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?

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    The question title makes it sound like you want to actively seek out bodies of water with lightning storms overhead... – Nick T Mar 8 '11 at 3:58
  • I suggest a rephrase to "Is it safe to swim during a thunderstorm?" – Joseph Weissman Mar 20 '11 at 14:49
  • Yes. It is not necessarily the highest object but the most conductive. Given a choice between a tree and a metal radio tower or a wet swimmer in the open. The latter are higher risks. However if submerged to a reasonable depth a direct strike energy is dissipated uniformly throughout the body of water. Fish survive thunderstorms. – resident_heretic Aug 15 at 4:44
up vote 26 down vote accepted

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.

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    I'm sorry, I had to sign up just to correct something here. Being in a car does not protect you because of rubber tires. That claim always bugs me. It protects you because the car body forms a crude Faraday cage around you. Rubber tires are useless. Think of it this way - if the lightning had no trouble going through a mile of air to get to your car, is it really going to have any trouble getting around 2 feet of rubber? – Tesserex Mar 7 '11 at 15:43
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    @Tesserex: I'm glad you made the effort, that makes way more sense. Never reflected on that. – David Hedlund Mar 7 '11 at 15:53
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    @David Although it should be noted, if you were in a car that was struck, you would still have some serious problems. The shockwave would likely blow out your eardrums, and possibly cause some internal bleeding. The windows may get blown in as well, right at you. But that's just a guess. – Tesserex Mar 7 '11 at 16:57
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    @Sklivvz, incorrect: it only matters if you're touching something near the outside of the conductive metal cage. You can touch the inside (so long as it's well within the skin effect depth; extremely small in lighting strikes) with no ill effect. – Nick T Mar 8 '11 at 1:33
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    Here's a video of how you'd react if your car got struck by lightning: youtube.com/watch?v=Z3WJt2shAms (language NSFW) – Michael Pryor Mar 8 '11 at 13:40

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:

  1. Amount of current flowing through the body (measured in amperes).

  2. Path of the current through the body.

  3. 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 [1998]. Worker Deaths by Electrocution; A Summary of NIOSH Surveillance and Investigative Findings. Ohio: US Health and Human Services. Greenwald EK [1991]. Electrical Hazards and Accidents - Their Cause and Prevention. New York: Van Nostrand Reinhold.

DeMaio and Demail, Forensic Pathology, provide, near bottom of page:

  • an example of household electrocution at 120V and 1000 ohms of skin resistance
  • "If the point of contact is thin moist skin, the resistance may be as low as 100 ohms."

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

  • more current kills faster, especially with respect to ventricular fibrillation
  • 1 mA tingle; 5 mA muscle tremor; 15mA muscle contractions
  • 50 mA all muscles contract, leading to respiratory arrest and death if sustained
  • 75-100 mA ventricular fibrillation, but recoverable if current stops
  • ~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.

  • I'm not buying #2. The damage would be a wider area when it strikes water because water conducts more easily than dirt, but it seems to me lightning should be striking water proportionately less than non-water if anything, because water is the lowest possible point. – user792 Nov 6 '11 at 13:18
  • @JoeWreschnig on the other hand, water is a good conductor and therefore the energy is faster spread around and grounded, if that makes some sense (I cannot express it better I'm afraid). Lightning strikes the water more likely because the water has higher conductivity and therefore provides better grounding. As well, in a slightly hilly area, the chance of a lightning to hit the lower parts are higher: The difference in the distance in air is low, but lower places are more humid (groundwater), therefore providing a better grounding. – yo' Feb 1 '13 at 23:15
  • This answer has been extensively revised. You may wish, therefore, to delete and/or repost revised comments if apropos. – Paul Jul 11 '13 at 11:56

The lightning will conduct through the water. Not for huge distances but farther than the ground. The larger risk is to watercraft. See NOAA Site for more information.

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    Interesting note: lightning damage is GREATER in freshwater than saltwater (because saltwater is more conductive). Seems non-intuitive: kp44.org/LightningAndSailboats.php and adminsr.com/blog/?p=127 – Michael Pryor Mar 7 '11 at 20:28
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    Seems intuitive to me. If you are in a car you are protected BECAUSE the metal is conducting. The more conductive the water, the less danger to things in the water. In freshwater YOU are the best conductor, as you are essentially a bag filled with saltywater. – Nick Aug 9 '12 at 9:40
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    @Nick However, the reasoning why in more conductive water the risk is lower is completely different from the car thingy: In a car, it is about Faraday cage. In water, it is about the fact that the electric current prefers ways with the highest conductivity. So higher the conductivity of the water is, lower amount of current chooses your body as the conductor. (This is not ultimately correct in the physical sense, but it catches the basic idea.) – yo' Feb 1 '13 at 23:07

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