​​What is St. Elmo’s Fire?

Abstract energy background of a blue ball or ring of plasma.

St. Elmo’s fire is a form of plasma. The electric field around the object in question causes ionization of the air molecules, producing a faint glow easily visible in low-light conditions. Conditions that can generate St. Elmo’s fire are present during thunderstorms, when high voltage differentials are present between clouds and the ground underneath.
The nitrogen and oxygen in the Earth’s atmosphere cause St. Elmo’s fire to fluoresce with blue or violet light; this is similar to the mechanism that causes neon lights to glow.
St. Elmo’s fire is named after St. Erasmus of Formia (also called St. Elmo, one of the two Italian names for St. Erasmus, the other being St. Erasmo), the patron saint of sailors. The phenomenon sometimes appeared on ships at sea during thunderstorms and was regarded by sailors with religious awe for its glowing ball of light, accounting for the name. Sailors may have considered St. Elmo’s fire as a good omen (as a sign of the presence of their patron saint).