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Senior Staff Writer & Space Correspondent

Electric fields in the atmosphere give rise to many spectacular phenomena such as lightning and red sprites. Among them, is the peculiar St Elmo’s Fire, a weather phenomenon that creates plasma between an object and the air around it. So, how does it occur, is it dangerous and why is it seen as both a blessing and an omen?

St Elmo's fire is not technically lightning, it's a continuous electric spark. It occurs when the atmosphere becomes charged and strong enough to cause a discharge of plasma between an object and the air around it. Luminous plasma forms around rod-like objects such as ship masts, spires, chimneys, cell phone towers, and in certain cases, animal horns. 

What people are seeing is a continuous corona discharge. Electric charges tend to accumulate around sharp edges instead of flat surfaces, and when there are enough of them (under a strong electric field), they can ionize air molecules. This makes the nitrogen and oxygen in the atmosphere fluoresce with a blue or violet glow, visible under low light conditions. If a neon sign tube was filled with a nitrogen-oxygen mix instead of neon gas (which glows orange) then it would light up blue-violet.

Given the need for a strong electric field for this plasma to be created, it is not surprising that it is often connected to electric storms. Scientists also discovered that objects on the ground and flying objects produce slightly different types of corona discharge, and, more recently, that stronger winds actually lead to a weaker glow.

Artificially, Nikola Tesla created St Elmo’s fire in 1899 while testing a tesla coil. The corona discharge was seen around the coil and allegedly also on the wings of butterflies flying about in the electrified air of the lab.  

The common English name for the phenomenon is St Elmo’s fire because sailors have reported seeing corona discharge on the tip of sailboats' masts for thousands of years. St Elmo, or Saint Erasmus of Formia, is the patron saint of sailors, and this was often seen as a good omen.

But observations of this phenomenon long pre-date Christianity. The Greeks referred to it as Helene – which means "torch" – or as Helen’s twin brothers Castor and Polydeuces if two instances were present. References to such phenomena have been found in Ancient Greek literature as well as observations recorded by Julius Ceasar and the Roman naturalist Pliny the Elder.

One of the earliest literary descriptions of St Elmo’s fire as a blessing appears in Ludovico Ariosto’s epic poem Orland Furioso (1516). In 15th-century China, Admiral Zheng He and his associates wrote about the phenomenon as a divine omen of Tianfei, a Chinese goddess of sailors and seafarers. Reports of it as a blessing are often found in the journals of seafarers.

“On a second night we witnessed a splendid scene of natural fireworks; the mast-head and yard-arm-ends shone with St.Elmo's light; and the form of the vane could almost be traced, as if it had been rubbed with phosphorous," Charles Darwin wrote in July 1832 in The Voyage Of The Beagle. 

"The sea was so highly luminous, that the tracks of the penguins were marked by a fiery wake, and the darkness of the sky was momentarily illuminated by the most vivid lightning."

Not all of its connotations are positive, however. Because it often accompanies stormy weather, many sailors saw it as a bad omen too. Famously, in Shakespeare’s The Tempest it is the very sign preceding the eponymous storm. St Elmo’s fire can also warn of an imminent lightning strike. Survivorship bias may link the phenomenon with a blessing because sailors that saw St Elmo’s fire atop their mast before a lighting strike may not have survived to tell the tale.

These days, sightings of the phenomenon are often linked to airplanes as it can occur along the wings and nose of aircraft. There have been sightings of it from airplanes that have crashed, like Air France flight 447, which crashed into the Atlantic in 2009 killing all on board, but it's not thought the St Elmo#s fire was a factor.

Natural phenomena are obviously neither good nor evil, but they can be useful. Corona discharge has plenty of technical applications, including helping protect airplanes from dangerous lightning strikes. 

In 2020, MIT scientists discovered that St Elmo's fire can actually be used to cushion airplanes during flights by using the phenomenon to control the electrical discharge of an aircraft.

It can also be used to alter the properties of surfaces, making them better at having ink coatings or adhesives stick to them. Corona discharge is also used as a disinfectant, making it safe to clean and reuse N95 respirators, which were in short supply during the COVID-19 pandemic. So maybe it can be a blessing.

Senior Staff Writer & Space Correspondent

Alfredo (he/him) has a PhD in Astrophysics on galaxy evolution and a Master in Quantum Fields and Fundamental Forces.

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