Why / how do electric arcs form? I've been looking online for a forum post, website article or indeed a physics stack exchange question that could help me understand why electric arcs form. I've seen plenty regarding the flow of electrons in an arc, their speed, the length of the arc etc. but, to the best of my ability, I was not able to find WHY an electric arc forms. 
Normally, air is rather non-conductive yet if you have enough electric potential difference between two electrodes, the air becomes conductive. Why? 
What is it about this large voltage that causes the air to become conductive?
I read that when the voltage exceeds the break-down voltage of the medium, air in this case, it becomes a plasma and starts conducting. How does this occur?  
Do the air molecules, or perhaps their constituents (atoms, electrons...), align themselves in such a way that they start to conduct? If so, what force acts upon them to behave as such? Is the air subject to an electric field that causes this to happen? If no current is yet flowing between the electrodes how can there be an electric field?
 A: I think that you are mixing up two things one is an arc and the other is a Townsend discharge.
An electric arc is often formed by two electrodes coming together, creating a lot of heat and then the electrodes being separated to form a small gap in which the arc is formed.
The very high temperature created causes a plasma (ionised gas) to be formed between the electrodes.
The plasma is maintained by the thermionic emission (electrons "boiled off" from the very hot electrode) and field (electrons pulled out of electrode by a large electric field) emission of electrons from an electrode.
Those electrons traverse the gap where the atoms are colliding with one another and the electrons and causing excitation and ionisation of the atoms due to their very large "thermal" kinetic energy.  
In another process (Townsend discharge) the mobile electrons which initiate the ionisation of the atoms are generated during the ionisation process itself.
Consider a gas which is between a positive and negative electrode.
Some electrons and ions are created in the gas by cosmic rays, ionising particles produced by radioactive decay etc.
The electrons (and ions) will be accelerated by the electric field between the two electrodes and in doing so gain kinetic energy.
These electrons will collide with the neutral atoms of the gas.
If the electric field is large enough (called the breakdown voltage and about $3\,\rm MV\,m^{-1}$ for air) then the energy gained by an electron between collisions with atoms will be sufficient to ionise an atom thus producing an ion and another electron which can then contribute to further ionisations.
So the number of electron and ions will multiply exponentially (cf avalanche) with the result that the gas now has a lot of mobile charge carriers within it - it has become a conductor.  
Air as a conductor video.
A: Let's assume you start with a very strong electric field between two electrodes. The chain of reaction might then be as follows:


*

*Free electrons are accelerated in the electric field (note that you always have free electrons for various reasons, e.g. from cosmic radiation, in the tail of a Maxwellian) 

*The accelerated electrons will ionize atoms once they have gained enough energy

*The electron which has collided with an atom, is again accelerated in the field, so is the additional free electron (from the atom) and the ion itself

*An avalanche process can start, creating more and more charged particles (assuming you have enough particles and the electric field is large enough)

*At the electrodes itself, additional charged particles can be created by particles hitting the electrodes

*The particles hitting the electrodes will then result in a current flowing from one electrode to the other

*Since a plasma is nothing else than an ionised gas, you have created a plasma (an arc is a plasma) and in a plasma charges are moving around (more or less) freely, thus the conductivity


Edit: just seen Farcher's answer, similar to mine
