St. Elmo's Fire is which phenomenon?

• A. A corona discharge lighting up aircraft surfaces where static discharge is greatest
• B. A method to measure airspeed
• C. A type of icing condition
• D. A pilot weather briefing acronym

Answer: (A)

St. Elmo's Fire is a corona discharge—a faint blue-green glow that appears when the electric field around sharp aircraft surfaces becomes strong enough to ionize the surrounding air. During thunderstorm activity or other high-electrical-stress conditions, the air around protruding points on the aircraft (like wingtips or antennae) conducts enough charge that electrons collide with air molecules, producing visible light. It lights up exactly where static discharge is most intense, and while it can look dramatic, it’s a harmless indication of strong atmospheric electrical activity. It isn’t a method for measuring airspeed, nor a type of icing, and it’s not an acronym used in weather briefings.