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St. elmo's fire (corposant)
This is a visible brush or glow discharge and generally occurs when there is interference in the atmospheric electrical field. During daylight and calm it is generally recognised by a gentle hissing and sizzling, at night it is also clearly visible. It occurs at points extending above or outside their surroundings. Interference with the atmospheric electrical field occurs as a rule during weather conditions combined with precipitation. St. Elmo's fire may occur during thunderstorms, snowstorms and hailstorms, but it is also sometimes observed during bright weather. During these discharges the earth may in one case be the positive, and in the other case the negative electrode (the other electrode is 'located' in the charged layers of air); during a thunderstorm the St. Elmo's fire may often change sign several times in a short time. On a positive pole the discharges take the form of helved wide-opened plumes, to a height of 1 to 10 cm; on a negative pole the form of discharge is more concentrated, so larger parts of objects extending out of their surroundings are surrounded by an even light. The separate light beams are then much smaller, around 1 cm high.