These conditions may apply.
Have you seen lightening? This is your answer.
Air doesn’t conduct electricity and acts as an insulation. However, just like all other things in nature, it has a limit (KV/cm).
Air is considered an insulator as long as there is less than 1cm between the conductors. If the voltage is higher than that, then the phenomenon of ‘insulator break down’ occurs. Air starts conducting. Sparking is common.
The electrons in an insulator begin to break down when they are exposed to high levels of electric field. They then start conducting electricity.
Although this is true, humidity can also play a major role in the process. Because water vapour molecules and air act together as an insulation media, the net strength of the insulator decreases (as water is a conductor for electricity).