Why Rain Clouds Get Dark: Water & Light

0.000000    5.960000     Welcome to everyday explained your daily 20-minute dive into the fascinating house and wise of the world around you.
5.960000    11.040000     I'm your host Chris and I'm excited to help you discover something new. Let's get started.
11.040000    16.400000     You know those days, right? You look up and you see these like big fluffy white clouds just kind of chilling up there.
16.400000    24.320000     Looking totally harmless, almost like popcorn maybe. But then, you know, often the distance or sometimes creeping right towards you, you see them, the dark ones.
24.320000    31.440000     Brewing, almost black sometimes. The ones that basically scream, "Get inside, rain's coming." Quite a switch, isn't it?
31.440000    41.560000     One minute, bright and airy the next. Well, gone full-goth, it seems. And it makes you wonder, "Okay, if water is clear, how come rain clouds get so dark?"
41.560000    45.520000     Well, all their, you know, fluffy white buddies just stay white. Let's unpack this.
45.520000    49.280000     That's a fantastic observation. Seriously, it's something people have wondered about forever.
49.280000    58.520000     And what's really cool here is, like you said, water is clear, so the answer isn't really in the water itself. It's all about how light plays with the water inside the cloud.
58.520000    65.520000     So yeah, our deep dive today is all about pulling back that curtain, you know? Getting to the bottom of this whole atmospheric mystery.
65.520000    68.640000     Why those rain clouds look so well ominous.
68.640000    75.760000     Okay, good. So before we jump head first into the gloom and doom of storm clouds, let's maybe start with the basics.
75.760000    81.840000     Like how do clouds even get up there? And why are most of them so brilliantly white usually? This is where it starts getting interesting, right?
81.840000    89.680000     Absolutely. Yeah, I got to start there. So the air all around us, it's full of water, but it's as a gas water vapor, you can't see it.
89.680000    98.960000     Now, when the air near the ground gets warm, it rises, like a hot air balloon basically. And it takes all that invisible water vapor with it, high up into the sky.
98.960000    108.400000     As it goes higher, the air expands because the pressure is lower and it cools down. And that cooling is the key. It makes the water vapor want to condense back into liquid.
108.400000    114.240000     But here's the kicker. It needs a little something to condense on to. It doesn't just happen in empty air.
114.240000    117.120000     Okay, so it needs like a starting point. We'll speck as something.
117.120000    122.560000     Exactly. We call them condensation nuclei. Tiny, tiny specks floating around could be anything really.
122.560000    128.400000     Microscopic bits of dust, volcanic ash, salt from the ocean, even sit from cars or factories, pollen too.
128.400000    136.960000     So the cooling water vapor grabs on to these tiny particles, forming zillions of minuscule water droplets. Or if it's really cold up there, tiny ice crystals.
136.960000    140.960000     And when enough of these droplets and crystals get together, bam, you've got a cloud.
140.960000    148.160000     Okay, so clouds are basically these huge collections of tiny water droplets or ice crystals claimed to dust. And water's clear dust is tiny.
148.160000    155.440000     So why white? Why do we see these bright, white shades most of the time, not like transparent or just ray? What makes them pop like that?
155.440000    160.400000     Yeah, right. That's the next piece. It's all down to something called light scattering. It's pretty neat actually.
160.400000    167.680000     Think about why the sky is blue, right? The tiny air molecules, nitrogen, oxygen, they're smaller than the light waves.
167.680000    172.560000     They scatter the shorter blue wavelength of sunlight more than the red ones, so blue sky.
172.560000    179.440000     But inside a cloud, you've got these water droplets and ice crystals. They're much bigger than air molecules,
179.440000    185.120000     closer in size to the wavelengths of light itself. And because of their size, they don't really play favorites with color.
185.120000    191.840000     They scatter all colors of visible light. Red, green, blue, everything pretty much equally in all directions.
191.840000    195.440000     So they're just bouncing all the colors around everywhere, like tiny disco balls?
195.440000    203.200000     Ah, kind of yeah. A less sparkly version maybe. But when all those scattered colors, the whole rainbow mix together reach our eyes at the same time.
203.200000    208.320000     Our brain just combines them and sees white light. So if a cloud is relatively thin, you know, not two packs,
208.320000    214.720000     sunlight can get in, bounce around a bit, and plenty of that white scattered light makes it out the other side to reach us on the ground.
214.720000    217.920000     That gives you that classic bright, fluffy white cloud look.
217.920000    222.240000     Okay, that makes perfect sense for the nice happy looking clouds, the fair weather ones.
222.240000    228.080000     But like we started with, what happens when they decide to get serious?
228.080000    232.880000     When you're thicken up, get all moody and dark, and you know you need to find cover, what's the switch?
232.880000    235.920000     Why go from brilliant white to that ominous gray or black?
235.920000    242.480000     Right, back to the main event. And the biggest reason really the number one factor is just their thickness and density.
243.040000    249.200000     Just picture it. You've got your thin, wispy, little cumulus cloud, maybe like a cotton ball floating by.
249.200000    255.040000     Now compare that to a monster cumulonimbus storm cloud. The ones that stretch miles up, you know,
255.040000    262.400000     anvils and everything. Brain clouds aren't just bigger overall. They are packed incredibly densely with water droplets and ice crystals.
262.400000    268.560000     Way, way more water packed into the same amount of space. And often the droplets themselves get bigger just before they fall.
268.560000    273.920000     So it's not like the water changes color of the cloud gets angry. It's just stuffed. It's really full.
273.920000    278.240000     Exactly, forget angry clouds. Think heavy clouds, really full clouds.
278.240000    286.000000     So sunlight tries to get through, right? But it hits this incredibly thick, dense maze of water droplets.
286.000000    292.560000     The light photons start bouncing around like crazy, scattering, refracting, that's bending when light goes from air to water.
292.560000    305.040000     Just bouncing and bending over and over and over again. And because there are so many droplets packed so tightly, very, very few of those light photons actually managed to make it all the way through the entire thickness of the cloud to come out the bottom where we are.
305.040000    310.480000     Most of the light just gets scattered around inside the cloud bouncing off the walls essentially. Or it even gets absorbed by the water.
310.480000    314.320000     It's like light trying to run a marathon through a packed crowd. It just can't get through easily.
314.320000    320.640000     Wow. Okay. That candy floss analogy mentioned earlier really clicks now or cotton candy. Yeah.
320.640000    328.320000     A small bit is light and airy. You see right through it almost looks white and fluffy, but squish a massive amount together really tight.
328.320000    334.480000     It suddenly looks way darker, denser, because the light just can't penetrate that thick ball of sugar. Same idea here.
334.480000    341.360000     Perfect analogy. That's exactly the density effect. The thicker and denser the cloud gets, the less light makes it through the bottom.
341.360000    347.680000     And there's another layer to it too, which is fascinating. As those water droplets inside get bigger and heavier,
347.680000    354.720000     right before they fall as rain or snow, they actually get better at absorbing some of the light, not just scattering it.
354.720000    362.000000     Little droplets are great scatterers. Bigger ones start to actually soak up some light energy and that absorption adds to the darkness.
362.000000    368.160000     So yeah, when you see a dark cloud, it really does mean it's packed with water, likely big droplets and rain is probably on the way.
368.160000    371.680000     That makes so much sense now. It's not a color change. It's a light blockage issue.
371.680000    379.680000     It reminds me of that story someone told, being on a plane, looking down, seeing this gorgeous blanket of brilliant white clouds below,
379.680000    385.600000     looked beautiful, and they land, look up from the ground, and those same clouds are suddenly all gray and threatening.
385.600000    389.360000     They joked about a cloud mood swing, but it's just perspective, isn't it?
389.360000    395.920000     That is a perfect, perfect real world illustration. Perspective is absolutely key here.
395.920000    399.600000     Think about it. From the plane, you're looking at the top of the cloud.
399.600000    407.040000     The top surface is getting hit directly by the sun. So it's scattering light like mad, looking bright white, just like a thin cloud.
407.040000    412.080000     But from the ground, you're looking at the bottom of that exact same very thick cloud.
412.080000    417.360000     Very low sunlight has managed to punch all the way through that immense thickness to reach the underside,
417.360000    422.080000     most got scattered or absorbed way up inside. So the base looks dark.
422.080000    431.920000     Gray, sometimes almost black, if it's really thick, it's density, thickness, and where you're standing. And that totally explains why even on big clouds, the bottoms often look darker than the sides, right?
431.920000    437.040000     Because the light that can't get through the bottom might leak out the sides where it's maybe a bit thinner, trying to escape.
437.040000    441.280000     Exactly. Path of least resistance for those light photons, always.
441.280000    445.680000     Now we've covered the main reasons, thickness, density, droplet size, viewing angle, big ones.
445.680000    451.360000     But are there any other little things that might make a cloud look dark, or is it always just about the water load?
451.360000    454.720000     Yeah, good question. Anything else messing with the color we see?
454.720000    463.680000     Well, yes, there are a couple of other things, they're usually less dramatic. Sometimes a cloud looks dark, simply because it's in the shadow of another cloud that's higher up.
463.680000    470.080000     Clouds often hang out in layers, you know, stratus below, cumulus above, maybe serious way up high.
470.080000    474.560000     So one cloud can literally cast a shadow on the one beneath it, simple as that.
474.560000    485.040000     And also our eyes complete tricks on us a bit. Contrast is a factor. A gray cloud will look much, much darker if it's sitting against a really bright blue sunlit sky.
485.040000    495.520000     The contrast makes the gray pop seem darker, but that same gray cloud might not look nearly as dark if the entire sky is already kind of overcast and grayish. It blends in more.
495.520000    501.120000     That makes sense too. That explains those weirdly dim afternoons where it feels like evening, but it's only 2 p.m.
501.120000    505.600000     The sun's still up there, but it's just completely hidden behind layers and layers of thick clouds.
505.600000    515.440000     So you have a whole mood swing idea for clouds is fun, but the real science, this interplay of light, water, density, perspective, it's actually way more fascinating as well.
515.440000    521.680000     It really is, just incredible physics happening constantly right over our heads, stuff we barely notice most days.
521.680000    531.040000     So, okay, next time we see those dark, you know, get your umbrella clouds rolling in, we'll know it's not a celestial tantrum, it's not magic black water.
531.040000    539.120000     It's just light, having a really, really hard time getting through a super packed, super dense cloud full of water.
539.120000    546.000000     It's a difference between light passing through a thin veil versus trying to punch through a, well, a giant water mattress in the sky.
546.000000    554.320000     Exactly, and it's just a great reminder, I think, these everyday things we see like a clouds color, they often have these amazing layers of science just waiting for us to get curious about them.
554.320000    558.640000     It's just this beautiful dance between light and water playing out on a massive scale.
558.640000    565.360000     So, here's something to think about next time, when you see that dark cloud, maybe try and guess just how dark it is.
565.360000    569.280000     What does that specific shade of gray tell you about how much water it's holding up there?
569.280000    577.680000     And then maybe just take a second to picture the other side, up above it all, where the sun is still hitting the top, making it brilliantly white against the blue.
577.680000    583.200000     Same cloud, totally different view, keep looking up everyone, and stay curious.
583.200000    589.760000     And that wraps up today's episode of Everyday Explained. We love making sense of the world around you, five days a week.
589.760000    594.800000     If you enjoyed today's deep dive, consider subscribing so you don't miss out on our next discovery.
594.800000    597.280000     I'm Chris, and I'll catch you in the next one.