Can lightning vaporize a swimming pool?

Factual Questions
21

You would not be protected by being in mid-air.

A “leader” would be sent upwards from the uppermost part of your body, looking like a faintly purplish trail of plasma snaking in the direction of the clouds. Another would stretch down from your lower extremities towards the ground. A path to ground through your body would offer less resistance, all else being equal, than through an equal distance of air, and the lightning would flow through your body. This would create large potential differences within your body, generating significant heat and causing major damage.

Whether you “feel anything” is largely down to whether you remain alive and conscious during this sequence of events.

22

Watts are not the same thing as Joules. A Watt is a Joule per second, or in other words, if you have a 1-Watt source of power, and you leave it on for one second, you’ve used 1 Joule of energy. Now, then, if we take the biggest numbers available for our lightning bolt, and water that’s almost boiling (let’s say it’s a Yellowstone hot spring, not a swimming pool), let’s see how much water we can vaporize.

Max voltage is 1 billion volts.
Max current is 100 000 Amperes.
Power is therefore 10[sup]14[/sup] Watts
Max duration is .2 seconds (never mind that this is incompatable with the 100 kAmps)
Therefore, the total energy in our superbolt is 2*10[sup]13[/sup] Joules.

The heat of vaporization of water is 2,260,000 Joules per kilogram, so our superbolt could vaporize 8.8 million kilograms of water. At one kilogram per cubic decimeter, this would be a cube of water about 20 meters on a side, which is a reasonable size for a swimming pool.

Of course, we made some unreasonable assumptions about our superbolt. Based on the information that Crunchy Frog posted, we should decrease either our current or our duration by a factor of at least a thousand. This brings us down to a cube of water about two meters on a side, which is much less than a swimming pool. Of course, the pool also won’t be on the point of boiling just before the strike, and the energy of the bolt won’t all be spent on vaporizing the water, so it’s even less yet.

23

>> and the energy of the bolt won’t all be spent on vaporizing the water, so it’s even less yet.

I think this is the main point. The bolt would give only a very minimal part of its energy to the water of the swimming pool and most would go to heat the air and water vapor in the air.

Roughly speaking: take the length of the bolt, apply the proportional part to the depth of the pool and you have a very small fraction of the total.

24

A watt is a joule per second… yep; the conversion comes out to 1:1, that’s why I used them interchangebly (guess I should have given a more detailed explanation - but hey, it was 3am). Looks like with all things combined the pool won’t vaporize or even come close. If lightning could vaporize a swimming pool, there would have been reports of it as lightning certainly has struck swimming pools at some point in history. Lightning can vaporize the sap in a tree though.

25

Tree sap probably has a much higher resistance than water. I think a miniscule portion of the energy in our lightning bolt would go towards heating the water–the vast majority probably goes right through the water and into the ground.

26

"The power in the stroke [of lightning] is three million megawatts, comparable to all the power generated in the United States at any one instant. " From <http://www.pbs.org/ktca/newtons/15/lightning.html>

Now, you tell me, do I need to do the math in order to show that that is enough energy to vaporize a pool of water? Of course that doesn’t mean that it will vaporize the pool, most of the bolt’s energy does other things. And you can survive a hit. Many people are hit by lightning each year in the U.S. Many, if not most, survive.

27

Wat a minute! I thought you said the poor sap was jumping into a pool. Confused.

I’ve seen a figure of 75% survive. don’t know if accurate.

28

What do you mean, enough energy? Three million megawatts isn’t energy at all. You might as well say “My car is twenty feet long. Is that enough energy?”. Three million megawatts is an awful lot of power, but power and energy are not the same thing. The conversion from watts to Joules isn’t one-to-one, because there is no conversion from watts to Joules. If you have a power, and you want to know the energy, then you also need to know the duration of the event. If that three terawatt lightning bolt lasted for an hour, then you’d have over ten exaJoules, which is more energy than a few thousand nuclear bombs… But on the other hand, if it lasted for a picosecond, then you’d only have three Joules, which is less energy than the water would get from one kid cannonballing into it.

29

He he, I guess it comes down to the wording of the question… “Can lightning vaporize a swimming pool?”

If you interpret that as “If all the energy in a lightning strike could theoretically be contained and applied to vaporizing a pool of water of X-volume under tight laboratory conditions with a 100% efficiency of power transfer, could it work?” - Answer: perhaps, depending on the power of the strike… which is variable at best.

If you see it as “In the real world will your typical lightning strike vaporize the water in my swimming pool?”
Answer: no, but stay the hell away from the pool during an electrical storm anyways.

30

To add one more hair-splitting post to my “a) interpretation” of the question…Crunchy Frog’s cite mentions that within 1/1000th of a second the current decreases, so the 3 million megawatts of power lasting that short gets reduced to 3 thousand megawatts/second (assuming that the cite meant 3 million megawatt-seconds, not megawatt-hours), and turning the megawatt-seconds into joules gives the 1:1 ratio. That’s only enough energy to vaporize 1.327 cubic meters (<1.5 cubic yards)of water already sitting at 100C, less if it’s at a realistic temp. Not quite swimming pool volumes there.

31

It says the lightning puts off 3,000,000 MW. Now, that doesn’t change no matter how long the bolt exists. One Watt=1 Joule/Second. Seconds is already in there. Now, knowing that a bolt lasts for about .001 of a second, we can figure out how much Power is in the bolt. What we do, is convert the MW to W and then divide by the amount of time. This ends up being:

(3.0010^6) MW(1.0010^6)=3.0010^12 W

(3.0010^12) W / .001 s=3.0010^15 W/s/s (Power)

(3.0010^15 W/s/s)/1000/3600(seconds/hour)=1.0810^16 kW*h (kilowatt hours)

Now that was a great demonstration of how much actual “power” is in a bolt. Lets figure out how much water that could bring to 100°C and then past the vapor pressure. (but first I will post this.)

32

Now to figure out how much H2O we vaporize. For this I will assume a few things. This is a heated pool. Who would want to swim in a cold one anyway? The pool’s temperature is 25°C which is 77°F. So we need to raise the temperature by 75°C.

We have 3.0010^16 Watts. This is equal to 3.0010^16 Joules. We know this because 1 J=1W/S, and we were given that a lightning strike contains 3.00*10^6 MW.

We also know that it takes 4.184 J to raise 1 cc of H20 1°C. That is, 4.184 J/°C/cc.

Now, we shall also assume that the pool is moderate in size. 2 m deep 10 m long and 5 m wide. This yields a pool (2 m * 10 m * 5 m)=100 m^3. That is 100 cubic meters of H20. To convert that into cc of H20 we simply need to take (100 m^3)100100100=1.0010^8 cc.

So, what we do is:

(1.0010^8 cc)(4.184 J/°C/cc) * 75°C=(3.14*10^10 J)

Now, many of you may think "wow, that is enough, it has reached boiling, but that is wrong. We still have to overcome a phase change, or the Latent Heat of Vaporization. It takes 2,257 J for 1 cc of H20 to vaporize.
So:

(2.25710^3 J/cc)(1.0010^8 cc)=(2.25710^11 J)

So, in the end we need to add up all these fun numbers and subtract them from the number of Joules the bolt of lightning gave us:

(3.0010^16 J)-((2.25710^11 J)+(3.1410^10 J))=3.0010^16 J

That means we have 3.00*10^16 J left over.

33

Oops. One mistake. I typed the wrong number.

I meant to type:

(3.0010^16 J)-((2.25710^11 J)+(3.1410^10 J))=(2.9999742910^16 J)

Which means that I can round the number up (following significant figures) back to the original 3.00*10^16 J. This means that the amount of energy needed to vaporize the pool is so insignificant compared to the total of energy in the bolt that it doesn’t show.

And please, do remember that we are all assuming that the energy does go to heating the water, rather than simply flowing through the water.

Have a nice day. :slight_smile:

34

So I guess the thing people are now disagreeing on is what “the power of the bolt is 3 million megawatts” means.

a) Puts out 3 million megawatt-hours for 0.001 s?
b) Puts out 3 million megawatt-seconds for 0.001 s?
c) Equivilent to 3 million megawatt-hours over 0.001s?
d) Equivilent to 3 million megawatt-seconds over 0.001s?

Assuming a, you’d divide 3 million MW by 3,600,000 to end up with 83,333 Joules of energy.
Assuming b, you’d divide it by 1000 to get 3x10^9 Joules of energy (what I did)
Assuming c you multiply 3 million MW by 3,600,000 to get 1.08x10^19 Joules of energy
Assuming d, you multiply it by 1000 to get 3x10^15 Joules of energy (what Grand Dream Ink did).

This lack of specific info is kind of thing that science students argue with to their profs (who leave stuff out so they can mark answers wrong and adjust the classes grades I think).

(ps Grand Dream Ink - you added and extra 0 to your total number of joules in the bolt starting in your second post: used 310^16J instead of 310^15J - not that it made much difference… rather insignificant)

35

All these responses to so simple a question.

Q- Can lightning vaporize a swimming pool.

A- No

But no, we need to beat it to death, discuss to the last BTU and kilojoule.

/patton voice/
I love it. God help me, I do love it so.
/patton voice/

36

Ha ha ha, now what kind of question board would it be if every thread was responded to with a mere “yes” or “no”? - I’d be bored stiff :D.
Sometimes it is most interesting to see why people say what they do and how they try to break a problem down… and how a simple yes/no question can bloom into something so much bigger.

37

mmmiiikkkeee: I think I’m going to cry.

Energy is the amount of capacity to do work that you have available. You can measure it in joules, foot-pounds, or gallons of gasoline if you like. Let’s go with gallons of gasoline, it’s easier to visualise. Power is how fast you burn the gasoline.

The power has NOTHING to do with the total energy! One gallon burned in 10 minutes gives the same power as half a gallon burned in 5 five minutes, gives the same power as a tenth of a gallon burned in a minute.

Another way to look at it is, energy is analogous to distance, power is analogous to speed.

"So I guess the thing people are now disagreeing on is what “the power of the bolt is 3 million megawatts” means."

No. “3 million megawatts” is completely unambiguous. Just like 3 million miles-per-hour is unambiguous.
3 million megawatts is the same power you get from burning 23100 gallons of gasoline per second. Or 1 teaspoon of gasoline per eighteen-millionth of a second. It doesn’t tell you how much gasoline you have.

"a) Puts out 3 million megawatt-hours for 0.001 s?
b) Puts out 3 million megawatt-seconds for 0.001 s?
c) Equivilent to 3 million megawatt-hours over 0.001s?
d) Equivilent to 3 million megawatt-seconds over 0.001s?"

No. You cannot assume that when someone refers to “power” measured in megawatts, they are talking about energy measured in megawatt-seconds or megawatt-hours.

Megawatt-hours and megawatt-seconds are units of energy, not power. A megawatt-hour is equivalent to 27.7 gallons of gasoline. So (a) and (b) make no sense. They are the same as saying “travels 3 million miles for 0.001 s.”

"A watt is a joule per second… yep; the conversion comes out to 1:1, that’s why I used them interchangebly"

No. That’s like saying “a knot is a mile per hour, the conversion comes out to 1:1, thats why I use knots and miles interchangably.” But one is a speed, the other is a distance.

You can use joules and watt-seconds interchangably because they are the same thing. You can use miles and “knot-hours” interchangably, because they are the same thing. You cannot use joules and watts interchangably, and you cannot assume that when someone refers to “power” measured in megawatts, they are talking about energy measured in megawatt-seconds.

(No-one need point out that a knot is a nautical mile per hour, by the way!)

38

Another thing about computing power dissipated in the pool.

Max voltage 1 billion volts
Max current 100,000 Amps

The 1 billion volts is between the bottom of the cloud and the earth (bottom of the pool). As has been stated:

P=I*V P is power, V is voltage, I is current.

Most of the voltage will be between the top of the pool and the bottom of the cloud so the actual power dissipated in the pool is much less than just volts time amps.

As far as tree getting vaporized it just ain’t so. Growing up in the mountains of New Mexico afford me the opportunity to see many a tree that was hit by lightning. The might be broken apart but vaporized is nowhere near the word for it.

39

To flog the question actually posed within the OP (" … would I hit concrete?") a little bit more, we can definitively say this: there’s NO WAY you’re going to hit the concrete at the bottom of YOUR POOL, anyway.

If we go with the scientific-rational explanation that the lightning can’t possibly vaporize it all, anyway, you’re going to fall in the water.

If we go with the superbolt-efficiently-transferred-instant-vaporization theory, the resultant steam explosion is going to blast you to kingdom come. As someone who works on 600psi boiler plants, I can tell you not to underestimate the awesome power of high-pressure steam. One half-second of this kind of steam blasting downwards into an open-topped drain funnel three feet below my hands created enough splashback to leave me with minor burns covering my exposed forearms. After your pool experiment, there’s a good probability that SOME of you would land on concrete… and some in trees… and some on roof shingle… and some on cars…

  • Dave
40

Yes, I think I’m gonna cry too. Look up any conversion table on watt-s to joules and you get 1:1 - I type in 10 watt-s and it gives me 10 joules… every time… sigh. Tired of arguing over definitions and electrical theory for a hypothetical problem that’s already been answered…:rolleyes: The lightning will not vaporize the pool; think I’ll go find something else to do.