[Joe L. Ogan]

What actually happens when lightning strikes?  Is there always a positive runner?  What does the damage?  Is it the negative runner?  Is it the positive runner?  Is it the heat?  Is there some other force exerted?  Thanks for comments.  Joe L. Ogan

[Joe L. Ogan]

Here are some things that I believe:  There is always a positive runner (from earth if the strike is toward the earth)  There is always a negative runner.  I must confess that I am a little puzzled that a positive runner can go from earth to cloud as the earth is supposed to be bassically negative.  I believe that the heat is what causes the damage.  I am not absolutely sure that there is not some other source of power.  Thanks for comments.  Joe L. Ogan

[David Cooper]

The cloud had an excess of electrons, and that pushes electrons in the ground that are free to move away, thereby making that part of the ground positive due to the relative lack of electrons. The Electrons in the cloud want to get away from each other and the ideal target is that positive ground below. I'm guessing that the positive runner will be formed out of ions and that it eventually forms what is in effect an ion wire from ground to cloud. The electrons in the cloud are then free to flow to the ground. The bang is the air being heated and expanding faster than the speed of sound (sonic boom), and the damage is directly from the heat (burning) and the high voltages flowing through whatever is struck.

[yor_on]

Joe, you might find this interesting too. http://www.thenakedscientists.com/forum/index.php?topic=41094.0

as well as The speed of electricity. 

[Voxx]

Alright, Lightning...  Just so we can throw some numbers out there:

How fast does lightning travel?  An average bolt of lightning travels at 224,000 MPH.  3,700 Miles Per Second and if we put that on an even smaller scale; the actual strike lasts about 30 millionths of a second.

How hot is lightning?  Lightning can rapidly heat the air in its immediate vicinity to about 20,000 C. (36,000 F.)  Lets just say the sun's surface temperature is 5505 C.  So an "average" lightning bolt for that split second is 4x hotter than the surface of the sun.  Positive lightning can even reach temperatures 5x hotter than the surface of the sun.

Now that that's said what about charges?  You see lightning can be Negatively charged, or Positively charged.

Which one is worse?  That would be Positively Charged Lighting, this is usually the cause of most forest fires.  Just to put it on a scale the average positive lightning is 10x that of negative lightning.

How does lightning kill you?  What kills you are the amps, and how many amps does it take to kill a person?  The answer is as low as a few milliamps passing through your heart.

Thunder?  Thunder is a result of the super-heating of the surrounding air, this is in fact two booms.  Just as in sonic jets, once you reach the sound barrier two resounding booms will echo through the surrounding area. 

Then why do people survive?  That is because most the time lightning travels in two paths through a human.  That is the surface of the skin and the pathway through your body.  After a lightning strike some people have damage to their outer skin, consider yourself lucky it didn't pass through your heart.   Lets just say an average "Negative" charge of lightning carries about 30,000 Amperes 30(kA)  Remember it only takes a few milliamps passing through your heart to kill you?

You see lighting is attracted towards the ground, now this also has a negative/opposing reaction.  That is what in the science community calls "Sprites."  See --->  http://en.wikipedia.org/wiki/Sprite_(lightning)

How does lightning form?  I could go on and on about ice crystals rebounding off graupel and causing super cooling, thus the generation creates a positive charge...  But the clouds gain a positive charge!  After that it has to wait for a golden path of ionized particles to lineup and create a pathway from its position towards however close it can get to the ground.

Just to be clear only 20%-10% of lightning strikes actually hit the ground.

Does lightning help anything?  Yes, in fact just to name one, "Plants."  Plants need nitrogen to grow and the air is about 77%-78% nitrogen, (give or take) but the nitrogen is in a form plants cannot use.  Lightning however transcends that barrier and preforms a process we call "flxation" of nitrogen.  Although...  Lightning only provides about 5% of the natural flxation of nitrogen.  Bacteria doing most the work...

I hope you learned a bit about lightning.  xD if you didn't find it too boring.  Anyone can correct me if I miss wrote any information.

[evan_au]

Another source of force is the magnetic field. I have heard of a lightning rod, where the earth conductor was run around a window, forming a half-loop.
When lightning struck the lightning rod, the extremely high currents (mentioned above) caused intense forces that ripped the earth conductor from the wall.

Another risk  is called "Earth potential rise". A lightning strike will increase the voltage for a region around the point of the strike, by 10kV or more, depending how close you are. A wired telephone, which is connected to a telephone exchange several miles away will have a very different voltage than an earthed person holding the telephone - so avoid using a fixed phone in a thunderstorm. This effect can cause failure of faxes & modems attached to the telephone line.

This can affect people and animals in the vicinity of a lightning strike: http://www.youtube.com/watch?v=u_6b6Bk6bM8

Lightning also causes interference to AM radio listeners - more a nuisance than a serious problem (unless the lightning actually hits your antenna!)