A luminous, often blue or violet electrical discharge that appears at the pointed tips of tall, grounded objects, such as ship masts, aircraft wings and church spires, during strong atmospheric electric fields associated with thunderstorms or blowing snow. It is a form of coronal discharge caused by the ionization of air around a sharply pointed conductor, rather than a lightning strike itself, and takes its name from Saint Erasmus of Formia, patron saint of sailors.
Facts
PeriodicityNot periodic. It appears only when the local electric field around a grounded pointed object reaches about 100 kilovolts per meter, chiefly during thunderstorms or volcanic eruptions. 1 ScaleA localized effect confined to the tip of a rod-like object such as a ship mast, aircraft wing, spire or chimney, not a large-scale atmospheric event in its own right. 1 First DescribedBenjamin Franklin proposed the underlying electrical mechanism in 1751, well after sailors had long regarded the glow itself with awe. 1 Identification
Distinguishing AppearanceA blue or violet glow around the tip of a grounded pointed object, often with a hissing or buzzing sound, most visible in low light such as during a nighttime thunderstorm. 1 Sources
1. St. Elmo's Fire (Wikipedia)
Wikimedia FoundationLead section
a blue or violet glow around the object, often accompanied by a hissing or buzzing sound
Cause section
In 1751, Benjamin Franklin hypothesized that a pointed iron rod would light up at the tip during a lightning storm, similar in appearance to St. Elmo's fire.
Cause section, field strength threshold
A local electric field of about 100 kV/m is required to begin an electric discharge in moist air.
Lead section, corona discharge object
a corona discharge from a rod-like object such as a mast, spire, chimney, or animal horn in an atmospheric electric field
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