through the atmosphere, and it’s getting bent various amounts depending on the moisture, the temperature that’s in the air, and that refraction is what we call this bending. This refraction of the light is causing different colors of light, first of all, to bend at slightly different angles, which is cool. That’s why we get rainbows. And then these colors are hitting the clouds. And so you end up with different parts of the sky where different parts of this rainbow have refracted. And where there’s different thicknesses, so you have different amounts of scattering, all of these different factors add up in really hard to calculate ways, which is why it’s 2016 and we, for the first time, have a website that you can go to.
Fraser: A web app, a website that can do it.
Pamela: And so now that we can go do this, we’re starting to be able to do this, it’s because we’re being able to start making the real-time calculations of at this altitude it’s this humidity, this temperature, these other factors that go into refracting the light and scattering the light, and it’s the combination of these two things.
Fraser: Right. So without – if there were no clouds, and you just had the sun, then all of – as it’s getting close to the horizon, it’s still going through all of this refraction, and I’m kind of imagining this light sort of exploding out of it, scattering off the sun in all these different directions and all these different colors, but there’s nothing to catch it. And so it’s all just sort of heading out into the air, and nobody sees it. But as soon as you get those clouds up from your perspective, it’s like you’ve now got a place for all that light to fall onto.
Fraser: And so then you’re getting these different colors, depending on, as you said, the thickness of the air, the moisture levels, the local temperature gradations, aerosols in the air, etc.
Pamela: And in figuring out exactly where to go to get the absolute best possible sunset, it looks for where are those high clouds. So you have two things that you’re taking into consideration – how much is the light getting scattered and refracted so that we end up with the potential for dynamic colors. And then once we have the potential for that, where are there the right amount of clouds? Because if you end up with too many clouds, it’s just kind of dull, blanket of grey. If you end up with just the right clouds reflecting the light – and so what they look for is nice, thin high-altitude clouds. Because first of all, those are tied to moisture. Moisture good. Second of all, beautiful scattering in your photos.
Fraser: And ideally volcano going off nearby.
Pamela: That does help. I recommend that over pollution. And to be clear, so Skyfire is the app I was looking at. That costs you money. It’s designed for photographers. But the one that’s free is sunsetwx, and I sort of combined that into one name with a cost. So sunsetwx is completely free. And then the app that’s designed for photographers is Sky Fire.
Fraser: That is really cool. I’m gonna play with this all the time.
Pamela: It’s really cool. So they have maps that allow you to look and figure out where are the cool places, and then they’re tracking their success using Twitter. So when people post things up on Twitter, they’re like, “Okay. This city with this weird sunset, do we have it right? Do we have it right? Yes! We have it right!” And when they don’t, they’re modelers. They refine their software. This is what scientists do, and these guys are scientists.
Fraser: It appears to only be for the United States and Canada and Mexico. So those of you in San Francisco at the time that I am recording this are gonna have a beautiful sunset, as well as the folks up around it looks like Winnipeg.
Pamela: So sunsetwx does the whole world.
Fraser: Oh, well this is the one I’m looking at right now. The sunsetwx, yeah, and it’s only the States, but maybe they more sections added. That is – this is super cool, though. Wow. And I guess you can put in some kind of – maybe get some announcements. Anyway. Alright, so I think we now understand sort of why we’re seeing the great sunsets. How does other worlds play into this. I mean, we talked about Mars. They’ve got a totally different sunset, right?
Pamela: Right. So Mars has much less atmosphere, and with less atmosphere, you get less scattering. And the atmosphere composition, it kind of is a little dry, you might say. It has water vapor in the atmosphere, but for the most part it’s not what we would call humid by a long shot. And it’s very cold, and so you have ice crystals, which are, again, a completely different effect. And so during the day, you have this beige, for lack of a better word, atmosphere. It’s kind of boring. Kind of white.
This is what happens when you don’t scatter enough light, and your eyeballs are like, “Meh. I’ll call it white.” But as the sun gets closer and closer to the horizon, you’re starting to get the blues scattered, and you end up with this beautiful grey/blue – if you’re watching this live, the color of my walls. You end up with this beautiful grey/blue color that you then end up with this pinpoint of white, slightly yellowish sunlight. And I think it’s just kind of magical to look at.
Fraser: It’s such a neat idea that the blue that we’re so familiar with because the sunlight is being scattered all the time, because of our thick atmosphere, can only really get rolling when sunset is happening on Mars, which is such a neat effect.
Pamela: The closest that you can get to a Mars sunset on earth is to go very high altitude in the middle of summer at the equator, or close to an equinox actually at the equator at high altitude. So get as much water vapor below you as you can. Get as little atmosphere above you as you can. Get the sun as close to straight overhead, and that color that you get down closer to the horizon is – it’s not right, but it’s as close as you get.
Fraser: Now there’s a really interesting phenomenon that can happen during sunrises and entering sunsets. Have you ever seen it – the green flash or the blue flash?
Pamela: I haven’t, and I have a friend, Robert Sparks. He’s a scientist down at the National Optical Astronomy Observatory – a former school teacher, a marathon runner, neat guy – who can catch green flashes on a regular basis taking pictures of Peak National Observatory from down in the city of Tuscon. And look up his photos if you can. They’re truly phenomenal.
Fraser: So what’s going on with the green flash and the blue flash?
Pamela: It’s at a very precise atmospheric condition. As the sun is below the horizon, the green light gets bent over the horizon from the sun, so it’s getting refracted through the atmosphere just right, so that if you’re at the surface of the planet, you’ll see for an instant this flash of green.
Fraser: That is really cool. And then it’s even trickier to get the one that’s blue, because it’s further down the spectrum. I’ve even heard that people can see a purple flash, but –
Pamela: They’re lying.
Fraser: Oh, really?
Pamela: I mean, I’m just – no, they may not be. But it just – that’s sort of like – s
Pamela: Yeah. It’s like winning the optical lottery.
Fraser: But say you wanna get a shot at it, right? You need to be looking – you need a nice clear view, right to the horizon, ideally at the ocean. And unfortunately, where I live, I’ve got mountains to the left of me and mountains to the right of me. And so I’ve got mountains in the east and mountains in the west.
Pamela: And the thing that amazes me because I didn’t know you could do it, but I’ve seen the photos, is Robert’s actually able to get pictures of the green flash with the light coming up over Kitt Peak. And so it’s not actually a flat horizon. He’s getting it as the sun goes below the mountain range.
Fraser: Oh, but it’s still low enough and still at the angle. Well then maybe all hope isn’t lost.
Pamela: I think it’s worth trying.
Fraser: I will give it a shot. Although it’s always cloudy, so we never see the sun anyway. We’re not even sure it exists here. But so one other, I think, kind of related is when we see the moon turn that beautiful red color during a lunar eclipse.
Fraser: Sort of a really –
Pamela: That’s the blood moon.
Fraser: Yeah, what’s going on with that, and how is that kind of related? Because it’s sort the same effect, right?
Pamela: It’s the exact same principle. So in this case, you have the sun’s light getting refracted around the planet, and as it goes from the daylight side of the earth through the atmosphere over to the eclipsed moon, the more stuff we have in our atmosphere, the more colors are removed. So if you have a really nice clean atmosphere, you end up with a grey moon during eclipse – doesn’t make for great photos. But if a volcano has recently gone off, or if there’s a lot of forest fires, those different things can add up to enough stuff in the atmosphere scattering the light out that you end up with this beautiful blood red moon. And what’s kind of cool is you’ll see different parts of the moon are slightly different colors based on different compositions and different parts of the atmosphere, and the sunlight’s refracting through in different places.
Fraser: It’s interesting. I know like every time that a lunar eclipse is coming we’ll be able to tell people, based on sort of where it’s gonna be moving through the atmosphere, the scientists will report on whether it’s gonna be a nice blood red one, or whether it’s gonna be a lot more muted.
Pamela: And we’re not always right.
Fraser: No, no. It’s like making predictions about meteor showers. But it’s really an atmospheric effect. It’s like the atmospheric modelers are the ones who are providing the data to say, “It should be a good one,” or “It’s gonna be pretty weak.” Although it’s always great. It’s always worth seeing.
Pamela: It’s true.
Fraser: Now there’s one other world that we got a chance to see, and we got a chance to see its sunset, which was Pluto.
Pamela: That’s true, and that was something we weren’t really expecting. Now we didn’t see it from the surface, but from looking at the sun passing all the way through the atmosphere to the spacecraft that’s experiencing sunset the same astronauts in the ISS experience sunset sort of. They saw the tholin-rich atmosphere of Pluto scatter enough light that you ended up with this grey/blue sunset.
Fraser: Yeah. It’s a beautiful –
Pamela: It was awesome.
Fraser: It’s a beautiful picture, yeah.
Fraser: It’s an amazing picture of Pluto itself, and not the disk is completely obscured because the sun is behind it. But you get this amazing atmosphere around it, and it really is this crazy blue circle that goes around the actual disk of the planet –
Fraser: and you could just see. So if you were standing on the surface of Pluto and the sun was just at the edge of the horizon, you would probably get something similar to what you see on Mars, right?
Pamela: Yeah. It’s not quite as rich of a blue, but it’s very similar.
Fraser: So if we could travel to other worlds around the universe and sample every possible atmosphere that was out there, what would make for the greatest sunsets and sunrises that you could sort of humanly imagine?
Pamela: I think – so you know how different liquids, like Italian salad dressing, you end up with different densities floating on top of one another? Well, you can imagine different atmospheric events that cause a highly stratified atmosphere. And with a highly stratified atmosphere, you actually have the chance for a stripey sunset, and I think that’s just a really cool idea.
Fraser: Oh, that would be cool. I mean, you could have different cloud layers and then different thicknesses and densities of the atmosphere at each one. Oh, that would be amazing.
Pamela: Yes. I’m a fan of stripey things.
Fraser: Does that happen naturally here?
Pamela: So we don’t have a highly stratified atmosphere in terms of very distinct layers, but one of the things that’s cool in some of the Pluto photos is there appear to be standing waves in the atmosphere of Pluto and we don’t know why. Don’t know why.
Pamela: And these are gravity waves, as opposed to gravitational waves, which LIGO detected. And these standing waves have these discrete density stripey bits, and you can see them through the atmosphere in the pictures. And so yeah, this happens. We’ve seen it.
Fraser: Oh, that’s amazing. Cool. Well, I think now we should all head out and look for the next great sunset.
Pamela: It’s true. So that free site is sunsetwx. They’ve given us no money. I just think it’s cool.
Fraser: Yep. And there’s an app.
Fraser: I want the app.
Pamela: The app is the other guys. The app is Skyfire, not free.
Fraser: That sounds great still. All right. Thanks Pamela.
Pamela: Thank you.
Fraser: Thanks for listening to Astronomy Cast, a non-profit resource provided by Astrosphere New Media Association, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at Astonomycast.com. You can email us at firstname.lastname@example.org. Tweet us @astronomycast. Like us on Facebook, or circle us on Google Plus. We record our show live on Google Plus every Monday at 12:00 pm Pacific, 3:00 pm Eastern, or 2000 Greenwich Mean Time. If you miss the live event, you can always catch up over at cosmoquest.org.
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