OFFICIAL52 What factor helps explain why no two snowflakes are alike?

[00:00.00]NARRATOR: Listen to part of a lecture in a chemistry class.[00:05.00]MALE PROFESSOR: OK, so today we're going to talk about the Arctic, ozone depletion, and… snowflakes.  [00:11.00]And it’s all related! [00:13.00]Let’s start with snowflakes.[00:15.00]Now, I find snowflakes fascinating. [00:17.30]To even begin to understand them, you need to understand physics, chemistry, and mathematics. [00:23.40]Even though there's been a lot of research, there's still actually a lot about snowflakes that we don't understand yet—[00:29.40]hard to believe, I know…
[00:31.10]Anyway, snowflakes have a particular form: there’s a six-sided center, with six branches or arms that radiate out from it. [00:39.30] But how do they get that way? [00:41.30] Well, you start with water vapor—you need a pretty humid atmosphere—[00:46.00]and that water vapor condenses directly into ice, into an ice crystal. [00:51.30] At this point it looks kind of like a thin dinner plate that, rather than being circular, is hexagonal, with six flat edges.[01:00.00]It's at this point in the process where we begin to see why each snowflake is unique. [01:05.50] Imagine this dinner plate is floating around in the wind, right, [01:09.20] and when it encounters water vapor, molecules from that vapor attach to each of the six sides. [01:15.50]You begin to see the development of six arms or branches radiating out from the center plate. [01:20.50]Each time the snowflake encounters water vapor, more molecules attach to it, leading to more and more complex structures—[01:27.50]and, of course, each snowflake takes a unique route through the clouds, on its way down…[01:32.10]and so the quantity of water vapor it goes through is gonna be unique for each one.
[01:36.60]Now, one important characteristic of snowflakes is that they have something called a quasi-liquid layer—the QLL.[01:45.00]Our snowflake is an ice crystal, right? [01:48.10]Well, we find a quasi-liquid layer on the surface of ice. It's basically a thin layer of water that's not completely frozen—[01:55.20]and it exists at temperatures well below freezing, though the thickness varies at different temperatures. [02:00.60]Now this quasi-liquid layer, it plays an important role in what we're going to talk about next…—
[02:05.60]uh, … yes, Mary?[02:07.00]FEMALE STUDENT: How can liquid exist below freezing? Why doesn't it freeze?[02:12.30]MALE PROFESSOR: Well… when water becomes ice, the molecules bond together and they get sort of, uh, locked into place. [02:19.30] They can't move around as much anymore. [02:21.60] So each molecule is surrounded by other molecules, and they're all locked together. [02:26.60]But what about the exterior of the ice? [02:29.50]There's a layer of water molecules on the surface… they're attached to molecules only on one side, [02:34.70]so they're a bit freer…[02:36.30]they can move around a bit more. [02:38.30]Think of a, think of a… brick wall. [02:42.00] Uh, the bricks in the wall, they have other bricks above and below them, and they're all locked against each other. [02:48.00] But that top layer, it only has a layer below it. [02:52.00]Now…[not impressed with the brick illustration] this can only be taken so far… because of course, bricks don't move at all—[02:57.40]they’re not liquid. [02:58.60] But if the bricks were water molecules, well, this top layer would be the quasi-liquid layer, [03:04.00]and it wouldn't be completely frozen. [03:06.00]Does that make sense?[03:07.60]So finally, we get to the connection between snowflakes and ozone. [03:12.30]Ozone is a gas found in the atmosphere of Earth. [03:15.50] Now, there's the ozone found in the stratosphere, which is the layer of the atmosphere from 6 to 30 miles above the Earth. [03:22.30]This is considered “good” ozone, which occurs naturally and helps block harmful radiation from the Sun.
[03:28.30]But there is also ground-level ozone. [03:31.20] It's exactly the same gas, but it's found closer to the surface of the Earth. [03:35.30] This ground-level ozone results from human activities, and at high concentrations it can be a pollutant.[03:42.30] Now, snowflakes' quasi-liquid layer plays an important role in some complex chemical reactions—[03:47.60] we're going to be looking at these in detail later today. [03:50.60] But basically, these reactions cause certain chemicals to be released, [03:54.74]and these chemicals reduce the amount of ground-level ozone. [03:58.20] So… the more branches you have in an ice crystal, the more quasi-liquid layer there is. [04:03.60]The more quasi-liquid layer, the more reactions… and the more chemicals that reduce ground level ozone. [04:09.20]So you can see why this is such an important system to study and understand.