A capacitor consists of two conducting plates with opposite charge separated by an insulator. When a wire is connected between the two plates, a large electric current flows the charges rapidly from one plate to the other, neutralizing the capacitor.

The charged regions of the clouds act as conducting plates while the air between them acts as the insulator. The same thing occurs between the lower section of the cloud and the ground. The air between these sections acts as the insulator, but when the forces exerted by the charges on the air molecules are large enough, they can rip the electrons from the molecules. The result is a positively charged molecule, called an ion, and a free electron. The air is changed from an insulator to a conductor. The mobile electrons gain more energy, creating more and more ions and additional free electrons. When the electrons and ions combine again light is emitted. The tremendous amount of energy released rapidly heats the surrounding air, producing thunder.