What is St Elmo’s Fire?

St Elmo’s fire is a familiar phrase to most of us (even if we ignore the film and song). But what is St Elmo’s fire and how is it caused?

(This article was originally published at Scienceray.com on 31 August 2009.)

The weather phenomenon known as St Elmo’s fire has been described by many writers. The logs and memoirs of the early European explorers, on the voyages led by Columbus, Magellan and others, mention the peculiar ‘fire’ on their ships’ masts. Charles Darwin witnessed the lights during his time on the Beagle.

St Elmo's Fire on a Ship's Masts

St Elmo's Fire on a Ship's Masts

Descriptions of St Elmo’s fire have also found their way into works of fiction, including those of William Shakespeare and Herman Melville. Even classical writers, such as Pliny and Julius Caesar, have given us first-hand accounts of the spectacle.

Often seen as an omen, or a manifestation of some mythological or religious presence, the phenomenon is sometimes known by the term ‘corposant’, meaning ‘holy body’. But despite its superstitious associations, St Elmo’s fire does have a scientific explanation.

Electrical Phenomenon

St Elmo’s fire belongs to a class of meteorological occurrences known as electrometeors. The UK Met Office describes an electrometeor as ‘a visible or audible manifestation of atmospheric electricity.’ Other examples include thunder, lightning, the polar aurorae (aurora borealis in the northern hemisphere and aurora australis in the southern hemisphere) and the wonderfully named sprites, blue jets and elves.

More specifically, St Elmo’s fire is an example of a corona or point discharge. Also, whereas the length of a lightning strike can usually be measured in milliseconds, an occurrence of St Elmo’s fire can last for minutes. To demonstrate what happens let us look at an example of a lightning conductor on the side of a cathedral, as in the photograph below.

The lightning conductor in this case is grounded, as is the rest of the building. This means that the electrical potential at point B is almost the same as at point A (subject to any electrical resistance in the material). In the air surrounding the building the electrical potential decreases as we go higher, by about one volt per centimetre in normal atmospheric conditions.

St David's Cathedral, Wales

St David's Cathedral, Wales

The result is a potential difference at point B between the lightning conductor and the air molecules around it. This causes an electrical current to flow from the building to the air. Under normal conditions this current is very weak and unnoticeable, but during periods of high electrical activity, such as a thunderstorm, the rate at which the electrical potential of the air decreases with altitude is greater, leading to an increased potential difference at point B and an increased current.

When the current is very high, the collisions between the free electrons and the air molecules provide enough energy for the air molecules to luminesce. They glow with a green, blue or violet light but produce no heat. The discharge may also be accompanied by a hissing or crackling sound. This is St Elmo’s fire.

Origin of the Name

But why is this phenomenon associated with St Elmo? As the bishop of Formia during the late 3rd and early 4th centuries, St Elmo (originally St Erasmus) was martyred during the Roman emperor Diocletian’s persecution of the Christians. He may be the same person as St Erasmus of Antioch.

As the potential for St Elmo’s fire to occur increases with height, it is more noticeable around the tallest of structures. In a time when most buildings had few storeys, the majority of the tallest structures in the world were the masts of ships. So, as the phenomenon usually occurred near the end of a thunderstorm and was seen mostly around ships, St Elmo’s fire was seen by sailors as proof that their patron saint was saving them. And the patron saint of Mediterranean sailors was St Elmo.

Sea, Land and Air

St Elmo's Fire Across an Aircraft's Windscreen

St Elmo's Fire Across an Aircraft's Windscreen

But St Elmo’s fire doesn’t just occur at sea. I have used the example of a lightning conductor above, but it can be seen elsewhere on land. Height amplifies the effect, but so does an object that comes to a sharp point. It has been observed around tree branches and objects high in the mountains.

The age of flight has taken St Elmo’s fire into the air. When an aircraft flies near to a thunderstorm, the phenomenon can be observed around its wing tips, nose, propellers or windscreen (as in the photograph to the left).

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