St Elmo's Fire
Later explainedSummary
A blue-violet glow appears on the mastheads of ships, the tips of spires, the horns of cattle and the wings of aircraft during storms, sometimes hissing. It is a corona discharge: air ionised by an electric field that concentrates at a sharp point. It burns nothing, it consumes nothing, and sailors were pleased to see it.
What is documented
What it looks like. A luminous blue or violet glow on the extremities of a pointed, grounded object during a thunderstorm or near a volcanic eruption. It is often accompanied by a hissing or buzzing sound. Its intensity rises with the strength of the surrounding electric field.
The physics. Air is an insulator until the electric field across it reaches roughly 30,000 volts per centimetre, at which point it breaks down: the field strips electrons from air molecules and the gas becomes a conducting plasma. In a thunderstorm a large potential difference builds between cloud and ground. The field is not uniform. It concentrates sharply at points of high curvature, which is why a mast, a spire, a lightning rod or a horn reaches the breakdown threshold at its tip while the air a metre away does not. The glow appears exactly where the geometry puts it. Excited nitrogen and oxygen give the discharge its blue-violet colour, for the same reason a nitrogen discharge tube glows blue-violet.
The record. It is one of the longest continuously documented natural phenomena in the archive. Julius Caesar's De Bello Africo describes it. Pliny the Elder records it in the Naturalis Historia. Alcaeus and Xenophanes allude to it earlier still. Antonio Pigafetta logged it on Magellan's circumnavigation. It appears in Camões, in Robert Burton's Anatomy of Melancholy, in Shakespeare's The Tempest, and in Moby-Dick, where Melville calls it by the sailors' name, the corpusants, from corpo santo, holy body. The Oxford English Dictionary's earliest citation for that term is Eden's Arte of Nauigation of 1561.
Darwin. Charles Darwin saw it aboard the Beagle, anchored in the Río de la Plata, and described it in a letter to J. S. Henslow, writing that the very masts were pointed with a blue flame while the sea itself glowed and the sky was lit by lightning.
Vermont and New Hampshire, 17 January 1817. During a luminous snowstorm, St Elmo's fire was reported as static discharges on roof peaks, on fence posts, and on the hats and fingers of people. This is a useful record because it puts the phenomenon on land, on ordinary objects, in front of ordinary witnesses.
Aircraft. Pilots report it routinely on windshields and leading edges. Aircraft carry static dischargers, small wicks on the trailing edges, specifically to bleed the charge away. In 1995 a University of Alaska research flight over the Amazon, studying sprites, observed St Elmo's fire and recorded its optical spectrum.
The name. From St Erasmus of Formia, also called St Elmo, patron saint of Mediterranean sailors. The Greeks called a single glow Helena, and the rarer twin discharge Castor and Pollux, after the Dioscuri and the two brightest stars of Gemini.
Leading explanations
There is only one, and it is not in dispute. St Elmo's fire is a corona discharge. The mechanism is understood, the threshold field is measured, the spectrum has been recorded in flight, and the effect is reproduced daily on high-voltage equipment, where it is a well-known engineering nuisance rather than a portent.
What the popular version gets wrong
"St Elmo's fire is ball lightning." It is not, and the difference is structural. St Elmo's fire is attached to a grounded, pointed object and persists as long as the field does. Ball lightning is reported as free-floating and mobile. The registry classes ball lightning as only partially explained. It classes this as explained. They are not the same phenomenon and conflating them muddies both.
"It is fire." There is no combustion. It is a cold plasma. It does not consume the mast, it does not scorch the hand it appears on, and it stops when the field collapses. The 1817 New England accounts have it sitting on people's fingers.
"Sailors dreaded it." Many welcomed it. It tends to appear toward the end of a storm, as the field peaks and then passes, and it was widely read as the saint's protection. The Greek convention was more precise still: the double form was the good omen, the single form the bad one.
"It only happens at sea." It has been recorded on church spires, power lines, cattle horns, fence posts, human fingers, and the windshields of jet aircraft.
Current status
Later explained. The case is closed and has been for a long time. It is kept in the register because it is the clearest example of what the register is for: an event that was genuinely anomalous to the people who saw it, documented continuously for two thousand years by observers who were not mistaken about what they were seeing, and eventually explained by a piece of physics that is now taught to undergraduates. Nobody was lying. Nobody was hallucinating. They simply did not have the field equations.
Sources
- Pliny the Elder. Naturalis Historia, book 2, paragraph 101.
- Caesar, Julius. De Bello Africo, 47.
- Pigafetta, Antonio. Journal of the Magellan circumnavigation, 1519-1522.
- Darwin, C. R. Letter to J. S. Henslow, 1832, and Narrative of the surveying voyages of His Majesty's Ships Adventure and Beagle, 1839.
- Contemporary accounts of the luminous snowstorm of 17 January 1817, Vermont and New Hampshire.
- University of Alaska sprite research flight, Amazon, 1995: optical spectrum of in-flight St Elmo's fire.
- Standard texts on gas discharge physics for the breakdown field of air and the mechanism of corona discharge.
Last reviewed: July 2026. Records are provisional. Where the evidence changes, the entry changes. Found an error? Tell us.