It's a common statement that passengers of a car are protected from lightening while inside because the car has rubber tires (which are insulators). An alternative theory often put forward is that the vehicle is made of a metal conductor.
The question is, then:
Are passengers protected from lightning while inside a car?
If so, is this protection from:
a. The rubber tires; or
b. the metal shell of the car?
Thanks for reading.
From the National Oceanic and Atmospheric Administration - TOP 10 Myths o Lightning Safety:
Lightning laughs at two inches of rubber!
Most cars are reasonably safe from lightning. But it’s the metal roof and metal sides that protect you, not the rubber tires.Thus convertibles, motorcycles, bicycles, open shelled outdoor recreational vehicles, and cars with plastic or fiberglass shells offer no lightning protection.
But closed cockpits with metal roof and sides are safer than going outside.
And don’t even ask about sneakers!
From NASA - Ask an Expert:
If the car is metallic -- not a convertible! -- then a person is shielded by the metal of the car and lightning would be safely conducted around the people inside.
This has nothing to do with the rubber tires! This is know in physics as a "Faraday cage".
From FEMA - Thunderstorms & Lightning:
... rubber-soled shoes and rubber tires provide NO protection from lightning. However, the steel frame of a hard-topped vehicle provides increased protection if you are not touching metal.
From Environment Canada - Lightning facts and fiction:
Rubber-soled shoes and rubber tires provide no protection from lightning. The lightning strike between the cloud and the ground has potentially traveled thousands of meters through thin air, therefore rubber soled footwear or tires are inconsequential.
However, the metal shell of a car provides a pathway for the lightning strike to flow around the vehicle provided the car has a hardtop metal roof (not a convertible). Although such vehicles do not offer you absolute protection from lightning, you and others are much safer inside a car with your hands on your lap, than outside.
From Weather Imagery - Do Rubber Car Tires Protect Me From Lightning?:
The truth is, the rubber tires don’t deter lightning in the least bit. By the time a lightning bolt reaches your car, it has been traveling for miles and miles through the air which is many orders of magnitude more resistant than a few inches of rubber.
From Lightning Safety - Vehicles and Lightning:
Rubber tires provide zero safety from lightning. After all, lightning has traveled for miles through the sky: four or five inches of rubber is no insulation whatsoever.
From POPSCI - An Electric Aviation Experience:
... the aluminum hull of an aircraft is highly conductive ... it forms a Faraday cage.
For the same reason, you don't get electrocuted when lightning strikes your car (provided your car is made of metal and not fiberglass, you don't have a cloth convertible roof, and you're not touching the outside surface).
It's a common misconception that the insulating rubber tires protect you. Not true. It's the Faraday cage.
Here is a video of a plane getting hit by lightning in mid-air.
None of the 500 people on board the Emirates Airlines Airbus A380 were injured as the plane flew through a storm as it approached London's Heathrow Airport on April 23 2011.
A United Emirates spokesperson told the newspaper lightning strikes are not uncommon and that every plane in its fleet is designed and certified to withstand a lightning strike.
... a closed metal surface, no matter what it's shape, screens out external sources of electric field.
Here is a video of MythBuster Adam Savage demonstrating the Faraday Cage.
Passengers inside a steel bodied car are protected from lightning because the car behaves like a 'Faraday Cage'. When lighting hits the car it wants to take the shortest path with the least resistance to ground. That is via the body shell and not via the occupants.
This was demonstrated several years ago on the BBC entertainment show 'Top Gear' where they put a hamster inside a car:
http://www.topgear.com/uk/videos/car-lightning
As for the tyres, made of rubber, which does not conduct electricity?
Well, the tyres are made of vulcanized rubber and the black stuff in them is carbon that does conduct electricity, as do the steel belts inside the tyre. The question is how much does this conducting-electricity-thing amount to?
Pneumatic tires which contain carbon black generally have a resistance of approximately 10^6 ohms, measured from the crown to the bead. There is a provisional specification for this measurement, this being laid down in the WDK guideline 110. According to this a pneumatic tire belongs to "electrostatically active Class I" if it has a leakage resistance of less than 10^6 ohms.
Pneumatic tires which do not contain carbon black do not satisfy this test; their electrical leakage resistance is approximately 10^10 ohms. Source
How does that compare to plain old air, as in that stuff we breath?
Generally we do not use air or any other gasses to conduct electricity and, for the sums, air is considered to be of infinite resistance. But, where there is a will there is a way and, if you place a big enough potential difference across air it will conduct. When this happens the little electrons in the air get scared and do a runner from the nuclei that they have been bonded to in a process called ionization. All of a sudden the air conducts.
So what does Mr Lightning make of this? Well, he knows he can ionize air as he has already done quite a lot of that already making his way through loads of sky. However, as he gets to the ground he wants to be lazy and discharge his electrical load as easily as he can. Spotting a car in his path he thinks 'I will go and hit that rather than the nearby ground as that is closer and less effort'. Then he gets to the bottom of the car and sees some more of that air stuff. Not wanting to strip all those air molecules apart again he cheats and goes for the tyres. The carbon-black tyres are planted on the ground and, although not exactly as easy a ride as copper would be, he does the maths (10000000 ohms resistance is better than infinite) so he goes that way.
Being a cheeky so-and-so Mr Lightning would dearly like to zap the hamster in the car, however, no matter how hard he tries he just cannot ionize the air inside the car in the way that he has been used to to so far. Not because the air conditioning is off, but he cannot get the required potential difference, what with that tin box conducting so readily.
So, to answer the question. The tyres play no role in protecting the occupant from lightning strikes. This is not because they are insulators, but because they conduct electricity instead. They have to conduct electricity to prevent static build-up in the car and there are standards in place to make sure that they do not have too high a resistance, even if they do not have 'carbon black' used in their manufacture.
It is the Faraday cage of the car that protects the occupant, a wrap-around lightning conductor that works very well so long as it is made of a metal.
