Thunder and Lighting

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boulderjohn

Technical Instructor
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I am sure we have all had vast afternoon thunderstorms interrupt dive days in the Rockies. Does anyone with a better knowledge of physics than I fully understand the actual threat to divers a) under the water or b) swimming on the surface?
 
great question, I've always wondered that myself. My understanding is that the further away from the strike the better, but the by being on the surface, you may the highest object in the water (especially with a metal tank on your back).
 
I'll explain my take, and I ask that anyone who disagrees is free to rebut anything I say.

If lightning strikes the water, and you are underwater, the risk to you (depending on proximity) should be relatively minor because the electrical current should travel through the 'shortest path.' Where the shortest path is defined as the distance of least total resistance. So if the lightning can travel through you, or through water, and you have greater electrical resistance than does the water, the electrical current will not pass through you.
 
I had a decent detailed answer to this on another board, but can't find it to paste in. I spent several years designing lightning grouding/bonding systems on NOAA sites.

The short answer is that in fresh water the lake bottom is the ground plane, not the water. To much less a degree this applies to seawater as well. If you are in the path in fresh water you get to be the conductor. Several have experienced this in Fla caves close to 100' underground.

Above ground get as small a footprint as you can while maintaining a low profile. Squatting on your heels with feet together is about as good as you can get if a strike happens close to you. Lying down, or even takng a step, puts contact points at different potentials due to the resistance of the ground. That little potential difference is enough to make you dead.

In seawater you are not the path as the water is a much better conductor. One documented case involved a strike directly hitting a scuba tank and blowing a pinhole in it without knocking out the diver. That said being over (or under) a steel or other conductive object in a thunderstorm in seawater would not be a good thing as those can attract a much larger strike!

FT
 
Similar thread here with more info.
 
https://www.shearwater.com/products/swift/

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