lightning and fish

Lori Meyer

How far does lightning travel on a body of water? Why don't a bunch of fish die when lightning strikes a lake? Would you be safe if you were scuba diving at a certain depth? These might be dumb questions, but I was listening to the extreme instability radio show last night and began to ponder these questions.

I called and asked Barometer Bob and he got a kick out of theses questions. He did say that on his radio show tonight he would try to come up with some answers as well.
 
Lightning has been documented to kill fish in close proximity to a strike. I'm assuming a scuba diver wouldn't be in any less danger.

Current from a lightning strike to a body of water will travel in a more or less hemispherical gradient away from the strike point (similar to what it does on land), with higher 'step' voltages closer to the strike. How close you'd need to be to be injured or killed is not clear though. At any rate, scuba diving during a storm would not be a good idea by any means.
 
Lori, not a dumb question because I've wondered the same thing. Dan, thanks for the answer. Would the water temperature in the immediate viscinity of the strike also be a factor, or is the hazard purely from the current?
 
Lori Meyer said:
How far does lightning travel on a body of water? Why don't a bunch of fish die when lightning strikes a lake? Would you be safe if you were scuba diving at a certain depth? These might be dumb questions, but I was listening to the extreme instability radio show last night and began to ponder these questions.
Click to expand...

While working as a Aquatic Director at a local YMCA, I witnessed a lightning strike on the lake where I had been teaching swimming. Fish did float to the top although I'm not sure if they were simply unconscious or dead. No trace of them was found the next morning so either they recovered and swam away or something has a wonderful snack that night!

Jerry
 
It's interesting how vague the research and guidance is on this. That's understandable, given that you can't have too much specific detail in information intended for general public safety. I don't presume to have any special expertise; but I do know some physics, and can read. Sometimes the general public guidance seems counter-intuitive, or even more risky. We all remember the times we were told to open all the windows and doors and seek shelter from a tornado next to the southwest corner of the basement.

Fact(?): Once lightning hits water or the ground Ohm's Law basically applies. The huge current (10k+ Amps) flows outward from the strike point, dispersing with distance and favoring low resistance.

Fact(?): The unclothed human body offers a resistance somewhere between fresh and salt water. The human body clothed in a wetsuit offers a very high resistance relative to water.

Not a situation that has any significant probability for me, but from all I read... if I were caught out in a scuba suit in a thunderstorm, I'd rather stay in the water away from the boat, unless the boat had enough shelter that the "skin effect" would protect me from a direct strike on the boat. FWIW.
 
Now all we need is some test subjects and a grant to be on the cutting edge of study on this subject.

lol
 
When I was young. I was at the beach in the water when lightning struck the water less than a mile away (Best guess is 1/4 mile) I didn't feel anything, but several of my friends said they felt a slight shock./
 
A lot of the lightning's path has to do with the mineral content of the water in a lake or pond or whatever. It would be an interesting study though. You could probably have a field day with this study in Florida by using metal rods on buoys...and of course some sort of wireless transfer of data from the buoy to shore.
 
I think it gets over-exaggerated how well of a conductor "water" is. Distilled water will not conduct electricity, but "normal water" with iron, some salt, calcium, other minerals, etc will conduct but not like solid copper or iron. Even with the enormous current a lightning strike has it will "dissipate" pretty quickly moving radially outward. IE: an entire lake is not going to die when lightning strikes it. :)
 
We've all been talking about the current associated with a lightning strike, but no one has mentioned the heat. Last I checked lightning was roughly 54,000 degrees (F) (give or take). So, if it struck water would some of the water be flash boiled?

In general, I wonder how easy it would be to roughly calculate how far away from the strike you would need to be to avoid feeling the heat/shock?
 
Just got a reply from my physics instructor...thanks to Dr Mason at OU :D !!!
The main factor is the amount of energy in the stroke. If the energy is given as 10^9 J, the energy will raise the temperature of 3000kg water (3m^3) from 20C to 100C and boil about 370kg (about 1/3m^3). The current spreads out over the surface of the water, and he thinks some (very little) vaporization would occur along the path of the current.
Now...to hit the library over spring break and see if there's any papers on this.
 
Curious about the research on this myself, I decided to send the following email to Dr. Martin Uman of the University of Florida Department of Electrical and Computer Engineering. He has done extensive field and lab research on lightning and its consequences.


> Hello Dr. Uman,
>
> I am a Gainesville area storm spotter and yearly vacationing storm
> chaser to the plains. My lifelong fascination with storms and storm
> structure has connected me via message board to many others who share
> the same interest. As a member of Stormtrack.org, I often find topics
> of personal interest discussed, and a recent thread appeared
> concerning lightning and water. Perhaps you have already studied the
> occurence in great detail or know of someone who has, but I thought I
> would pass along the following thread as a potential project for
> someone's study. Perhaps your rockets tethered to floating bouys?
> Just trying to be helpful.
>
> http://www.stormtrack.org/forum/showthread.php?t=11386
>
> Respectfully,
>
> Paul D. Austin


His reply was brief, but insightful. I have not read any of his books or research, but it might be very interesting.


> Thanks. Lightning does kill fish near the strike point in
> water, and it often produces arcing ( separate visible
> channels) near the strike point which at some radius (
> meters in fresh water) turns into a uniform outward.
> current flow.
> Martin
>
>
> ~~~~~~~~~~~~~~~~~~~~~
> Martin A. Uman
> Distinguished Professor
> Department of Electrical and
> Computer Engineering
> University of Florida
> ~~~~~~~~~~~~~~~~~~~~~

 
Prof. Mason's expert computation is similar to my back-of-the-napkin calculation. However the complicating factor is, I think, that most of the power is released in the arc channel. Sources I found report an arc channel reactance of a few-tenths Ohm/m and an average voltage drop on the order of 500 Volts/m. Over a channel of a few thousand meters that's a pretty substantial resistance. [For data wonks, see http://radarmet.atmos.colostate.edu/~kwiens/rm3/kwiens/ICAE2003/icae2003/proceedings/ThC1-003-101_Larsson.pdf]

What this suggests is that the ground current will spread out laterally in proportion to the areal resistance of the "earth" ground. Thinking of it from the "lightning's point of view", the current will splatter out as a disk-shaped ionized channel until the resistance of entry decreases enough and the arc current enters the surface.

I think all this means that most of the voltage drop and thereby most of the power is released in the arc channel. This should particularly be true of a direct strike to water where the resistance to ground is relatively low, IMO.
 
Thanks for the insight everyone. So it seems that scuba divers would be theoretically safe assuming they were not near the surface, correct?
 
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