The negatively charged electrons are pulled in one direction and the positively charged ions in the other. When electrons are removed from a nucleus, it becomes positively charged. When air molecules become ionized in a very high electric field, the air changes from an insulator to a conductor. Sparks occur because of the recombination of electrons and ions. It produces the electric field that exceeds the dielectric strength of air.
Ionized air is a good conductor and provides a path where by charges can flow from clouds to ground.
Its exact value varies with the shape and size of the electrodes and increases with the pressure of the air.