《那些古怪又讓人憂心的問題》第23期:閃電(1)
LIGHTNING
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?
人們總說閃電會擊中附近最高的東西。這種極其含糊的說法簡直就是個招人質疑的活靶子，“附近”是多遠？我說，好像不是所有的閃電都會擊中珠穆朗瑪峯吧。但閃電會擊中人羣中長得最高的人嗎？我所知道的最高的人應該是瑞恩•諾斯3了。那出於安全考慮，我是不是隻要站在他旁邊就不會被閃電擊中了呢？如果是其他原因呢？也許我應該回答問題，而不是提出問題。那麼閃電是如何選擇目標的呢？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.
閃電最先會形成一個叉狀電荷，又被稱爲先導。先導會從雲端伸出向下延伸，以每秒幾十到幾百千米的速度向地面傳播，在幾十毫秒的時間內穿過幾千米長的空氣到達地面。
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 .
先導中的電流相對較小，大約在200安培這個數量級，這點電流也足夠電死你了，但與之後發生的相比就有點相形見絀了。一旦先導接觸到地面，雲層和地面之間就會發生劇烈的電荷中和過程，放電電流將會超過2萬安培。在這個過程中你就會看到一道亮眼的閃光，接着先導會以光速的幾分之一速度原路返回，在不到1毫秒的時間內就能回到雲端。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.
我們看到的閃電“擊中”地面的地方，就是之前先導最先接觸到地面的地方。先導在空氣中呈跳躍狀向下傳播，它最終會向着（通常是）地面上的正電荷前進。不過，它在決定下一步跳向哪裏時只能“感知”尖端附近幾十米範圍之內的電荷。如果在這個範圍內地面上連着什麼東西，那麼閃電就會跳到這個東西上，否則就會半隨機地挑選一個方向，然後重複這一過程。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.
爲了找出閃電會擊中哪裏，你在地面上滾動着這個假想的半徑60米的球。5這個大球會爬到樹上或建築上面而不會穿過它們（也不會把它們捲進去）。與大球表面接觸的地方——樹頂、柵欄以及正在打高爾夫的人——都是潛在的被雷擊的目標。
