St. Elmo’s fire is a small, bright, stationary electrical discharge that appears around pointed objects during a thunderstorm. It is a corona discharge: a strong atmospheric electric field ionizes nearby air, producing a faint blue or violet plasma glow.
The effect is most visible at the tips of masts, spires, chimneys, lightning rods, aircraft surfaces, and sometimes animal horns. Pointed shapes concentrate electric fields, so they can produce a visible discharge at lower voltages than rounded objects. A hiss or buzz may accompany the glow.
The name comes from St. Erasmus of Formia, also known as St. Elmo, the patron saint of sailors. Sailors historically regarded the glow as an omen or warning of electrical danger. It is not ball lightning: St. Elmo’s fire clings to an object, whereas ball lightning is reported as a free-floating luminous sphere.