St. Elmo's Fire- Name given to a faint, soft blue-green glow around objects (particularly on airplanes or the masts of ships) during a thunderstorm. St Elmo's fire is thought to be a form of corona discharge caused by the high electrical potential created by a thunderstorm. The presence of St. Elmo's Fire on objects on the ground often indicates that a very close or direct cloud-to-ground lightning strike is imminent.