考满分 toefl

1 .Listen to part of a lecture in an Earth science class.

听一节地球科学课上的部分内容。

1 .Lightning is one of the most common meteorological events on Earth. It occurs somewhere on the planet roughly 40 times a second. Although the most common type of lightning occurs between clouds, the type of lightning we are most familiar with is so-called cloud-to-ground lightning, the kind we see during thunderstorms. And that's what I want to focus on today.

闪电是地球上最常见的气象事件之一。地球上大约每秒会发生 40 次闪电。虽然最常见的闪电类型是云间闪电，但我们最为熟悉的闪电类型是所谓的云地闪电，也就是我们在雷暴期间看到的那种。这就是我今天想要重点讲解的内容。

2 .Despite its frequency, lightning is difficult to study, since the process only takes around half a second from beginning to end. But here's the generally accepted scenario of lightning formation.

尽管闪电发生的频率很高，但它却很难研究，因为从开始到结束这个过程只需要大约半秒钟的时间。不过，以下是关于闪电形成的普遍被接受的情况。

3 .Now, a storm cloud is a very turbulent environment, with powerful updrafts and downdrafts occurring continuously. And these air movements carry drops of water and ice particles that are constantly colliding. This causes negatively charged electrons to be released from the rising water particles and to collect on the solid falling particles.

现在，一片雷暴云是一个非常动荡的环境，其中持续发生着强大的上升气流和下降气流。这些空气运动携带着水滴和冰粒，它们不断地相互碰撞。这使得带负电的电子从上升的水粒子中释放出来，并聚集在下落的固体粒子上。

4 .So we end up with a positive charge at the top of the cloud and a negative charge at the bottom. At the same time, this negative charge at the bottom of the cloud creates a corresponding shadow of positive charge on the surface of the Earth below it.

所以最终云的顶部带正电，底部带负电。与此同时，云底部的负电荷在其下方的地球表面产生了相应的正电荷 “影子”。

5 .Now, the atmosphere between the cloud and the Earth's surface does not conduct electricity very well. It's a very good insulator. This means that it normally prevents a connection and electrical circuit from forming between the two.

现在，云层和地球表面之间的大气导电性能并不好。它是一种非常好的绝缘体。这意味着它通常会阻止云层和地面之间形成连接和电路。

6 .But at some point, the cloud's charge becomes too great for the air to restrain it. So much energy builds up within the cloud itself that the negative charge at the bottom eventually becomes much greater than the ground's positive charge. The electrical imbalance between the two reaches a tipping point, and the resulting lightning flash is an attempt to equalize them.

但在某个时刻，云层的电荷变得太大，空气无法再抑制它。云层内部积累了如此多的能量，以至于云层底部的负电荷最终远大于地面的正电荷。两者之间的电荷不平衡达到了一个临界点，由此产生的闪电就是试图使它们达到平衡的一种表现。

7 .So in cloud-to-ground lightning, a pathway forms through the atmosphere. This pathway is called a leader, because it leads a trail of negative charges from the cloud to the ground below. And the leader progresses downward in short steps, maybe 50 meters at a time.

所以在云地闪电中，会在大气中形成一条通道。这条通道被称为先导，因为它引领着一股负电荷从云层流向下方的地面。而且先导会一小步一小步地向下延伸，每次可能延伸 50 米。

8 .Now, each step doesn't go in exactly the same direction. It's not predictable, but the steps are generally downward, attracted by the positive charge on the Earth's surface, directly below the cloud. This is what gives lightning its characteristic, jagged, forked appearance.

现在，每一步的方向并不完全相同。这是不可预测的，但这些步骤总体上是向下的，被云层正下方地球表面的正电荷所吸引。这就是闪电呈现出其独特的锯齿状、叉状外观的原因。

9 .Now, once the leader reaches the ground, the circuit between the negative charge in the cloud and the positive charge on the ground is complete. But interestingly enough, we don't actually see this leader as it forges a pathway down through the atmosphere. The lightning bolt that we do see is a powerful bolt of positive electricity called the return stroke.

现在，一旦先导到达地面，云层中的负电荷和地面上的正电荷之间的电路就完成了。但有趣的是，当先导在大气中开辟出一条向下的通道时，我们实际上看不到它。我们看到的闪电是一股强大的正电，被称为回击。

10 .Now, this return stroke leaps back up the pathway from the ground. It distributes a positive charge to the cloud, balancing the electrical field. The energy in this return stroke can be intense. It's hotter than the surface of the sun. Frequently, after the return stroke, there will be another leader that will recharge the same pathway, which leads to another return stroke. And this fast succession of leaders and return strokes is what causes lightning to flicker. And all this happens, as I said, in about half a second.

现在，这种回击会沿着通道从地面向上跳跃。它向云层分布正电荷，平衡电场。这种回击中的能量可能非常强烈。它比太阳表面还要热。通常，在回击之后，会有另一个先导为同一条通道重新充电，这就会导致另一次回击。而先导和回击的这种快速交替就是闪电闪烁的原因。正如我所说，这一切都发生在大约半秒钟内。

11 .Now, I hope you noticed I said that the lightning we do see is going from the ground to the cloud. I know when we see lightning, it seems to be going the other way, from the cloud to the ground, and it also seems to last longer than one second. But both of those observations are untrue. Remember that human eyes are really not the best scientific instruments, especially when it comes to things that only last a fraction of a second.

现在，我希望你们注意到我说的我们看到的闪电是从地面射向云层的。我知道当我们看到闪电时，它似乎是反方向的，从云层射向地面，而且它似乎持续的时间也超过了一秒钟。但这两种观察都是不正确的。要记住，人眼并不是最好的科学仪器，尤其是在观察只持续零点几秒的事物时。

12 .Now, because the process of electric buildup and discharging clouds is so quick, it's virtually impossible to detect to find evidence of it, which is why I said earlier that what I'm describing is the generally accepted explanation.

现在，因为云层的电荷积累和放电过程非常迅速，几乎不可能检测到相关证据，这就是为什么我之前说我所描述的是普遍被接受的解释。