The breakdown of air is roughly three kV/mm; however, the precise dielectric strength of air depends on the conditions of the air involved. If, for instance, the air pressure is higher, the air's dielectric strength will also be higher.
Dielectric breakdown in the air occurs following an accumulation of energy up to the level of three kV/mm. An example occurs in conditions leading up to a lightening storm. Electrons from the ground, which act as mobile charge carriers, try to reach the positively charged ions in a cloud. The air in the cloud initially acts as an insulator, preventing electrons from entering. A section of the cloud slowly moves from an insulated state to a state of conduction as electrons begin to interact with the air in the cloud. The electrical fields in the cloud now have the potential to create a conducting path straight to the ground. When the electrons and ions move toward each other, they collide, creating sparks on a non-observable scale. These little collisions charge a section of air in the cloud, enabling more dramatic action. Once this energy has increased to a certain threshold, a lightning bolt is produced. This occurs when the electric field exceeds the dielectric strength of the air.