Following on from the recent discussion of lightning strikes what is the expert opinion here of what to do when a storm arrives and you're swimming?
The instant reaction of almost everyone today was to get out of the water but I have a nagging doubt. In the water I was not going to provide a preferential path to earth, although a strike would have caused the potential of the whole lake to rise surely I would have simply floated with it. Out of the water I was standing on the bank soaking wet and probably more of a target for a strike, a few mm of wet neoprene certainly wasn't going to protect me.
I was swimming in a lake last week as a thunderstorm approached. I got out well before it arrived.
Water does not have zero resistivity. If there is a strike on the water a potential difference will appear across you in the water. This will most likely be enough to pass more than 30ma through your body. This is highly likely to be fatal.
Best place either in a car, or curled up in a ditch out of the water.
A strike in water is almost bound to be fatal, the voltage developed across the water will be high. Same thing happens on a far smaller scale when a heater is dropped in a bath.
If there really is nothing higher than the water around, youre in trouble either way: stay in and get fried, or get out and become the tallest object in the area.
It is possible to estimate how far away you need to be from a lightning strike while immersed in water and be reasonably safe.
The typical current in a lightning strike is about 30kA with a worst case of about 300kA. The resistivity of seawater is 0.1 to 0.5 Ohm.m, but the resistivity of lake waters varies from 100 to 400 Ohm.m.
Assume the lightning strike is a point source of current spreading into a uniformly resistive hemisphere. There will be some distance at which the potential difference between two points separated by say 2m is at a level considered to be safe. A "safe" voltage is probably somewhere in the region of 10 to 50V.
From "The Art and Science of Lightning Protection" by Uman, the voltage between two points at distances a and b from the lightning strike is
Vab = rho.I(1/a - 1/b)/2.pi
So in seawater with 0.5 Ohm.m resistivity for a 30kA lightning strike at a distance of 20 to 22m the peak voltage drop will be about 11V.
However, in a 400 Ohm.m lake at 20m from the strike, the voltage drop across 2m of water will be about 9kV.
At a range of 100m, in the 0.5 Ohm.m seawater the voltage drop will be
0.5V whereas in the 400 Ohm.m lake it will be about 370V.
So how many people have actually been killed by a 'heater' (presumably electric) being dropped in the bath? I suspect that in reality the person dropping the heater is more at risk than the person in the bath.
Your body, being loaded with salts, is a better conductor than fresh water, so the current will preferentially go through you.
They are. A standard fishing technique is to generate a current through the water which stuns/kills the fish, and you simply pick up the ones which float to the top.
More likely a popular myth. In one situation, lightning struck water containing scuba divers. Those who were touching the bottom were shocked. Those floating freely in water apparently did not know lightning struck. Makes sense once one learns the underlying principles.
We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember snipped-for-privacy@isbd.co.uk saying something like:
According to the movies and cheesy whodunnits, lots. In reality, I've no recall of it actually happening, but the newspapers love to perpetuate old myths.
Circa late 1970s/ early 1980s, the late Lady Barnet of What's My Line? TV program fame killed herself by arranging for a wall elec heater to drop into her bath.
She lived in a very pleasant late Victorian house with its own grounds in Cossington, Leicestershire, opposite the vets. I know because my cat escaped into her garden from the vets!!
Her suicide occurred after a widely reported case when she was accused of shoplifting.
At the time both the case and the inquest received detailed coverage - including an indication as to how she had made the elec fire fall off the wall above the bath - in the local rag (the Leicester Mercury), as well as passing notes in the nationals.
Unfortunately the on-line Merc archive doesn't stretch back that far (by a long way). So if you really doubt my memory you need to either (1) visit the Merc's offices & ask to check back issues (if they will let you) or (2) visit the Leicester Ref Library - or perhaps the Leics Record Office or (3) visit the Brit Lib Newspaper Reading Rooms in Collingdale.
I didn't often see the Loughborough Echo which also has Cossington within its area, but should have expected that also would have given the matter detailed coverage. So that's a second source for you. And aren't there official inquest records somewhere too?
Fortunately I moved away from Leics in the mid 80s, otherwise I'd do the lookup for you.
Even so the matter still sticks in my craw. Two reasons. One was that my cat's escape & several searches of her garden imparted a vivid recollection of the 'crime scene'. T'other was that electrocution seemed, and still seems, an extraordinary suicide choice for a doctor to make.
And even if by some bizarre circumstance the fire fell off the wall by itself & it wasn't suicide, there is no doubt she was electrocuted in her bath.
Funny though, the doubt I have about the ref quoted above lies not in the suicide, but IIRC Lady Barnet was found not guilty by the local magistrates - not convicted by a jury. Which factoid also still sticks in my craw. Possibly because reports in the local rag left one with a definite feeling that she had had a lucky 'got off' (or 'let off'?). Not that that was a great surprise as she was, or had been, a magistrate herself.
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