《那些古怪又让人忧心的问题》第23期:闪电(1)

LIGHTNING

闪电

They say lightning strikes the tallest thing around. That's the kind of maddeningly inexact statement that immediately sparks all kinds of questions. How far is “around”? I mean, not all lightning hits Mount Everest. But does it find the tallest person in a crowd? The tallest person I know is probably Ryan North.2 Should I try to hang around him for lightning safety reasons? What about other reasons? Maybe I should stick to answering questions rather than asking how does lightning pick its targets?

人们总说闪电会击中附近最高的东西。这种极其含糊的说法简直就是个招人质疑的活靶子，“附近”是多远？我说，好像不是所有的闪电都会击中珠穆朗玛峰吧。但闪电会击中人群中长得最高的人吗？我所知道的最高的人应该是瑞恩•诺斯3了。那出于安全考虑，我是不是只要站在他旁边就不会被闪电击中了呢？如果是其他原因呢？也许我应该回答问题，而不是提出问题。那么闪电是如何选择目标的呢？

The strike starts with a branching bundle of charge-the “leader”-descending from the cloud. It spreads downward at speeds of tens to hundreds of kilometers per second, covering the few kilometers to the ground in a few dozen milliseconds.

闪电最先会形成一个叉状电荷，又被称为先导。先导会从云端伸出向下延伸，以每秒几十到几百千米的速度向地面传播，在几十毫秒的时间内穿过几千米长的空气到达地面。

The leader carries comparatively little current-on the order of 200 amps. That's still enough to kill you, but it's nothing compared to what happens next. Once the leader makes contact with the ground, the cloud and the ground equalize with a massive discharge of more like 20,000 amps. This is the blinding flash you see. It races back up the channel at a significant fraction of the speed of light, covering the distance in under a millisecond .

先导中的电流相对较小，大约在200安培这个数量级，这点电流也足够电死你了，但与之后发生的相比就有点相形见绌了。一旦先导接触到地面，云层和地面之间就会发生剧烈的电荷中和过程，放电电流将会超过2万安培。在这个过程中你就会看到一道亮眼的闪光，接着先导会以光速的几分之一速度原路返回，在不到1毫秒的时间内就能回到云端。

The place on the ground where we see a bolt “strike” is the spot where the leader first made contact with the surface. The leader moves down through the air in little jumps. It's ultimately making its way toward the (usually) positive charge in the ground. However, it “feels” charges within only a few tens of meters of its tip when it's deciding where to jump next. If there's something connected to the ground within that distance, the bolt will jump to it. Otherwise, it jumps out in a semi-random direction and repeats the process.

我们看到的闪电“击中”地面的地方，就是之前先导最先接触到地面的地方。先导在空气中呈跳跃状向下传播，它最终会向着（通常是）地面上的正电荷前进。不过，它在决定下一步跳向哪里时只能“感知”尖端附近几十米范围之内的电荷。如果在这个范围内地面上连着什么东西，那么闪电就会跳到这个东西上，否则就会半随机地挑选一个方向，然后重复这一过程。

To figure out where lightning is likely to hit, you roll the imaginary 60-meter sphere across the landscape.4 This sphere climbs up over trees and buildings without passing through anything (or rolling it up). Places the surface makes contact-treetops, fence posts, and golfers in fields-are potential lightning targets.

为了找出闪电会击中哪里，你在地面上滚动着这个假想的半径60米的球。5这个大球会爬到树上或建筑上面而不会穿过它们（也不会把它们卷进去）。与大球表面接触的地方——树顶、栅栏以及正在打高尔夫的人——都是潜在的被雷击的目标。