A Staff Report from the Straight Dope Science Advisory Board

What is St. Elmo's fire?

January 12, 2015

Dear Straight Dope:

While listening to the classic eighties song "St. Elmo's Fire," it occurred to me I had absolutely no idea what St. Elmo’s fire was. Please help me so that when I sing it in front of my friends, I can back up what I say!

SDStaff Bricker replies:

BILLY: "Jules, y'know, honey… this isn't real. You know what it is? It's St. Elmo's fire. Electric flashes of light that appear in dark skies out of nowhere. Sailors would guide entire journeys by it, but the joke was on them …there was no fire. There wasn't even a St. Elmo.”

Rob Lowe’s character in the eponymous 1985 brat-pack movie offers the foregoing explanation of St. Elmo’s fire, but don’t worry if you missed the scene: he's not entirely right. If you really want to follow your tune with a talk about ionization of air molecules, you’re starting at the right place by reading this.

First, there was in fact a St. Elmo. Erasmus of Formiae, who lived in the third century, was one of Christianity’s earliest saintly intercessors on behalf of humanity. In addition to being renowned for efficacy against intestinal aliments, Erasmus, also called Elmo, was the patron saint of sailors.

His life, chronicled in Jacobus de Voragine’s “Golden Legends,” a medieval compendium of lives of the saints, says he continued preaching the Gospel even after a lightning bolt struck the ground beside him, leading sailors, often at risk from lightning themselves, to see him as an appropriate patron. Since the atmospheric electrical display at issue here was often seen from the tops of ships’ masts, it was only natural for sailors to associate the glowing display with their patron saint.

As far as we know, however, sailors didn’t guide their journeys by the eerie light, except to the extent that they saw it as an omen for good or for ill. The Roman Pliny the Elder, commenting on the phenomenon well before St. Elmo came along, wrote that one fiery apparition foretold a bad storm, two during a storm were a good sign, and three appearances were an even worse sign than one.

In general, though, since St. Elmo’s fire tends to appear as a storm is ending, the appearance of the flame-like discharge has been taken as a positive sign — an answer to the prayers of storm-tossed sailors.

So what exactly is St. Elmo’s fire? It's what’s known as point discharge, which occurs when the potential strength of an electrical field reaches a certain threshold — typically 100,000 volt per meter (specifically vertical meter in the case of St. Elmo’s fire, since the field strength of atmospheric electricity varies with height).

We see this sort of thing all the time, not just during storms. Fill a glass tube with gas, apply an electric charge to both ends, and the gas glows as the electrons travel through it, making possible the familiar phenomenon known as a neon sign. When the same thing happens in the open air, it’s St. Elmo’s fire.

Under normal conditions, the atmosphere has an electrical field strength of only 100 volts per vertical meter. Immediately prior to a lightning discharge, on the other hand, the electrical potential of the atmosphere can reach 1,000,000 volts per meter.

The greater the distance from the ground, the greater the difference in electrical potential. If the potential changes at, say, 100 volts per meter, then a point 20 meters up will have a potential difference of 2000 volts relative to the ground.

Suppose a mast is 20 meters tall and that it’s electrically conductive. The tip then has the same potential as the base — let’s say it’s zero volts. Meanwhile, the adjacent air is at 2000 volts. That difference is enough to cause electrons to leap from the tip to nearby air molecules.

You see the same thing when you shuffle across a carpet, then touch a doorknob during winter. When there’s enough difference in electrical potential between two objects, sparks fly.

The flow of electricity isn’t necessarily visible. But when the electrical field is sufficiently strong — that is, in the 100,000 volts per meter range — the electrons don’t simply leap from one atom to another and get captured. Instead, they carom around like pool balls, knocking other electrons in air molecules from their neutral orbits into higher ones and leaving the molecules electrically charged, a process called ionization.

When an excited electron drops from that higher orbit back into a stable orbit, the process releases a photon — what we see as light. If enough collisions occur in the small area surrounding the tip of a mast, the result is the glowing, flame-like luminescence of St. Elmo’s fire: excited atoms in air molecules releasing photons as a result of collisions with energetic free electrons.

Considering how hard it is to rhyme “energetic free electrons,” I suppose we can give songwriter John Parr a pass for not including the term in his 1985 theme for the movie St. Elmo’s Fire. But now that you’ve got the explanation and are ready to combine karaoke and physics, the world can brook no more delay. Here’s hoping your friends appreciate your efforts.

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Staff Reports are written by the Straight Dope Science Advisory Board, Cecil's online auxiliary. Though the SDSAB does its best, these columns are edited by Ed Zotti, not Cecil, so accuracywise you'd better keep your fingers crossed.

References

“Golden Legends, or Lives of Saints,” compiled by Jacobus de Voragine, Archbishop of Genoa, c. 1275, at http://www.fordham.edu/halsall/basis/goldenlegend/, retrieved 9/23/2009

“Weather Phenomenon and Elements: The Fire of St. Elmo,” by Keith C. Heidorn, PhD, at http://www.islandnet.com/~see/weathe...nts/stelmo.htm, retrieved on 9/23/2009

"What causes the strange glow known as St. Elmo's Fire? Is this phenomenon related to ball lightning?", by William Beatty, Scientific American at http://www.scientificamerican.com/ar...uses-the-stran retrieved on 9-23-2009.

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