Everybody out of the
pool!
 Q: Does lightning strike indoor swimming pools? If
the building has lightning rods? Has anyone ever been hurt or killed?
Ann, Oberlin, Ohio
A crawl swimmer, indoors. Photo courtesy of the US Army, Fort Sill
and Wikipedia.
A: Lightning strikes indoor swimming pools. That's why lifeguards yell, "Every body out of the pool!" when lightning gets within six
to eight miles.
Current from a lightning strike can invade
an indoor pool, and hurt people. Here's how: Lightning strikes a power pole
outside the pool building. A surge of current races along the electrical
wires from the hit power pole to the pump in the building. The pump
moves water and, therefore, contains water; water conducts electricity. The
current surge flows from
pump wires into pump water, which conducts the electric current into pool water
and water splashed around the area.
This endangers people in the pool and walking on the wet floors, because the
current then flows through people to ground, along any of various paths: A
person standing in the pool, one climbing on the ladder, another touching the
underwater lights. The huge current could injure or kill all such people.
When lightning is within five miles of an indoor pool, safe practices dictate
leaving the pool, and moving to a dry area of the building — well away from any metal (such as, phone lines, electrical wires or buried pipes).
Metal can conduct a bolt's current.
If the "Flash-To-Bang" delay (length of time in seconds between a lightning
flash and its subsequent thunder) is 30 seconds, the lightning is five miles
away.
Current flowing from a lightning strike is fearsome — averaging about 25,000 amps.
For comparison: a reading lamp shines with only one amp of current. In the year 2000, lightning killed
two boys in an outdoor pool in Florida.
"The enormous power of a lightning bolt does make the kind of precautions you
outlined appropriate, although they may seem extreme to the kid who really wants
to go swimming," emails physicist
Rod Nave, professor at Georgia State University.
"We could find no reports of deaths or injuries in indoor pools related to
lightning causes," says
Richard Kithil of the National Lightning Safety
Institute and Kevin Johnston, a senior consultant. They suspect such
incidents happen, but go unreported. They have observed: a main
circulation pump destroyed, injuries to employees touching electrical panels and
the concrete footing of a water slide blown apart.
"There are many reports of injuries/deaths in bathtubs from lightning. This
is indirect evidence, of course, but it seems reasonable
the same danger exists in (larger) swimming pools," emails Kithil.
Kithil and Johnson conclude: suspend all pool activities (including showers) until 30
minutes after the last observed thunder or lightning.
By the way, over the past 30 years, lightning killed more people in the USA than
tornadoes or hurricanes, according to
John Jensenius of the NOAA and the National Weather Service.
Further Reading
Lightning and aquatics safety: a cautionary perspective for indoor pools by
Richard Kithil and Kevin Johnston, National Lightning Safety Institute
Weather
fatalities, NOAA
Lightning safety,
NOAA
Lightning current by Rod Nave, HyperPhysics
Lightning safety, National Weather Service
Lightning
FAQ, National Severe Storms Laboratory (NSSL)
Reader's Answer
According to the National Severe Storms Laboratory (NSSL),
both indoor and outdoor pools are unsafe during a lightning storm. Reason being
that any part of a home or outdoor facility that is struck by lightening can
affect the entire location. According to this website, there have been no
reported deaths due to swimming indoors during a lightning storm.
Karishma, Kaajal, and Shanya, West Sacramento, California USA
(Answered Oct. 9, 2007)
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