I was cautioned by mom that I mustn't go swimming when there is lightning; the theory is that it is more likely for lightning to hit you when you are in the water.
Is this fear justified?
The theory, the way I've understood it, is that you're more likely to be the highest elevated point, when you're in the water, since there are rarely any surrounding tall objects, such as trees. This is more likely to be true for a boat than for a person swimming, obviously. This is the same warning as that with regards to standing near a tree during lightning. The reasoning is that you should get away from tall objects, and certainly not comprise one yourself, during lightning.
The rationale for this is that an electrical discharge always seeks out the shortest path to ground. Shortest path, here, refers in part to the physical distance between the lightning source and the target, but also to resistance - if there's something more conductive in the vicinity of the highest point, this might be the lightning target instead. There is no reason to expect this shouldn't hold true for lightning - indeed, it is by this premise a lightning rod works - but to the extent that it does, it does so rather crudely. Not infrequently does lightning strike very close to a lightning rod, without hitting it, despite the rod being the highest elevated point. When swimming, attracting lightning strikes to your direct vicinity can be just as fatal as attracting a strike immediately to yourself, which is my next point:
So much for the risk of being hit. Another concern with swimming during a lightning storm is the danger of any lightning strike, regardless of whether or not it hits you. On land, it's quite easy to see to it that you're relatively safe. Inside a car, for instance, you're protected since you're inside a conducting enclosure (see Gauss's Law). Non-distilled water is a good conductor, which means you could be in danger if lightning strikes in your vicinity, not only if it hits you. The best source I've found for that is some dude who's apparently a lightning researcher in Argonne, who says:
My guess is that the average lightning strike would electrify a few hundred feet worth of water from it's strike point sufficiently to electrocute someone. Note that the Straight Dope suggests a radius of twenty feet or so.
No, you shouldn't swim in a lightning storm. Electricity is more hazardous to a wet person because the body's electrical resistance (ohms) is lower when wet. Also, the body of water can conduct the lightning's electricity from a strike.
According to US National Weather Service Science and Operations Officer Ted Funk, Lightning strikes can generate 300 million Volts and 10,000 - 200,000 Amps (see his PDF presentation "Lightning: Facts, Fiction, Safety Issues..." slide #15).
Slide #20 of the same presentation provides that Water-related activities rank 3rd in danger among the studied categories, causing 13.6% of lightning fatality with a 35 year study period, behind being in an open-field (45%) or under a tree (23%), and way ahead of most work-related categories (e.g. telephone workers 4%).
I would think that being at a park with a pool or lake actually could raise all 3 of the top concerns, because there could be an open field or flat parking lot surrounding the pool or lake, and trees, but we also know that electricity has greater effect on a wet human body.
In terms of lightnings affect on the body I'm going to provide two references that both indicate that the human body's resistance to electricity is lower when wet, and provide a range of effects. It is assumed that the reader is familiar with Ohm's law.
We'll compare a notice from a OSHA, a workplace safety agency to an excerpt from a college textbook Forensic Pathology, 2nd ed. by DeMaio and DeMaio.
The USA Department of Labor Occupational Safety and Health Administration, a national regulator of workplace safety, in "How Electrical Current Affects the Human Body" states:
Wet conditions are common during low-voltage electrocutions. Under dry conditions, human skin is very resistant. Wet skin dramatically drops the body's resistance.
Dry Conditions: Current = Volts/Ohms = 120/100,000 = 1mA a barely perceptible level of current
Wet conditions: Current = Volts/Ohms = 120/1,000 = 120mA sufficient current to cause ventricular fibrillation
Three primary factors affect the severity of the shock a person receives when he or she is a part of an electrical circuit:
Amount of current flowing through the body (measured in amperes).
Path of the current through the body.
Length of time the body is in the circuit.
[numbers added, text unchanged - Paul]
Other factors that may affect the severity of the shock are:
The voltage of the current. The presence of moisture in the environment. The phase of the heart cycle when the shock occurs. The general health of the person prior to the shock. Effects can range from a barely perceptible tingle to severe burns and immediate cardiac arrest. Although it is not known the exact injuries that result from any given amperage, the following table demonstrates this general relationship for a 60-cycle, hand-to-foot shock of one second's duration:
Current level (Milliamperes) Probable Effect on Human Body
1 mA Perception level. Slight tingling sensation. Still dangerous under certain conditions.
5mA Slight shock felt; not painful but disturbing. Average individual can let go. However, strong involuntary reactions to shocks in this range may lead to injuries.
6mA - 16mA Painful shock, begin to lose muscular control. Commonly referred to as the freezing current or "let-go" range.
17mA - 99mA Extreme pain, respiratory arrest, severe muscular contractions. Individual cannot let go. Death is possible.
100mA - 2000mA Ventricular fibrillation (uneven, uncoordinated pumping of the heart.) Muscular contraction and nerve damage begins to occur. Death is likely.
2,000mA Cardiac arrest, internal organ damage, and severe burns. Death is probable. References
NIOSH . Worker Deaths by Electrocution; A Summary of NIOSH Surveillance and Investigative Findings. Ohio: US Health and Human Services. Greenwald EK . Electrical Hazards and Accidents - Their Cause and Prevention. New York: Van Nostrand Reinhold.
DeMaio and Demail, Forensic Pathology, provide, near bottom of page:
Additional points to summarize the linked page of the Pathology text (which is difficult to cut/paste from Google Books and in any event is copyrighted material that I would rather summarize than copy directly) and compare to the OSHA article
~1A or more ventricular arrest
This is fairly consistent with the OSHA article.
In summary, lightning produces incredible amounts of voltage and current when compared with household voltages and currents. The currents needed to stop the heart, though miniscule when compared to lightning currents, are further reduced when the body is wet because resistance is lower, which increases the danger of being around any kind of electricity.
Swimming during a thunderstorm is unsafe, both based on prior observation of lightning fatality and the known effects of greater exposure and greater susceptibility to electricity's effects.
Thank you for your interest in this question.
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