Why stay in the car during thunderstorms?

So there appears to be quite a bit of misinformation on the web as to why people should stay in their cars during a thunderstorm. So I'd like to clear some things up. One such non-nonsensical answer is that cars have rubber tires which insulate you from the ground. I believe this contributes little to nothing to the actual reason.

The "correct" answer appears to be because the car acts like a Faraday cage. The metal in the car will shield you from any external electric fields and thus prevent the lightning from traveling within the car.

However, what happens if you have an imperfect Faraday cage around you? Say for example, you had a window open. I think the car would still protect you a) because it still acts as a Faraday cage, albeit a bit not perfect and b) because electrons will travel the path of least resistance which would be through the body of the car and not through you.

Now going with my b) reasoning, wouldn't you be just as safe standing next to a giant conductive pole (i.e. a lightning rod)? Wouldn't the lightning just go through the lightning rod and you'd be 100% safe?

Also a side question: lightning is essentially just an huge electric arc from the clouds to the ground, correct?

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UK car show Top Gear did a segment about this. Quite fun. youtube.com/watch?v=ve6XGKZxYxA –  Fara Jul 5 '11 at 20:10

Yes, if it is not a plastic covered car it is an effective Faraday cage.

If the tires are such that the car is insulated electrically, if it is hit it will take some time to discharge to the ground, but still the passengers would be safer than standing next to it outside. I have learned that modern tires are particularly constructed so that the static charge generated by the friction on the road is discharged so that would also help. ( in olden times they used to have chains trailing from the trucks in order to discharge the static. Recently I saw a car with a discharger too, trailing on the road!).

lightning is essentially just an huge electric arc from the clouds to the ground, correct?

Wrong, the current actually may start from the ground. That is the rational of the lightning rods, to create a path for a current to be generated by the potential difference to the cloud and to meet the current from the clouds in a prefered location instead of a random one. It is not wise to stand next to a rod, read in the link the amount of power dissipated by a bolt.

The average peak power output of a single lightning stroke is about one trillion watts — one "terawatt" (1012 W), and the stroke lasts for about 30 millionths of a second — 30 "microseconds".[18]

And it is not wise to stand, because you may also give rise to leaders that will meet the lightning path. If in the open it is best you fall on the ground as much sheltered as possible.

A colleague once was about 20 meters from a lightning bolt, and he was so shocked by the sound and fury, it took him a week to come down to normal.

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In answer to your second question: "Now going with my b) reasoning, wouldn't you be just as safe standing next to a giant conductive pole (i.e. a lightning rod)? Wouldn't the lightning just go through the lightning rod and you'd be 100% safe?" No. A lightning strike can carry a high current, up to 200 kiloamps has been recorded. Not only can the current be high, but the rate of change in this current is high as well. That means the induced magnetic field around the pole and the path of the lightning can be quite high. This can induce high voltages in the vicinity of the strike. In fact many people who are reported to have been "hit" by lightning, have not taken a direct hit, but suffer from induced currents.

You are correct about the widely misunderstood thing about rubber tires. A spark that has enough voltage to travel thousands of feet, will not be stopped by the few inch gap between the conductive part of the car and the ground.

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I've heard that being close to where the lightning strikes is dangerous mainly because of the resistivity of the ground: the huge current traveling along the surface can cause a significant potential difference between your feet. This risk would be minimized by keeping your feet as close to one another as possible, and cows are more at risk than humans. –  Edgar Bonet Jul 6 '11 at 13:24
Edgar, thats an additional risk, although the O.P. assumed a well grounded pole. Both induced currents, and current spreading along the surrace are issues. –  Omega Centauri Jul 6 '11 at 15:58
There is also a danger of secondary strikes, if the grounding of the pole or it's conduction isn't perfect, or there is a sharp point on the pole near you - there may be a better path formed from an arc from the pole through you to the ground. That's how a lot of people near trees get zapped. –  Martin Beckett Jul 6 '11 at 16:13

Be careful about your definition of giant conductive pole (ie be exact in you description).

Some people may consider trees as a conductive pole. The danger here is not only from electrical discharge but from exploding tree (due to sap boiling in an inclosed space).

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I would stay in my car for reasons of comfort. The car will attract a little more lightning than you would if you were standing up; but it might (or might not) protect you from said lightning.

Lightning strike locations are psuedo-randomly dispersed. If a thunder storm develops over, say, a 100 square kilometer PERFECTLY FLAT area, and throws out 100 strikes, then the expected strike coverage is 1 strike per sq. kilometer. Given that a strike will occur in that square kilometer, the chances of it striking any given 1m x 1m spot is (1x1)/(1000x1000) = 1E-6 = 0.0001%. Literally and exactly one chance in a million.

But the earth is not flat. It has been shown that taller objects attract more lightning. Taller conductive objects attract lighnting in direct proportion to the area they "protect." For a long slim object, that area is the area covered by a cone, which has its apex at the top of the object and which slants out at a $45^o$ angle in all directions. So, in this example, a 10m tall metal light pole would protect an area of ${(Pi)}{10^2} = 314 m^2$. It chances of getting struck by lightning in this storm would be 314 in a million, or roughly 0.3 in a thousand.

Anything which is fully within the volume of the cone of this light pole is considered protected (though I would not stand RIGHT NEXT to the light pole).

A 10m tall and 100m x 100m warehouse would "protect" an area of about 120mx120m = $14400 m^2$. Its chances of taking that sole lighning bolt are 14400 in a million, or about 14.4 chances in a thousand: 1.44%. Now we are talking about real chances of taking a hit.

Let's go back to you in your car, out in flatland. Assume you are 1.6m tall and your car is about 1.4 m tall x 2m wide x 5m long. For purposes of a lightning strike, you are considered a conductive object. If you are out standing in the rain, like a dummy, you "protect" an area of about $8m^2$. Your car "protects" an area of about 4.8m x 7.8m = about $22m^2$. So, you have about 3 times greater chance of getting hit by lightning in your car.

If hit by lightning out in the open, you are obviously toast. If you are in the car... well... lightning does weird things. It is better than a direct hit to your cranium; but I would definitely not say that being in a car getting hit by lightning is safe. Let's say the path from the roof of your wet car to ground is 1 Ohm, and the path tfrom the roof of your car to ground THROUGH YOU is about 1000000 Ohms (1M Ohm, includes air gap, since your dry-skin body only provides about 20K-30K resistance). Say the lightning strike is 30000 Amps. About 29999.97 amps go through the car structure. About 0.03 amps = 30mA go through you... and you die. Oh yeah, you might not be charred and smoking, but dead nonetheless.

If I had to take a choice between a 35% chance of getting hit in the head by lighning while standing out in a field, or a 100% chance of my car getting hit while I was in it, I think I would take the standing out in the field option.

The best protection would be to lie in a ditch in the field. But that gets you very wet and uncomfortable. (Can you imagine flying down the freeway with your family, and you run into a thunderstorm; and you say, "Hey, honey, lets pull over and go lay out in the ditch in the pouring rain until this is over."?) I like to stay dry, so would take my 22 out of a million shot in the car. Of course, if I was REALLY concerned, I would try to greatly lower those odds by finding a parking spot about 4m out from the base of that 10m tall light pole (or tree or whatever).

EDIT: I miscalculated the area protected by a standing person... fixed.

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Nice to stay dry, especially in a safe Faraday cage. –  Michael Luciuk Jul 7 '11 at 22:07
A car with windows is a Faraday cage with huge holes in it, which is the same as saying it is not a Faraday cage at all. I would agree that GIVEN you will be struck by lightning you would prefer the current shunt which the car will surely provide. But, will it be enough? And, yes, agreed, nice and dry. –  Vintage Jul 7 '11 at 22:36