pictures on the Internet. I like it when things catch fire and explode, which means I do not have your best interests in mind. Th e authorities on lightning safety are the folks at the US National Weather Service:
http://www.lightningsafety.noaa.gov/
Okay. With that out of the way . . .
To answer the questions that follow, we need to get an idea of where lightning is likely to go. Th ere’s a cool trick for this, and I’ll give it away right here at the start: Roll an imaginary 60-meter sphere across the landscape and look at where it touches. 1 In this section, I answer a few different questions about lightning.
Th ey say lightningstrikes the tallest thing around. Th at’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? Th e 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 shouldstick to answering questions rather than asking them.
So how does lightning pick its targets?
Th e 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.
Th e leader carries comparatively little current — onthe order of 200 amps. Th at’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. Th is 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 . 3
Th e place on the ground where we see a bolt “strike” is the spot where the leader first made contact with the surface. Th e 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 connectedto the ground within that distance, the bolt will jump to it. Otherwise, it jumps out in a semi-random direction and repeats the process.
Th is is where the 60-meter sphere comes in. It’s a way to imagine what spots might be the first thing the leader senses — the places it might jump to in its next (final) step.
To figure out where lightning is likely to hit, you roll the imaginary 60-meter sphere across the landscape. 4 Th is 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.
Th is means you can calculate a lightning “shadow” aroundan object of height h on a flat surface.
Th e shadow is the area where the leader is likely to hit the tall object instead of the ground around it:
Now, that doesn’t mean you’re safe within the shadow — often, it means the opposite. After the current hits the tall object, it flows out into the ground. If you’re touching the ground nearby, it can travel through your body. Of the 28 people killed by lightning in the US in 2012, 13 were standing under or near trees.
With all this in mind, let’s look at possible lightning pathsfor the scenarios in the following questions.
Q. How dangerous is it, really, to be in a pool during a thunderstorm?
A. Pretty dangerous. Water is conductive, but that’s not the biggest problem—the biggest problem is that if you’re swimming, your head is poking up from a large flat surface. But lightning striking the water near you would still be bad. Th e 20,000 amps spread outward—mostlyover the surface—but how much of a jolt it will give
http://www.lightningsafety.noaa.gov/
Okay. With that out of the way . . .
To answer the questions that follow, we need to get an idea of where lightning is likely to go. Th ere’s a cool trick for this, and I’ll give it away right here at the start: Roll an imaginary 60-meter sphere across the landscape and look at where it touches. 1 In this section, I answer a few different questions about lightning.
Th ey say lightningstrikes the tallest thing around. Th at’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? Th e 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 shouldstick to answering questions rather than asking them.
So how does lightning pick its targets?
Th e 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.
Th e leader carries comparatively little current — onthe order of 200 amps. Th at’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. Th is 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 . 3
Th e place on the ground where we see a bolt “strike” is the spot where the leader first made contact with the surface. Th e 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 connectedto the ground within that distance, the bolt will jump to it. Otherwise, it jumps out in a semi-random direction and repeats the process.
Th is is where the 60-meter sphere comes in. It’s a way to imagine what spots might be the first thing the leader senses — the places it might jump to in its next (final) step.
To figure out where lightning is likely to hit, you roll the imaginary 60-meter sphere across the landscape. 4 Th is 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.
Th is means you can calculate a lightning “shadow” aroundan object of height h on a flat surface.
Th e shadow is the area where the leader is likely to hit the tall object instead of the ground around it:
Now, that doesn’t mean you’re safe within the shadow — often, it means the opposite. After the current hits the tall object, it flows out into the ground. If you’re touching the ground nearby, it can travel through your body. Of the 28 people killed by lightning in the US in 2012, 13 were standing under or near trees.
With all this in mind, let’s look at possible lightning pathsfor the scenarios in the following questions.
Q. How dangerous is it, really, to be in a pool during a thunderstorm?
A. Pretty dangerous. Water is conductive, but that’s not the biggest problem—the biggest problem is that if you’re swimming, your head is poking up from a large flat surface. But lightning striking the water near you would still be bad. Th e 20,000 amps spread outward—mostlyover the surface—but how much of a jolt it will give
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