The voltage tears apart the air molecules and the gas begins to glow. St. Elmo's Fire is plasma. A normal gas is composed of molecules. The molecules are composed of atoms, which in turn are composed of electrons and clusters of proton particles. If the electric force applied to each bit of gas is greater than a certain level of voltage, it causes the electrons and protons of the gas molecules to be pulled away from each other.
High voltage transforms the gas into a glowing mixture of separate proton clusters and electrons.
It also fluoresces with light. The color of the glow depends on the type of gas involved. No one knows, because no one knows what ball lightning is, and it might not be a spark at all. St. Elmo's fire is sometimes mistaken for ball lightning. Among other differences, ball lightning can drift around like a soap bubble, while St. Elmo's Fire always remains attached to an object.