[00:01.00]Listen to part of a lecture in a chemistry class.
[00:05.00]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’re 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 did they get that way? [00:41.30] Well, you start with water vapor. [00:43.80] 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 were 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 attached to each of the six sides. [01:15.50]You begin this development with six arms or branches radiating out from the center plate. [01:20.50]Each time the snowflake encounters water vapor, more molecules attached to it, leading to more and more complex structures. [01:27.50]And of course, each snowflake takes unique route through the clouds on its way down. [01:32.10]And so the quantity of water vapor that it goes through is going to 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 is basically a thin layer of water that’s not completely frozen.[01:55.20] And the existed temperature is well below freezing, though thickness varies at different temperatures. [02:00.60]Now this quasi-liquid layer, it plays an important role on what we are going to talk about next.
[02:05.60]Ah, yes, Mary?
[02:07.00]Mary:How can liquid exist below freezing? [02:09.80]Why doesn’t it freeze?
[02:12.30]Professor:Well, when water becomes ice, the molecules bond together and it gets sort of…locked in the 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 are all locked together. [02:26.60]But what about the exterior of the ice? [02:29.50]There is a layer of water molecules on the surface, they attached molecules only on one side. [02:34.70]So, they are 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]The bricks in the wall, they have other bricks above and below them, and they are all locked against each other. [02:48.00]But that top layer, it only has a layer below it. [02:52.00]Now this can only be taken so far because of course bricks don’t move at all. [02:57.40]They are not liquid. [02:58.60]But the bricks of 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 is 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, at high concentrations it can be a pollutant. [03:42.30] Now snowflake’s 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.
vt. 欣赏；感激；领会；鉴别 （appreciate的第三人称单数）
They don't have any confidence that houses will appreciate in value.(TPO49-20)