Sunday, June 1, 2008

Apple #321: Lightning

Yesterday some friends and I were outside, talking about swimming, and then it started to sprinkle. A few of us agreed that we had enjoyed swimming in the rain once upon a time, but then I asked, "Isn't it bad to go swimming during the rain, or something?" Yes, my friends assured me, because lightning could strike the water and you could be electrocuted.

Sometimes the physics of this world impress the heck out of me.

  • The National Lightning Safety Institute recommends that you NOT go swimming while it's raining or during a thunderstorm or any time you think lightning might strike. As one of my friends said, water is a conductor, so if lightning strikes the water and you're within about 20 feet of the strike, the electricity is probably going to shoot through the water and zap you.

If the sky looks like this, better not go swimming.
(Photo of the Hamilton Fish Pool in New York City, by Charlotte Cooper)

  • If you're swimming in salt water, that's potentially more dangerous, since salt water is an even better conductor of electricity than fresh water.
  • Or, what could be more likely, is that your head, sticking up out of the flat surface of the water, might be the highest point in the nearby area, so the lightning might strike you directly.
  • The National Lightning Safety Institute says that swimming in outdoor pools is also hazardous. If the lightning strikes the water, the electricity could travel not just through the water but also down to the base of the pool where there are pipes or perhaps the metal drain or maybe some wiring, the electricity would travel along that conductor as well as through the water, and then you're super-toast.
  • Even swimming in indoor pools during storms is not as safe as you might think. The lightning could strike some pipe that goes from the ground outside to the pool inside, or it could strike some wiring that supplies power to the heated pool, and then there goes the electricity along that metal conduit into the pool, which is filled not just with water but also with other chemicals, and then zappo! you're cooked.
  • To prevent such zap-you're-cooked scenarios, the National Weather Service recommends that you count the number of seconds in between the flash of lightning and the clap of thunder. They call this the Flash-to-Bang count. Each five seconds in between the lightning flash and the thunder represents one mile. As soon as you count 30 seconds between lightning and thunder, that's considered the danger zone, and you should get out of the water or away from any exposed location. If my memory of thunderstorms serves me correctly, that's probably in most cases when you can see lightning and hear thunder.

This is what it looks like when lightning hits water. You do not want to be around when this happens.
(Photo by Francis Schaefers and Daniel Burger, at

  • The best places to take shelter are in substantial buildings (not lean-tos but structures with walls and a roof) or a vehicle that has a metal-topped roof.
  • They also say that you should wait at least 30 minutes after you hear the last rumble of thunder before going back in the water. This is because most lightning comes from the back edge of a thundercloud than from its leading edge.
  • While it is not possible to have lightning without thunder (lightning causes thunder), it is possible to see lightning and not be able to hear the thunder because it is out of earshot.

  • Lightning travels about 1 billion feet per second.
  • A lightning strike actually has two components: the cloud-to-ground strike and the ground-to-cloud strike in return, but it all happens so fast, it looks like it's a single event. (Storm Highway has animations that show this process in slow motion.)

A pretty clear cloud-to-ground strike, with several branches
(Photo from Wikimedia)

  • Lightning can heat the air anywhere from 18,000 degrees F to 60,000 degrees F. That's at minimum 7,000 degrees hotter than the surface of the sun.
  • Thunder is the sound of lightning expanding the air super-fast to such super-hot temperatures.
  • A typical lightning flash is about the thickness of a person's thumb.
  • Lightning strikes objects that stick up the highest in a landscape. This is why standing under a lone tree during a thunderstorm is probably the stupidest place to be.
  • It is not true that metal things will attract lightning to them. Lightning will typically strike an object that sticks up highest before it will "find" something made of metal.

Lightning striking the Eiffel Tower multiple times in 1906.
(Image from NOAA, sourced from Wikipedia)

  • 8% of deaths due to lightning strikes happened to people in or on the water. Almost 14% of lightning-strike fatalities happened to people standing under trees.
  • Lightning that hits a tree will travel down the trunk and turn the water in the tree to steam.
  • If it strikes the ground, it will fuse the dirt and clay into silica rocks called fulgarite. Those rocks often take the shape of the path the lightning took across the ground.

A piece of fulgarite. This photo and more of fulgarite in different shapes from the Glendale Community College's Earth Science Image Archive.

  • It is possible to be struck by lightning while using the toilet. Electricity from the lightning could theoretically travel along the plumbing or through the water. Less than 3% of lightning strikes occur inside a building, so it's a very low-risk situation, but there is some risk.
  • NOAA recommends that if you are outside and isolated during in a thunderstorm, ditch the umbrella, and get into a crouch with your feet together and your head tucked in between your arms. To reduce the amount of you that is touching the ground (soil with its metals and other ingredients makes a good conductor), lift your heels from the ground, too. Though I can't imagine this is a position you could sustain for very long.
  • The state with the highest number of cloud-to-ground flashes is Texas, with an average of almost 3 million flashes per year from 1996 through 2005. Florida is a distant second, with about half that amount.

Global map showing the number of lightning flashes per year, averaged over 5 years. Central Africa has the highest number of lightning strikes.
(Map generated by NASA's Optical Transient Detector, sourced from NASA's Remote Sensing Tutorial page on Hurricane Andrew)

  • The good news is that 80% of people who have been struck by lightning do survive.

This man was struck by lightning, which made branching patterns across the surface of his skin.
(Photo from the New England Journal of Medicine, sourced from Stoneridge Engineering's page What are Lichtenburg Figures)

You might also be interested in my entry about thunderstorms.

National Lightning Safety Institute, Indoor/Outdoor Swimming Pool Safety and Lightning and Aquatics Safety
USA Today, Answers Archive, Lightning safety and survival
NOAA, Top 10 Myths of Lightning Safety
NOAA, National Severe Storms Laboratory, Lightning Basics and Lightning FAQs
Vaisala, Number of Cloud-to-Ground Flashes by State from 1996 to 2005


  1. Great caption under the "running" photo!

    Interesting post, but I'm curious, did you happen to come across, what actually causes lightning to strike? I thought I had heard somewhere once that we didn't actually know that, but I've never confirmed that since.

  2. Hey, Dustin, nice to hear from you! And thanks for asking!

    Basically, lightning happens when storm clouds build up pockets of oppositely charged particles. Then all the updrafts & downdrafts swirling around mix the polarized particles together and zappo! you've got a spark.

    But scientists don't really understand how those charges build up in the first place.

    Here are a couple of sites that talk about that process:

    Short description:

    Slightly longer description:


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