Sunday, June 03, 2007

what are the chances?

i drove home from smallish state to even smaller state last night through the most amazing lightning storm. it got me thinking. what is it about a car that makes it a safe place to be? what are the chances that you'll get struck by lightning? are you more likely to get struck by lightning or to get swept away by an avalanche? well, i suppose the latter would depend on whether you lived in a place where avalanches occur.
at any rate, i googled lightning strikes this morning and found all the answers to my questions and then some.
-you have a 1:5000 chance of getting struck by lightning during your lifetime
-9 out of 10 people survive a lightning strike
-a lightning bolt strikes the earth a hundred times a second, heating the air around it 5 times hotter than the surface of the sun
-lightning strikes kill more people each year than tornadoes or hurricanes (about 2000 people worldwide each year versus 150 people killed by avalanches each year)
-cars are safe not because they aren't struck but because the tires conduct the charge to the ground
-lightning bolts start as negatively charged particles that extend towards the earth and are met by positively charged particles called streamers extending up from the earth; when the two oppositely charged particles meet, an electric current is formed that makes the flash (for a really neat demo, check out this national geographic interactive site (yes, i am aware that i am a geek).

some safety tips from national geographic (what can i say, i'm a pediatrician ;):
  • If outside, seek refuge in a car or grounded building when lightning or thunder begins.
  • If inside, avoid taking baths, or showers, and washing dishes. Also avoid using landline phones, televisions, and other appliances that conduct electricity.
  • Stay inside for 30 minutes after you last see lightning or hear thunder. People have been struck by lightning from storms centered as far as 10 miles (16 kilometers) away.
  • If caught outside away from a building or car, stay clear of water bodies and tall objects like trees. Find a low spot or depression and crouch down as low as possible—but don’t lie down on the ground. Lightning can move in and along the ground surface, and many victims are struck not by bolts but by this current.


daedalus2u said...

A few corrections and comments. I have done a lot of work with high voltage electricity, and have also read a lot about it, including the codes on lightningn protection.

Cars are safe because they are made of metal. The metal acts as a Faraday cage, and all the current flows on the outside. It is current that kills you, usually by stopping your heart. Being inside any metal object would be equally safe (provided it was sufficiently conductive). It has nothing to do with the tires, and nothing to do with the car being grounded. An airplane is safe because it is made of metal too. Airplanes get hit by lightning all the time.

The reason that most people survive a direct lightning strike is because your body is not a very good conductor. A lightning bolt is many thousands of amperes. When that much current tries to flow through something that has as much resistance as your body, it requires a high voltage to do so. A voltage high enough that the air you are standing in "breaks down" and becomes conductive. Air that has "broken down" is many orders of magnitude more conductive than your body, so all the current flows through the broken down air, and not through you. If you were in water instead of air, the water would not "break down", and fresh water isn't as conductive as you are. If you were hit while emmersed in fresh water it would kill you with virtual certainty. Probably salt water too. If you were immersed in liquid metal, you would certainly survive.

Standing near something that is hit is much more dangerous than being hit. The ground isn't that conductive, so when the lightning bolt puts hundreds of thousands of amperes in it, it requires significant voltage to drive that current through the ground. That generates a substantial voltage drop across the surface, and current can flow up one leg and down the other, stopping your heart. If you stand on one leg, there is no path for the current to take, and you should be unharmed.

If you stand on something conductive, such as a piece of aluminum foil, the aluminum foil is highly conductive, so there are no voltage gradients across it, so the current flows through the metal and not through your body. In a lightning storm, you should take off your aluminum foil hat and stand on it. That will protect you from ground currents.

It is well known that lightning strikes can kill a whole herd of animals. I suspect that is why many herd animals stampede. When running, a four legged animal has at most one foot on the ground at a time. In that state, the animal cannot be killed by ground current. An individual animal might be killed by a direct strike, but all the others in the stampeding herd would survive.

About avoiding electrical stuff inside a house. There are 2 risks here, the direct lightning strike and surges conducted in by wires. Any kind of material, even non-conductors such as wood or brick provide a little protection from a direct strike. The reason is that it takes a higher voltage to breakdown wood than it does air, so the lightning will (usually) contuct around the wood or brick. However, any condutor above the surface of the Earth provides a closer target and will be preferentially hit by lightning. Usually this effect doesn't extend much more than a distance equal to the height of the object. But an object doesn't need to be a "good" conductor, most any object is "good enough". A house is "good enough".

If a house contains conductive object, like pipes or wires, and lightning hits the house, it will be conducted in those items. Similarly, if lightning hits wires outside the house, it can be conducted in. This is by far the largest danger, that lightning will hit phone, power, or cable TV wires and be conducted along them into your house. Water pipes are usually in direct contact with ground, so they won't conduct lightning in, they would be the "sink" for the currrent to go out. Unless you are directly between the source (the wire) and the sink (a water pipe), it is not likely you would be injured.

I see no risk from watching TV, so long as you use a remote and are not in direct contact with it. If lightning hits your house, it is more likely to damage electronic appliances if they are on, but the only danger to you is if you are in direct contact with them. I have had stuff damaged by lightning even when it was off.

The safest place to be is one floor above the lowest one (i.e. on the first floor, not the basement). A ground current could conceivably flow across the basement floor. The normal metal in a house, electrical wiring, plumbing, etc. is likely enough to have a Faraday cage effect, so long as you are a couple of feet away from those conductors. Usually the conductors are in walls, so being a few feet away from the walls, in a central location in the house would be the safest.

There is zero risk from cell phones or other radio operated equipment, or battery operated equipment.

You are not even close to being a geek. A true geek (such as myself), knows all of this off the top of their head. ;)

Just Suzanne said...

Being a geek is one of the many reasons I've read your blog since the beginning. :) Nice to see that every walk of life has our presence. :)

Just got back from a (much-needed) longish trip west (for the first time!) and never appreciated how being in a remote area can restore one's sense of balance. Driving through Wyoming, not seeing another car for long periods of time, having no cell coverage -- bliss, for a short time, at least. :)

Welcome back! And what do you think of "How Doctors Think"? That's on my list to check out at the library.

GirlTuesday said...

as a long-time reader/lurker, i concur (and second) suzanne's geek compliment. i really enjoy your writing & observations.