If a powerful lightning bolt strikes the ocean, how far can the charge travel underwater or across the water and still be deadly or dangerous to someone floating in the water?
Decently far…my WAG is 20+/- a few feet. But I am not sure. Living on the ocean, I’ve seen lightning stike the ocean dozen’s of times, and it creates quite a bit of energy. I really wish I could photograph it…<-thats how often I have seen it.
…the Lightning Bolt cast in water covers the area of a Fireball (20’ radius).
If the swimmer can interrupt the spell, there is no danger :o
NOAA suggests everyone follow the 30-30 Rule:
Source: http://www.nssl.noaa.gov/edu/ltg/
Assuming water is a better conductor than the ground and/or air, and you can cover a greater distance running on the ground compared to swimming, if you find yourself in the water and the time is less than 30 seconds between flash-bang, you are in serious trouble.
Absolute nonsense. A 30 second flash-bang interval implies about 6 miles between you and the last lightning stroke. Are you going to get rained on? Probably. If a later bolt lands within a few feet (10? 30? 100? I don’t know) of you will you get killed? Yes. But what are the odds you’ll be safe? Damn good. Would I go swimming into the teeth of an approaching storm? No. Would I consider myself doomed if I was swimming / rowing in the middle of a lake and couldn’t get ashore before the heavy rain hit? Hardly.
NOAA is in the business of warning people. If we all followed their advice 100%, entire cities would screech to a halt as everyone cowered in their basement for an hour every time anyone heard thunder. In May and June here in St. Louis and most of the Midwest, we’d all be spending 2 or 3 hours most days in the basement. Imagine all the freeways empty, every outdoor worker runnning for shelter, every indoor worker abandoning their machines or desks, terrified schoolkids shrieking for Mommy, no work going on anywhere. Hour after hour, day in and day out.
That silly scenario is exactly what NOAA’s preaching. If we actually followed their advice, they’d be doing more damage to the US economy than Al Qaeda did. And doing it every day several months out of the year.
Naturally, we don’t follow really that advice 100%. And guess what? Ligthtning does kill a few people nationwide every year.
The odds on a lightning strike getting you are something like 0.000000001. Following the 30-30 rule will reduce that to 0.00000000000001. I feel sooo much safer. YMMV.
Hmm. I had always assumed (with no evidence), that lightning striking a large body of water nearby to you submerged in it would cause no damage; with the water acting as a Faraday cage around you.
The warning against boating/swimming in lightning would therefore be because in either case (assuming you come up for air when swimming), you’ve become the highest point around on a wide, relatively flat surface, hence dramatically increasing the liklihood of hitting YOU DIRECTLY on it’s way to the water.
An academic point, certainly, since the REASON that you’re dead might matter relatively little to the newly departed, but will this theory, as they say, hold water?
“Few” might be understating it; it’s 50-100 killed most years. Lightning is the number one weather-related killer in the United States- more than hurricanes, tornados, and exposure (golfers, apparently get the majority of the "hits’), and only about 1/4 of the people hit die, so we’re actually talking about as many as 200-400 people hit a year. Random cite: NASA Earth Observatory - Newsroom There are a few people who have been hit multiple times, even a few whose job doesn’t involve the words “human lightning rod.”
I’ll be the first to admit that out of a population of 300 or so million that this isn’t an epidemic (and I suspect that the “weather-related killer” stat doesn’t include auto accidents caused by the weather), but the claim that it’s unreasonable to fear nearby lightning–especially when swimming, boating, golfing, or otherwise presenting an obvious local maximum height–seems to understate the danger somewhat.
Duckster, I’m surprised to hear about the 30 second rule. Down here, thunderstorms come upon us so fast that I cannot recall ever seeing a 30 second differential. By the time we see lightning, the thunder is an almost immediate jolting sub-woof rip in space-time that opens with a distinct, sharp crack. Under the guidelines you’ve cited, we should never swim in the summer.
Well, LSLGuy, you are most welcome to take the advice and do with it what you want. That’s all it is. While attempting to find the cite provided, I found another one stating lighting can also strike ten miles away from the observed storm.
So while you may scoff the the NOAA advice, you could very well be strucd dead by a lightning bolt out of a clear blue sky from a storm ten miles away.
And yes, I am from the Midwest and grew up with thunderstorms.
Hey, it’s what I found! Take it up with NOAA. 
TimeWinder, I think it’s important to point out that your cite is for the United States, where hurricane tracking has become a fairly decent science. Hurricanes spawn tornadoes and include severe lightning along with the winds and storm surges. Lives are saved because people take shelter or evacuate. The Galveston hurricane alone, which occured before modern tracking, killed 6,000 people. And it’s even worse for the third world. A cyclone in 1970 in Bangladesh killed 600,000.
Not to speak for astro, but I think y’all are drifting away from the question asked in the OP and I’m curious myself as to the answer.
If you are in the ocean, how close to you would a lightning bolt have to strike the water to be dangerous or deadly?
Agreed… where are all of our science and math experts to break it down for us with numbers and formulae? 
According to this site its unknown but people who were injured or killed by lightning strikes in the water were within a few tens of yards of where the lightning struck.
Stupid question, but how do you do that? When I’ve seen storms, the lightning strokes have generally been closer together than 30s, so to count you’d have to memorize the sequence time gaps of strokes, and wait for thunder with similar gaps. What am I missing?
If lightning can kill a human in water from great distances, how come all the fish don’t die when lakes or ponds are struck?
Isn’t it obvious? They follow the NOAA’s 30-30 rule.
Well, if we assume that the lightning spreads out uniformly through the water in all directions (not quite true), then the current that flows through you will be proportional to your cross sectional area vs. that of a semi-sphere centered on where the lightning bolt hit the water, with a radius equal to your distance from the lightning bolt. Lets make lots of assumptions here. First, the cross sectional area of you that matters is the part that goes across your heart, or about 1 square foot to make the math easy. Next, let’s assume your typical Joe Average lightning bolt has about half a million amps.
There’s a couple of points we are interested in. First, I would guess about 1000 amps through you would be enough to put you in a serious hurting, even if it was for the short duration of a lightning bolt. I’ll use 1000 amps as the “almost guaranteed death” point.
The second point we are interested in is the 5 mA point, because 5 mA is the figure commonly used to indicate how much current is “safe” to pass through the human body (i.e. this is the spec for isolated power in hospital electrical systems).
The surface area of a sphere is 4 (pi) r (squared) (one of these days I’m going to have to look up how you do superscripts). We’ve only got half a sphere, since the current is only going through the water (if you’re having trouble with this, imagine a sphere centered where the lightning bolt hits the water, now remove the top half of the sphere that is sticking up out of the water). So, the current through any given square foot is going to be half a million divided by 2(pi)r(squared).
For 1000 amps, the radius r works out to be (… punches numbers on calculator… ) roughly 8.9 feet.
For 0.005 amps, r works out to be… 3990 feet,
For 15 amps, which is the biggest shock you’d get from a typical home outlet before the breaker goes, r works out to be about 72 feet. So at about this distance (which is a fair distance from ye ol lightning bolt) you’d feel the equivalent of sticking your entire body directly into the light bulb socket, then having the breaker blow. Might kill you, might not, but it would definately be unpleasant.
So, under 10 feet away, you’re guaranteed to be toast. Farther out, the probability of death decreases. You’re only safe when you’re about 3/4 of a mile away from the bolt.
It’s late, I’ve been hanging drywall all day, and I’m beat. Someone might want to check my math.
FWIW, Wet n’ Wild evacuates all of its pools (and slides, obviously) when lightning is observed within 6 miles. Whether this is because you’d get fried if lightning struck a pool or simply because people in the water means people out in the open, I can’t say.
On a practical level, the answer to the OP doesn’t matter, because if you’re in the middle of a lake, then you’re the highest thing around, and lightning will probably strike you directly, rather than heading ten yards away to a featureless spot of water.
That’s why it’s dangerous to be swimming or boating in a thunderstorm, not because of the conductivity of water.
CompGeek – you have to remember that the human body has a much lower conductivity than water, so much less current will be going through your body than through the rest of the half-sphere of water.