Everyone thinks sunset is just, you know, a simple
off -switch. Right. Day turns to night. 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? 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 machine 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
afterglow, and its morning equivalent, the foreglow.
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. OK.
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
afterglow, that broad arch of whitish or pinkish
sunlight that just kind of lingers in the twilight,
you aren't just seeing light. Right. You are
witnessing a highly complex mechanical collision
between specific light frequencies and that atmospheric
soup. OK, 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 afterglow 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, it 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 thinnest
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. Yeah, 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. But 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. 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.
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. That backscattering is exactly
what paints the sky with that lingering reddish
-pink glow. So 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 2 and 6 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. Down low, 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 thinner. 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 sets in. Yeah, the bright
segment is kind of the last gasp of that forward
scattered sunlight. Yeah. 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. because the amount
of particulate matter in the air is the ultimate
variable in an afterglow. So let's talk about
the phenomenon of volcanic purple light, specifically
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. That'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.
Not 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.
Exactly. It took the standard physics of the
afterglow and turned 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. Okay. They actively alter our visual
reality on a global scale. And that altered visual
reality permanently changed art history. It really
did. Think about Edvard Munch's famous painting,
The Scream. You know, the one, the agonizing
bald figure holding its face on a bridge. Right,
set against this swirling, chaotic, blood red
and orange sky. Exactly. Yeah. And for decades,
people assumed 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 -back scattering
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 incredible. 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 shortly after the
1991 eruption of Mount Pinatubo in the Philippines.
Wow. You see the exact same particulates scattering
over skyscrapers in Taiwan, housing estates in
Krakow, a pier 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, sure, 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 Alpenglow. Right. So
imagine standing deep in the Triglav 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 snowfields. Those
high elevation snowfields are still physically
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 Alpenglo is
what really fascinates me, because the absolute
final stage of Alpenglo occurs specifically when
the purple light is reflected. Right, because
those peaks are so incredibly high. They catch
that specific 2 to 6 degree intersection of backscattered
red and high -altitude blue light. So you are
standing in darkness down below. Yeah, and the
mountaintops... Miles above you are glowing a
surreal, vibrant purple. OK, 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? Ah, the
South Pole foreglow. This is where the geometry
of the Earth stretches the physics we've been
talking about into something almost unrecognizable.
How so? At the Earth's poles, the sun doesn't
rise and set every day. It only appears at 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's 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.
But 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. That 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 forglow,
you get this atmospheric machinery stretched
to its absolute temporal limits. 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 bigger picture. It proves
that our perception of light, color, and even
time is entirely relative. Totally relative.
The physics are identical. The railings scattering,
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 master class in physics happening right
above our heads every single day completely ignored
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.
A 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 particulate
dust into that fragile equation like Krakatoa,
violently exploding into the stratosphere, can
paint global masterpieces across the sky. Leaving
observational records and iconic art like the
Scream. Exactly. And we discovered that your
location on the planet, whether you're using
the jagged peaks of the Alps as a projection
screen for Alpenglow or watching the agonizingly
slow dimmer switch of a South Pole foreglow and
a six -month night, completely dictates how you
experience those constant physics. The afterglow
is just a beautifully complex, infinitely variable
system. It really is. So here's our challenge
to you. Tonight, after the sun goes down, don't
just walk inside and flip on a lamp. Wait for
it. Just wait a bit. Wait until you think the
sun is roughly two to six 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. But think about the atmospheric soup
hovering over us today. What kind of invisible
modern particulates from global industrial emissions,
constant commercial air travel, or the sprawling
footprint of urban life are suspended up there
right now, actively scattering the light in our
skies? When 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 twilight
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 Steep Dive.