St Elmo's fire. Historical artwork of St Elmo's fire glowing from the tops of a ship's masts. St Elmo's fire is a continuous electrical discharge that occurs towards the end of thunderstorms. The air between the surface and the clouds becomes strongly electrically charged. The electrical fields are most intense at the tips of pointed objects, such as a ship's masts. These very strong electrical fields ionise, or excite, the air molecules, changing them to a plasma state and causing them to release energy as light, resulting in a glow. The ship in this illustration is Christopher Columbus's caravel.