The physics of the purple afterglow

0:00You're listening to a podcast right now, driving, working out, walking the dog. If you're into podcasts, chances are you have something to say too. With RSS.com, starting your own is free and easy. Upload an episode, and we distribute it to Apple podcasts, Spotify, Amazon Music, and hundreds more. Track your listeners, see where they're from, and start earning from ads like this. Even with just 10 listeners a month. If you've been thinking about starting a podcast, this is your sign. Start free at RSS.com. Everyone thinks sunset is just, you know, a simple off-swit. Right. Day turns tonight. Exactly. You watch the sun drop below the horizon, the sky gets dark, and you just assume the show is completely over. I mean, it's the ultimate binary, right? Yeah. We're totally wired to see it as this clean break between light and new light. If you know exactly when to look, specifically when the sun hits that sweet spot between two and six degrees below the horizon, you can actually see this massive, hidden atmospheric

1:04machine that controls our sky. Yeah. It really is a machine. It is. And welcome to this deep dive. Today, our mission is to completely transform how you look up after the sun goes down. We're pulling from this fascinating Wikipedia deep dive on the atmospheric optical phenomenon known as the after glow, and it's morning equivalent, the fore glow. Yeah. We're going to unpack the hidden physics of light frequencies, some pretty dramatic historical events that, you know, permanently altered our skies, and the visual masterpiece is hiding right there in the quiet light. But to really grasp what is happening above our heads, we have to stop looking at the sky as an empty void. Okay. We have to start looking at it as a physical medium. The atmosphere is basically this dense soup of gases suspended dust and aerosols. A soup. I like that. Yeah. So when you see an after glow, that broad arch of whitish or pinkish sunlight that just kind of lingers in the twilight, you aren't just seeing light, right?

2:05You are witnessing a highly complex mechanical collision between specific light frequencies and that atmospheric soup. Okay. Let's unpack this. Because to truly understand this phenomenon, we literally have to dismantle the sky. Exactly. We have to look at the mechanical ingredients. So let's start with the baseline physics before we even get to the after glow itself. Sure. Let's talk about the golden hour right before sunset. Why does the light suddenly look so, I don't know, heavy and red? Well, really comes down to the physical shape of the light waves. Sunlight looks white to us, but it's actually a spectrum of different colors, which are really just different frequencies of energy. Like a rainbow. Right. So blue light is high energy. Its waveforms are short, tight and choppy. Red light is the opposite. It's low energy with long, rolling waveforms. Now when the sun is high in the sky at noon, its light is shining straight down through the finish part of the atmosphere. So it's taking the shortest possible path to our eyes? Right. But as the sun drops toward the horizon, the angle changes drastically.

3:07Yeah, obviously. So now that exact same sunlight has to punch horizontally through a much thicker slice of the Earth's atmosphere to actually reach you. Like the thicker the atmospheric onion, the more brutal the filter. That's a great way to put it. And this is where this mechanism called Rayleigh scattering takes over. Rayleigh scattering. Yeah. Remember those short choppy blue light waves? Right. The high energy ones. They cannot survive that long journey through the dense, lower atmosphere. They basically crash into oxygen and nitrogen molecules and just get scattered away in every single direction. Oh, wow. So they just get bounced out. Exactly. By the time the sunlight reaches your eyes at the golden hour, all those fragile, blue and green wavelengths have been completely obliterated from the direct beam. Leaving only the heavy hitters. You've got it. The long, rolling, low frequency red waves that can just bulldoze their way through the thickest part of the atmosphere. That intensely low frequency red component is what survives.

4:07But the machinery we really care about today kicks into high gear during what's called civil twilight. Right. When the sun physically drops below the horizon line. Exactly. The direct beam is gone. Yeah. Yet the sky is still glowing red. Wait. Let me make sure I'm visualizing the mechanics here. Sure. If the light source like the sun itself is physically blocked by the curvature of the Earth, how is that red light still reaching my eyes? Backscattering. Backscattering. I remember that atmospheric soup we talked about. The dust and aerosols. Right. The air is full of particulate matter, dust, pollen, volcanic ash, pollution. Even though the sun is technically below your horizon, its low frequency red rays are still shooting straight across the lower atmosphere, just above your head. Oh, I see. So those red rays hit the suspended particles in the air, bounce off them, and scatter back down toward the ground. That makes total sense. Yeah, sometimes they reflect off clouds or even high snow fields. The backscattering is exactly what paints the sky with that lingering reddish pink glow.

5:09So we have red light bouncing off dust in the lower atmosphere. But the absolute showstopper of this whole sequence is something called the purple light. Yes, the purple light. And this happens specifically during civil twilight when the sun is exactly between two and six degrees below the horizon. That's the sweet spot. But wait, if the red light we just talked about is low energy, and the blue light is high energy, how on Earth are they mixing to create purple? It's a bit of a mind-bender. Because if the atmosphere is acting like this brutal filter obliterating the blue light, where is the blue coming from? Well, you have to think about the atmosphere in three dimensions, not just as a flat ceiling above us. OK. At that highly specific two to six degree angle, you get this perfect structural overlap of two completely different atmospheric layers. Two layers. Download, near the ground, you have the surviving low frequency red light backscattering off the dust, just like we discussed. Our surviving heavy hitters. Right. But look, higher up, miles above your head, up in the upper atmosphere, the air is much

6:12thinner. Oh, right. The sun is still illuminating that high altitude layer. And because the air is thinner up there, the short high energy blue light waves aren't getting obliterated yet. They're surviving. Yeah, they're doing their normal thing, just scattering broadly across the sky. And this broad, high altitude blue light actually belongs to the darker phase of twilight known as nautical twilight. Oh, I see. So your eyes are playing a trick on you. Basically, yeah. You're standing on the ground, looking up through a lower layer of backscattered red light straight into a much higher layer of scattered blue light. Exactly. Our eyes compress those miles of vertical depth into a single flat visual canvas. We just see the physical intersection of two completely different energy frequencies stacked on top of each other. Red layered over blue. Which equals the purple light. It's literally the visual mixing of civil twilight and nautical twilight. That is so cool. And anchoring this whole visual display is the bright segment, which is that part that lingers right at the horizon until nautical twilight completely takes over and true darkness

7:15sets in. Yeah. The bright segment is kind of the last gasp of that forward scattered sunlight. And while we are breaking down the cold physics of this, we really can't ignore the human impact. Definitely. This specific scattering, particularly when the high frequency blue light dominates just before or after that reddish glow creates what we call the blue hour, the blue hour. It's this fleeting window of time that painters and photographers have just treasured for centuries. The ambient sort of omnidirectional quality of that scattered blue light provides an atmospheric depth that is physically impossible to replicate with a flash or artificial lighting. So we have this highly delicate balance of red and blue light acting as our baseline, a canvas provided by the natural dust and geometry of the earth. Right. Here's where it gets really interesting. What happens to that fragile equation if you violently dump billions of tons of ash into the stratosphere? Well, it completely breaks the standard physics. You move instantly from everyday meteorology into the realm of world changing historical events.

8:19Because the amount of particulate matter in the air is the ultimate variable in an after glow. So let's talk about the phenomenon of volcanic purple light. The 1883 eruption of Krakatoa in Indonesia. Oh, Krakatoa. This wasn't just a standard geological event. It was an explosion so monumentally massive that it literally ruptured the sky. Yeah, Krakatoa blew this incomprehensible volume of exceedingly fine, pulverized rock and ash straight up into the stratosphere. It's massive amounts. We are talking about altitudes where commercial jets fly today and even higher than that. And it obviously didn't just stay hovering over Indonesia. But at all, high atmospheric winds caught that unbelievably fine dust and basically used it to wrap the entire planet. It became a global diffusion. It's like imagine throwing a massive handful of glitter straight into a projector beam. That's a great visual. Suddenly the light isn't just passing through empty space. It's aggressively bouncing off millions of new surfaces. The Krakatoa ash provided trillions of microscopic mirrors suspended globally in the stratosphere.

9:22Exactly. The standard physics of the afterglow and turn the volume all the way up to maximum. Wow. For months, people all over the globe were suddenly seeing a worldwide series of intensely magnified red sunsets and volcanic purple light. The low frequency red light just had this unimaginably vast, new canvas of particulate matter to backscatter against. See, I want to push back on this a little though. How is it that an event so incredibly violent, so destructive at its epicenter translates into a lasting, beautiful, visual phenomenon halfway across the globe? What's fascinating here is that monumental geological events don't just shape the ground beneath us. They actively alter our visual reality on a global scale. And that altered visual reality permanently changed our history. It really did. Think about Edward Munch's famous painting, The Scream, the one the agonizing bald figure holding its face on a bridge. It's set against this swirling chaotic blood red and orange sky.

10:23Exactly. And for decades, people assume that sky was purely psychological, like a metaphor for the figure's internal dread, but it's actually an observational record. The timeline aligns perfectly with the global diffusion of the Krakatoa dust cloud over Europe. Munch was known to paint exactly what he saw. He wasn't painting an emotional metaphor. He was documenting the literal, hyper-intensified red backscattering of the volcanic purple light forced into the atmosphere by Krakatoa. A geological rupture in the southern hemisphere becomes an iconic visual masterpiece in the northern hemisphere. It's credible. It just reinforces this profound idea that everything in the natural world is deeply physically interconnected. And this isn't just a 19th century phenomenon, either. Right. If you look at modern photographic records of afterglows, you see an intensely vivid, bruised purple sunset over Hong Kong. Exactly after the 1991 eruption of Mount Pinotubo in the Philippines. Wow. You see the exact same particular scattering over skyscrapers in Taiwan, housing estates

11:27in Krakow, up here in Australia, a college track in Maine, and the city of Dresden in Germany. Because the physics are entirely agnostic. Exactly. A long red wavelength will backscatter off a volcanic ash particle exactly the same way over a housing estate in Poland as it will over a track in Maine. The mechanical constants of the atmosphere do not care what is on the ground below. The physics are a constant shore, but where you are standing on the ground drastically alters how you perceive those physics. We've explored how a global extreme, like a massive volcano, injects chaos into the afterglow. Let's look at how geographic extremes warp the experience. Well, elevation is the most immediate modifier. In mountainous regions, we see a specific variation of the afterglow called Alpine Glow. Right. We're standing deep in the Treglow Lakes Valley in the Julian Alps. The sun has already dipped behind the jagged peaks. You are standing in the cold, dark shadows of civil twilight. But when you look up at the highest snow fields, those high elevation snow fields are still

12:32physically catching the direct or scattered light that the valley floor has already lost. Oh, wow. The mountains are acting as a massive geological projection screen. The golden red light reflects brightly off the peaks, functioning very similarly to another optical phenomenon called the belt of Venus, where that pinkish band sits just above the horizon. But the timeline of Alpine Glow is what really fascinates me, because the absolute final stage of Alpine Glow occurs specifically when the purple light is reflected. Right. Because those peaks are so incredibly high, they catch that specific two to six degree intersection of backscattered red and high altitude blue light. So you are standing in darkness down below. Yeah. And the mountain tops miles above you are glowing a surreal, vibrant purple. Okay. So elevation creates a projection screen. But what happens if we push the geography to the absolute extreme, like the bottom of the Earth, the South Pole? The South Pole for Glow. This is where the geometry of the Earth stretches the physics we've been talking about into

13:33something almost unrecognizable. How so? Here it's poles. The sun doesn't rise instead every day. It only appears that the horizon around the equinox. Which marks the transition between a half year long polar night and a half year long polar day. Exactly. So put yourself in the boots of a researcher stationed out there on the ice. You haven't seen the sun in nearly six months. That's intense. Your body circadian rhythm is completely starved of daylight. You are living in a permanent, freezing, nautical twilight or total darkness. The psychological weight of that environment has to be staggering. Then, days before the sun even physically breaches the horizon, the sky starts changing. Because of the equator, the sun drops vertically. Sunset takes what? 20 minutes? Yeah, pretty quick. But because of the severe tilt of the Earth at the poles, the sun approaches the horizon at an incredibly shallow, agonizingly slow angle. So what does this all mean? It's like the Earth's geometry is acting as a massive slow motion dimmer switch.

14:34It is exactly it. The low energy red light begins pushing through the thickest possible horizontal slice of the atmosphere. The heavy atmospheric refraction bends that light over the curvature of the Earth, allowing it to reach the South Pole longer for the physical body the sun does. So instead of a 20-minute four-glow, you get this atmospheric machinery stretched to its absolute temporal limits. Yeah. You get days of this agonizingly slow build up of refracted red and purple light, just this glowing promise on the horizon, fighting against a six-month night. If we connect this to the bigger picture, it proves that our perception of light, color, and even time is entirely relative. Totally relative. The physics are identical. The Rayleigh scattering is stripping away the blue light, the low frequency red waves punching through, the back scattering off suspended ice crystals or dust. It is the exact same mechanical process happening over a driveway in Ohio. Right. But the extreme geography of the South Pole turns a passing daily moment into a prolonged monumental event. It really is a masterclass in physics happening right above our heads every single day, completely

15:38ignored by most of us. Oh, absolutely. We started this deep dive talking about how simple a sunset seems, day to night. But when we actually dismantled the sky, we found a highly sensitive machine. On delicate balance. Yeah. We found that the golden hour is just a brutal filtering process, leaving only long, heavy red wavelengths. We found that when the sun hits that specific sweet spot of two to six degrees below the horizon, the surviving red rays in the lower atmosphere stack perfectly beneath the scattered blue rays of the upper atmosphere, tricking our eyes into seeing the purple light of civil twilight. And we saw how injecting massive amounts of fine, particular dust into that fragile, equation-like crackatoa, violently exploding into the stratosphere, can paint global masterpieces across the sky. Having observational records and iconic art like the screen, and we discovered that your location on the planet, whether you're using the jagged peaks of the Alps as a projection screen for Alping Glow, or watching the agonizingly slowed dimmer switch of a South Pole 4 glow

16:40and a 6 month night, completely dictates how you experience those constant physics. But the afterglow is just a beautifully complex, infinitely variable system. It really is. So here is our challenge to you. Tonight, after the sun goes down, don't just walk inside and flip on a lamp. Wait for it. Wait until you think the sun is roughly 2-6 degrees below the horizon. Look up. Look past the darkness and actively try to spot the structural overlap of the purple light. Try to find that bright segment lingering right at the edge of the world before nautical twilight finally takes over. Look for the hidden machinery at work. And as you look up at that glowing horizon tonight, I want to leave you with one final thought to ponder. Okay. We know that the incredibly fine natural volcanic dust from an eruption in 1883 drastically altered the colors of the afterglow, right? Creating skies so vivid they changed the course of art history. Yeah. But think about the atmospheric soup hovering over us today. What kind of invisible modern particulates from global industrial emissions, constant

17:43commercial air travel or the sprawling footprint of urban life are suspended up there right now, actively scattering the light in our skies. Can you look at a brilliantly vivid, bruised purple sunset tonight? Are you just seeing the natural, untouched physics of the atmosphere? Or are you actually seeing the invisible footprint of our modern world suspended in the air and illuminated in red and blue? That is a wild thought. It really makes you realize that the Quilette sky was never just a simple off switch at the end of the day. It is a massive, complex, brilliantly illuminated mirror reflecting exactly what is suspended in our world. Thanks for joining us on The Steepdive. You're listening to a podcast right now, driving, working out, walking the dog. If you're into podcasts, chances are you have something to say too. With RSS.com, starting your own is free and easy. Upload an episode and we distribute it to Apple podcasts, Spotify, Amazon music and hundreds more. Track your listeners, see where they're from, and start earning from ads like this.

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