Safe Depth for a Lightning Strike
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fire_diver
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Last month my buddy and I were doing a beach dive at Tablerock lake in MO. It had been storming almost the whole week and I missed out on most of my planned dives. I got up early one morning and it was a beautiful, clear sunrise. I checked the weather. Storms up north. I figured I had plenty of time to get this last dive in. So down we go. We were cruising around at about 20-25 feet when I thought I heard thunder. I figured it was just a power boat firing up nearby. Then a few minutes later it looked like someone fired a flash-strobe at me. I stopped, looked at my buddy, and he saw it too. We ended the dive right then, and got into our car as fast as possible. First time for me. If any of you know what the water is like there, that strike must have been damn close to light up the bottom like that.
FD
FD
PerroneFord
Contributor
In open water, you kind of have an advantage. You can hear or see it coming. We exited an overhead into the middle of a lightning storm. Apparently, just the tail end of one because after completing the safety stops, we exited the water quickly and found it had been raining.
It was sunny with clouds when we went in 40 minutes earlier.
It was sunny with clouds when we went in 40 minutes earlier.
eclipse785
Guest
PerroneFord:
Well it must not be too wrong. Your alive to tell us about getting hit three times in 15-20ft of water. Personally I think that speaks volumes about how accurate my statement really is.
Charlie99
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There's a big difference between being 15 or 20' directly below the lighting strike and being in 15-20 ft of water at some distance laterally from the strike point. PerroneFord says that he got hit three times. It's unlikely that all 3 were in the same spot. (Unless he was sitting under a tower -- I did get 14 separate strikes directly above me in one single night, but that was while running a TV transmitter attached to a tall tower in the flatlands of mid-Michigan).eclipse785:Well it must not be too wrong. Your alive to tell us about getting hit three times in 15-20ft of water. Personally I think that speaks volumes about how accurate my statement really is.
I suspect that if any one of the three lightning strikes had been directly above him, PerroneFord wouldn't be posting anymore.
Charlie99:
Since the current will tend to spread in a hemispherical pattern, the current density (the killing component of the lightening) is probably going to dissapate in 1/R^3 fashion. Whereas I would not volunteer to test my hypothesis, I'm inclined to believe Perrone.
PerroneFord
Contributor
All three "hits" were while we were in a freshwater spring doing a safety stop between 15 and 20 feet. I suspect if the storm was right over us, we would have had some real issues. We surfaced less than 5 minutes after feeling the last hit, and the storm appeared to be some distance away.
I only felt 1 of the 3 hits strongly, and generally through the chest cavity. My dive buddy inidcated he felt the hits mostly in his head. Again, it wasn't enough to jolt me, but I think most people know the feeling of putting the 9volt battery on the tongue. That is what my chest felt like. It caused an involuntary tensing of my muscles and I yelled into my regulator.
I didn't break my hover though!
It's not something that I'd volunteer to do again even though I didn't seem to have any lasting effects from it. I wish I could say the same for my dive buddy. He's now seeking cave training, so it must have messed him up badly!
I only felt 1 of the 3 hits strongly, and generally through the chest cavity. My dive buddy inidcated he felt the hits mostly in his head. Again, it wasn't enough to jolt me, but I think most people know the feeling of putting the 9volt battery on the tongue. That is what my chest felt like. It caused an involuntary tensing of my muscles and I yelled into my regulator.
I didn't break my hover though!
It's not something that I'd volunteer to do again even though I didn't seem to have any lasting effects from it. I wish I could say the same for my dive buddy. He's now seeking cave training, so it must have messed him up badly!
PerroneFord:
Sounds like it fried his brain.
Charlie99
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I agree with all of your post, but ....... if you look at Perrone's post just after yours you will find that he doesn't believe that the lightning was hitting right above him.Desert_Diver:
lightning strikes can be 100,000 amps. Fibrillation or stoppage of the heart can be trigger by as little as 0.05amps, and 1A through the body has a very high likelyhood of causing a fatality. This means that you need an attenuation factor on the order of 100,000 to 2 million to be safe. The current flow through your body is driven by the differential potential, which in turn is driving by both the resistance of the water (obviously much less for freshwater) and the distance from the strike. If you say the danger goes down as the inverse of cube of distance from the strike I'll believe you --- I could even easily be convinced that the effect on the body decays as the 4th power of distance.
What's missing is the scale factor on distance. Clearly, a high conductivity material like seawater attenuates/disperses the strike in shorter distance than does a low conductivity (high resistance) material like freshwater. Another factor not yet mentioned is that a lightning strike is a very high frequency event, and things like inductance and skin effect will alter the dispersion pattern.
It is conceivable to me, just barely, that 20' of saltwater would be enough protection from a strike directly above, but there is no way that 20' of freshwater is going to attenuate the current sufficiently.
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