Narrator: Listen to part of a lecture in a chemistry class.

旁白：请听一段化学讲座的节选片段。

Male professor: OK, so today we're going to talk about the Arctic, ozone depletion, and… snowflakes.

男教授：好的，所以呢，今天我们要讲讲北极圈，臭氧损耗和……雪花。

And it's all related! Let's start with snowflakes.

这三者都是有关联的！我们从雪花开始。

Now, I find snowflakes fascinating. To even begin to understand them, you need to understand physics, chemistry, and mathematics.

现在，我发现雪花很迷人。甚至在我们开始研究它们之前，我们就需要懂得物理、化学和数学。

Even though there's been a lot of research, there's still actually a lot about snowflakes that we don't understand yet. Hard to believe, I know…

即使已经有了大量的研究，但我们对于雪花还是有很多不了解的地方。我知道这可能很难相信……

Anyway, snowflakes have a particular form: there's a six-sided center, with six branches or arms that radiate out from it.

无论如何，雪花有某种特定的形态：有一个六边的中心，有六个分支或臂从中心向外辐射。

But how did they get that way?

但是它们为什么会以这个形状出现呢？

Well, you start with water vapor. You need a pretty humid atmosphere. And that water vapor condenses directly into ice, into an ice crystal.

嗯，我们要从水蒸气开始说起，我们需要一个相当潮湿的环境，水蒸气会直接凝结成冰，凝结成冰晶。

At this point it looks kind of like a thin dinner plate that rather than being circular, is hexagonal, with six flat edges.

这个时候它看起来就像是一个带六个平边的薄薄的六边形餐盘，而不是圆形的。

It's at this point in the process where we begin to see why each snowflake is unique.

这个时候我们就需要看看为什么每一片雪花都是独一无二的了。

Imagine this dinner plate is floating around in the wind, right? And when it encounters water vapor, molecules from that vapor attach to each of the six sides.

想象一下这个餐盘是漂浮在空中的，对吗？当它遇到水蒸气的时候，水蒸气分子就会附着在六个边上。

You begin to see the development of six arms or branches radiating out from the center plate.

你可以看到六个分支从冰晶主体上辐射向外发展。

Each time the snowflake encounters water vapor, more molecules attach to it, leading to more and more complex structures.

每一次当雪花遇到水蒸气时，更多的水蒸气分子会附着其上，这就形成了越来越复杂的结构。

And, of course, each snowflake takes a unique route through the clouds on its way down…And so the quantity of water vapor it goes through is gonna be unique for each one.

当然，每一片雪花在飘落时候的飞行路线都是独一无二的……所以附着在每一片雪花上的水蒸气的量也是各不相同的。

Now, one important characteristic of snowflakes is that they have something called a quasi-liquid layer, the QLL.

雪花的重要特征之一是它们拥有一种叫做类液态薄层，缩写为QLL。

Our snowflake is an ice crystal, right? Well, we find a quasi-liquid layer on the surface of ice.

雪花是一种冰晶，对吗？我们在冰的表面发现了一层类似液体的层。

It's basically a thin layer of water that's not completely frozen. And it exists at temperatures well below freezing, though the thickness varies at different temperatures.

它基本上是一层薄薄还没有结冰的水。它存在于远低于冰点的温度下，尽管在不同的温度下厚度不同。

Now this quasi-liquid layer, it plays an important role in what we're going to talk about next…uh… yes, Mary?

现在这层类液态薄层，在我们接下来的讲解中将占据重要地位……啊……有什么问题吗，Mary？

Female student: How can liquid exist below freezing? Why doesn't it freeze?

女学生：为什么会有液体在0度以下保持液态呢？为什么它不结冰呢？

Male professor: Well… when water becomes ice, the molecules bond together and they get sort of, uh, locked into place.

男教授：嗯……当水结成冰的时候，水分子会连接到一起，然后就被固定住了。

They can't move around as much anymore. So each molecule is surrounded by other molecules, and they're all locked together.

他们不能再到处移动了。所以每个分子都是被其他水分子包围着的，它们都被固定在一起了。

But what about the exterior of the ice?

但是冰的外层呢？

There's a layer of water molecules on the surface, they're attached molecules only on one side, so they're a bit freer… they can move around a bit more.

在冰的表面上有一层水分子，它们只和其他分子保持一面的接触，所以，它们就会稍微更自由一点...它们可以稍微动一动。

Think of a... think of a… brick wall. Uh, the bricks in the wall, they have other bricks above and below them, and they're all locked against each other.

你们可以想象一下……想象一下……一堵砖墙。墙里的每一块砖块上下都有其他的砖块，它们就被彼此锁住了。

But that top layer, it only has a layer below it. Now, this can only be taken so far… because of course, bricks don't move at all. They're not liquid.

但是最上面的一层砖，它们就只和下面的一层砖相邻。但是这个例子就只能类比到这种程度了……因为砖块本身并不能移动。它们不是液体。

But the bricks were water molecules, well, this top layer would be the quasi-liquid layer, and it wouldn't be completely frozen. Does that make sense?

但是水分子集结成的"墙"，嗯，它的上层就是类液态薄层，这一层不会是全部冻住的。我说明白了吗？

So finally, we get to the connection between snowflakes and ozone. Ozone is a gas found in the atmosphere of Earth.

所以，最后我们就得到了雪花和臭氧的联系。臭氧是一种在地球大气层里发现的气体。

Now, there's the ozone found in the stratosphere, which is the layer of the atmosphere from 6 to 30 miles above the Earth.

在距地面6到30公里高的平流层存在臭氧。

This is considered "good" ozone, which occurs naturally and helps block harmful radiation from the Sun.

这一些臭氧被认为是好的臭氧，是自然形成的，可以帮助我们抵挡太阳的有害辐射。

But there is also ground-level ozone. It's exactly the same gas, but it's found closer to the surface of the Earth.

但是也有地面臭氧。它和前者是同一种气体，但是它被发现的位置是更加靠近地球表面的。

This ground-level ozone results from human activities, and at high concentrations it can be a pollutant.

这种地面臭氧是由人类活动产生的，如果其浓度过高的话，它可以变成一种污染物。

Now, snowflakes' quasi-liquid layer plays an important role in some complex chemical reactions. We're going to be looking at these in detail later today.

雪花的类液态薄层在一些复杂的化学反应中起到重要的作用。我们晚些的时候会仔细地讨论这一点。

But basically, these reactions cause certain chemicals to be released, and these chemicals reduce the amount of ground-level ozone.

但是基本上，这一些反应会导致某些化学物质被释放出来，这些化学物质会减少地面臭氧的含量。

So… the more branches you have in an ice crystal, the more quasi-liquid layer there is.

所以……你在一片冰晶里看到的分支越多，其上面的类液态薄层就越多。

The more quasi-liquid layer, the more reactions and the more chemicals that reduce ground level ozone.

类液态薄层越多，会导致地面臭氧减少的化学反应和化学物质就越多。

So you can see why this is such an important system to study and understand.

所以你们就可以明白，为什么我们需要学习并理解如此重要的一个系统。