St. Elmo’s Fire Explained: Why Masts, Planes and Mountaintops Glow Blue in Storms

In the middle of a thunderstorm, the windshield of a big military jet starts to crawl with light. Thin violet threads branch across the glass like frost forming in fast motion, flicker, fade and come back. Nothing has hit the plane. Nothing is on fire. The crew has just flown into one of the oldest weather phenomena people have ever written down: St. Elmo’s fire.

Sailors saw the same eerie glow for thousands of years, perched like a candle flame at the tops of their masts during storms. Some took it as a saint’s blessing, others as a death sign. Hikers know it as the buzzing of an ice axe or hair standing straight up on a ridge, and pilots see it on windshields and wingtips. It is real, it can be photographed, and it is one of the few kinds of plasma you can see in nature without looking at the sky.

Here is what St. Elmo’s fire actually is, why it always shows up on points and edges, where the name comes from, the most famous sightings at sea, on land and in the air, and what it means if you ever see it near you.

What St. Elmo’s fire actually is

St. Elmo’s fire is a weather phenomenon in which a glowing plasma forms around a pointed or rod-like object, such as a ship’s mast, a church spire, a lightning rod, an aircraft’s nose or even an animal’s horns, when the air around it sits in a very strong electric field. Physicists call it a corona discharge. It looks like a blue or violet glow, sometimes a steady flame-like tuft and sometimes flickering streamers, and it often comes with a hissing, crackling or buzzing sound.

Because its brightness depends on how strong the electric field is, it shows up mainly during thunderstorms, in snowstorms with heavy electrical activity, and around volcanic eruptions, when huge differences in electrical charge build up between clouds and the ground. It is not lightning, and it does not jump from place to place like a spark. It stays attached to the object for as long as the field stays strong, which can be seconds or many minutes.

The physics: why sharp points glow first

Air is normally a good insulator. But a thunderstorm can charge the ground and everything standing on it relative to the clouds above, and that charge collects most densely on sharp points and edges. The electric field around a point is far stronger than the field around a flat or rounded surface carrying the same voltage, because the field lines crowd together where the surface curves tightly. That is why a mast top or a needle glows long before the deck or the hull does.

Once the local field gets strong enough, about 100 kilovolts per meter is enough to start a discharge in moist air, stray free electrons near the point are accelerated hard enough to knock electrons off nitrogen and oxygen molecules. Those new electrons do the same, and a small avalanche of ionization builds up right at the tip. The air there becomes a weakly ionized gas, a plasma, and the excited molecules give off light as they settle back down.

The color comes from the air itself. Nitrogen and oxygen glow blue and violet when excited this way, by the same basic mechanism that makes a clear gas-discharge tube glow without any phosphor coating. A neon sign is red because it is filled with neon; St. Elmo’s fire is blue-violet because it is made of ordinary air. The hiss and crackle are the sound of countless tiny discharges flickering on and off at the tip. Electricity in air makes light in odder places too, such as the triboluminescent flash of a crunched wintergreen candy.

Benjamin Franklin saw the connection in the 1750s. In 1751 he suggested that a pointed iron rod would light up at its tip during a lightning storm, in a way that looked like St. Elmo’s fire, an idea tied to his work on the lightning rod. Engineers deal with the same effect today on high-voltage power lines, where corona rings are designed to spread the field out and keep the air around fittings from glowing, humming and wasting energy.

Phenomenon	What it is	How long it lasts	Where you see it
St. Elmo’s fire	A steady corona discharge on a pointed object in a strong electric field	Seconds to many minutes	Masts, spires, aircraft noses and windshields, ice axes, cattle horns
Lightning	A giant electrical breakdown that jumps between cloud and ground or within clouds	A fraction of a second	Anywhere under a thunderstorm
Ball lightning	A rare, still poorly understood floating glowing sphere often reported during storms	Reported as seconds	Mostly eyewitness reports, often confused with St. Elmo’s fire
Ash glow on aircraft	Light flashes from ash particles striking a plane’s leading edges	While flying through the ash cloud	Aircraft in volcanic ash, as on British Airways Flight 9 in 1982

Where the name comes from

The name honors St. Erasmus of Formia, a bishop martyred around the year 303 and a patron saint of sailors, whose name was shortened over the centuries to Elmo, or Telmo in Spanish. Spanish and Portuguese seamen had a second St. Elmo too: Peter González, a 13th-century Dominican friar from Castile who devoted the end of his life to preaching to mariners. He was also popularly called San Telmo, and paintings often show him holding a ship and a candle with a flame.

Portuguese sailors called the glow corpo santo, the holy body, and English sailors turned that into corposant. The ancient Greeks had their own names: a single flame was called Helene, a torch, and a pair was called Castor and Pollux after the twin brothers of Helen, protectors of seafarers. Roman writers knew it as well. Pliny the Elder described it in his Natural History, and the account of Julius Caesar’s African war mentions it too.

A sailor’s omen: famous sightings at sea

For people far from land in a storm, a quiet glow at the top of the mast was impossible to ignore. In 15th-century China, Admiral Zheng He and his associates carved an inscription at Changle, honoring the sea goddess Tianfei, that describes how during a hurricane "suddenly a divine lantern was seen shining at the masthead" and the danger seemed to pass.

Antonio Pigafetta, who sailed with Ferdinand Magellan on the first voyage around the world from 1519 to 1522, recorded the lights several times off the coast of South America, and the crew took them as good omens. In 1788, on the voyage that would end in the famous mutiny, William Bligh wrote in the log of HMS Bounty in the South Atlantic: "Some electrical Vapour seen about the Iron at the Yard Arms about the Size of the blaze of a Candle."

A young Charles Darwin saw it from the Beagle in 1832, while the ship lay in the estuary of the Rio de la Plata. In a letter to his mentor John Stevens Henslow he described lightning in the sky, glowing particles in the water, and how "even the very masts are pointed with a blue flame." In The Voyage of the Beagle he added that the masthead and yardarm ends shone with St. Elmo’s light.

Not everyone found it comforting. Richard Henry Dana wrote in Two Years Before the Mast that sailors watched to see whether a corposant climbed or sank in the rigging, believing that a light coming down meant a storm, and that it was a fatal sign to have its pale light fall on your face. Herman Melville gave the corposants a whole chapter of Moby-Dick, "The Candles," where they burn on the masts of the Pequod during a typhoon.

On land: glowing horse ears and cattle horns

St. Elmo’s fire is not just a sea story. On the night of February 20, 1817, the Scottish surgeon James Braid, later known for his work on hypnosis, mounted his horse during a severe electrical storm at Leadhills in Lanarkshire and noticed that the tips of both of the horse’s ears were glowing, along with the edges of his own hat. He watched a swarm of minute sparks darting toward them until wet snow put the glow out.

Weeks earlier, a luminous snowstorm in Vermont and New Hampshire reportedly left glowing discharges on roof peaks, fence posts, and even people’s hats and fingers. Blue fire on the horns of cattle during storms on the trail became a staple of westerns, from the TV series Rawhide to Larry McMurtry’s novel Lonesome Dove.

In the air: windshields, wingtips and a famous night in 1945

Today the people most likely to see St. Elmo’s fire are pilots. When a plane flies through a charged storm cloud, its pointed nose, wingtips, propeller tips and the edges of its windshield become exactly the kind of sharp, exposed conductors where corona forms. On windshields it usually looks like branching violet tendrils that crawl across the glass. It looks alarming, but it is a familiar sight to crews and usually harmless to the aircraft.

One of the most vivid descriptions comes from August 9, 1945. The New York Times reporter William L. Laurence was aboard The Great Artiste, an instrument plane flying with the B-29 Bockscar on the mission to Nagasaki, when he looked out and saw that "the whirling giant propellers had somehow become great luminous discs of blue flame." Worried about the bomb in the plane ahead, he asked the pilot, Captain Frederick Bock, who told him: "It is a familiar phenomenon seen often on ships. I have seen it many times on bombing missions. It is known as St. Elmo’s Fire."

Eight years earlier, Princeton physics professor Mark Heald was watching the airship Hindenburg land at Lakehurst, New Jersey, on May 6, 1937. About a minute after its lines dropped, he noticed a dim blue flame flickering along the top of the ship’s backbone, and moments later the hydrogen caught fire. In 2009, the crew of Air France Flight 447 is understood to have seen St. Elmo’s fire about 23 minutes before the crash, though investigators found it played no part in the disaster.

Not every glow on a plane is corona. On British Airways Flight 9 in 1982, when a Boeing 747 flew into ash from Mount Galunggung in Indonesia and lost all four engines, passengers and crew saw glowing flashes along the wings and windshield. They looked like St. Elmo’s fire, but the light came from ash particles striking the leading edges, much like the glow seen in sandblasting.

Researchers still study the effect. In 2020 a team at MIT led by Carmen Guerra-Garcia tested an electrically isolated model wing in strong winds in the Wright Brothers Wind Tunnel. On grounded structures like towers, wind makes corona stronger because it blows away the cloud of ions that normally shields the tip. On the floating wing, the opposite happened: as the wind rose, the wing charged negatively, and the glow weakened. The team suggested that a controlled corona could be used to give aircraft a protective negative charge and possibly reduce the chance of a lightning strike.

Is St. Elmo’s fire dangerous?

The glow itself carries only a tiny current and does not burn. What matters is what it tells you. St. Elmo’s fire only forms when the electric field near the ground is extremely strong, and that is the same condition that comes right before a lightning strike. The Wilderness Medical Society’s lightning guidelines list a blue haze around objects or people, static on hair or skin, an ozone smell and a nearby crackling sound as signs that a strike may be imminent. The glow is not the danger. It is the warning.

Mountaineering groups give the same advice. The British Mountaineering Council and Mountaineering Scotland say that if your gear starts buzzing or giving off a blue glow, or your skin tingles and your hair stands on end, you should get off summits and ridgelines, move away from metal gear, and crouch low on your rucksack or a pad with your feet together rather than lying flat. An American Alpine Club article on lightning hazards to mountaineers noted that climbers could hear corona crackling on peaks even when the nearest thunderclouds seemed far away, and advised treating it as a danger sign.

The glow is not the danger. It is the warning.

On flat ground the rule is simpler. The U.S. National Weather Service says no place outside is safe when thunder is within earshot: get into a substantial building or a hard-topped vehicle, and wait 30 minutes after the last rumble of thunder before going back out.

How to see St. Elmo’s fire safely

Most people never see St. Elmo’s fire in person, and you should not go looking for it outdoors in a storm. The safe ways are indirect. Pilots and airline passengers sometimes catch it on windshields in storm clouds, especially at night. Videos and photos from cockpits are plentiful. And you can see the same physics in a science museum or classroom, where a Van de Graaff generator, a Tesla coil or a plasma ball makes violet corona and streamers from a sharp point on demand.

If you are ever outdoors and see a blue glow on a fence, a pole, a hiking pole or a friend’s hair, treat it as an urgent instruction to get to shelter or get low, not as a photo opportunity. Save the sky-watching for calmer strange sights, like a Fata Morgana mirage over cold water or the colors of the northern lights.

St. Elmo’s fire FAQ

Is St. Elmo’s fire the same as ball lightning?

No. St. Elmo’s fire is a corona discharge that stays attached to a pointed object. Ball lightning is a rare, floating, glowing sphere that is still poorly understood, and many old reports mixed the two up.

Why is St. Elmo’s fire blue?

It is made of excited nitrogen and oxygen in ordinary air, which glow mostly blue and violet, the same way a clear gas-discharge tube glows with the color of the gas inside it.

Can St. Elmo’s fire damage a plane?

The glow is usually harmless to aircraft and familiar to crews. It means the plane is in a strongly charged part of a storm, which is something pilots try to avoid for other reasons.

Who was St. Elmo?

St. Elmo is St. Erasmus of Formia, an early bishop and patron saint of sailors. Spanish and Portuguese sailors also gave the name San Telmo to Peter González, a 13th-century friar who preached to mariners.

Further reading