Lightning, Electrical breakdown, and Current Recently a discussion came up about this gif: http://i.imgur.com/r9Q8M4G.gifv
This lead to a few questions about electrical breakdown of insulators. The disagreement we're having is whether or not a current is present before a lightning strike occurs. I personally don't think there is any measurable current between the clouds and the ground prior to the electrical breakdown of the air, but other disagree with me. Others went as far as to say that there needs to be a current for lightning to happen. Considering that you don't need a current to polarize a dielectric, that sounds totally wrong to me, although the amount of disagreement I'm receiving has caused me to second guess myself. 
Unfortunately I can't give any more context to this without posting individual comments, so to sum about the question:


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*Is there a current between the air and ground prior to a lightning strike, or is the potential difference solely responsible for the electrical breakdown and discharge?

 A: before the lightning strike occurs, this happens: 
the local field strength inside the cloud is strong enough to cause the creation of a thin column of the field which protrudes out of the cloud towards the ground (or from the ground towards the cloud, depending on the charge polarity). enough current to ionize the path flows during the creation of that protrusion. 
once that protrusion has traveled ~ tens of feet, it temporarily halts until the field ahead of it readjusts to its presence and then the process repeats. the resulting concatenation of protrusions is called a "stepped leader" and it progresses in this manner through the air towards the ground (or cloud). when it comes close enough to the ground (or cloud) to complete the discharge circuit, then a gigantic pulse of charge flows through the pre-existing stepped leader and the lightning strike develops along that path. 
the usual observation is that after the stepped leader has closed the circuit, a surge of extremely high amperage flow called the return strike goes through that ionized channel and discharges the potential difference between the cloud and ground. the return strike can be composed of several discrete pulses which follow the original channel before the ions in it have an opportunity to recombine and quench the channel, causing the lightning strike to hit the same place several times. 
