The storm watch had been set, and all hands awaited the onset of the tempest that raged to windward. Rushing ahead of gale winds, heavy seas rocked the brig. Then suddenly, at the mastheads and bowsprit, ghostly blue flames leapt into the somber night, lighting the masts like candles. The atmosphere of dread anticipation split at the sight. The sailors to a man breathed a sigh of relief, for their patron saint Elmo had come to watch over the brig and see her safely through the storm.
St. Elmo's Fire has long served as an omen of heavenly intervention to sailors. The ancient Greeks termed a single jet of the fire, Helena, and a double jet, Castor and Pollux. It has also been known by the names St. Nicholas and St. Hermes, corpusante and Corpus Santos. The name of St. Elmo is attributed to an Italian derivation of Sant 'Ermo or St. Erasmus (circa 300 A.D.), the patron saint of the early Mediterranean sailors challenging the powers of storm and sea in small sailing vessels.
Julius Caesar wrote in his Commentaries: "In the month of February about the second watch of the night, there suddenly arose a thick cloud followed by a shower of hail, and the same night the points of the spears belonging to the Fifth Legion seem to take fire."
Of all the varied names attributed to this phenomenon, St. Elmo is the one most often passed down in English language chronicles. Mention of St. Elmo's Fire can be found in the journals of sailors from crews of the early explorers Columbus and Magellan, the tales of illiterate sailors as well as those of Shakespeare and Melville, and the notes of Charles Darwin during his voyage on H.M.S. Beagle.
A chronicler of Magellan's voyage to circle the globe, observed:"During those storms the holy body, that is, to say St. Elmo, appeared to us many times in light...on an exceedingly dark night on the maintop where he stayed for about two hours or more for our consolation."
Darwin wrote in a letter to J.S. Henslow that one night when the Beagle was anchored in the estuary of the Rio Plata: "Everything was in flames, the sky with lightning, the water with luminous particles, and even the very masts were pointed with a blue flame."
The appearance of St. Elmo's Fire was regarded as a good omen, for it tended to occur in the dissipating stages of severe thunderstorms when the most violent surface winds and seas were abating. Thus, it was interpreted as the answer to the sailors' prayers for heavenly intervention. Its appearance preceding a storm or during fair weather portended that the guiding hand of St. Elmo would be present.
According to Francis Bacon quoting Pliny (Roman naturalist): "If it [St. Elmo's Fire] be single, prognosticates a severe storm, which will be much more severe if the ball does not adhere to the mast, but rolls or dances about. But if there are two of then, and that, too, when the storm has increased, it is reckoned a good sign. But if there are three of them, the storm will become more fearful."
In Melville's Moby Dick, Ishmael observes: "All the yardarms were tipped with a pallid fire, and touched at each tri-potential lightning rod with three tapering white flames, each of the three tall masts was silently burning in that sulphurous air, like gigantic wax tapers before an altar....in all my voyagings seldom have I heard a common oath when God's burning finger has been laid on the ship..."
With the advent of Franklin's lightning rod, church spires and metal weather cocks, St Elmo's Fire came inland, especially in the thundery weather of the North American continent, inspiring tales of ghosts and spirits. With the age of flight, the Fire has appeared along the wing tips, propellers, and antennae of aircraft, often disrupting radio communications. There is even a theory that the Hindenburg Zeppelin disaster may have been sparked by St. Elmo's Fire igniting leaking hydrogen.
Physical descriptions of St. Elmo's Fire have ranged from a ghostly dancing flame to natural fireworks. It usually is of a blue or bluish-white colour attached to fixed, grounded conductors and has a lifetime of minutes. The flame is heatless and non-consuming, occasionally accompanied by a hissing sound. These latter properties promote the myths of spiritual presence. The biblical burning bush that was not consumed may have been displaying one form of St. Elmo's Fire.
Despite the mythology of divine intervention that has arisen from this natural phenomenon, St. Elmo's Fire has a scientific explanation. Benjamin Franklin first correctly equated the Fire to atmospheric electricity in his 1749 description of the lightning rod which he believed could draw the electrical fire "out of the cloud silently before it could come near enough to strike; and a light would be seen at the point like the sailor' corpuzante" (St. Elmo' Fire).
The phenomenon is scientifically known as a corona or point discharge. It occurs on objects, especially pointed ones, when the electrical field potential strength reaches about one thousand volts per centimetre. (When the electrical potential field is great enough to overcome the resistance of medium across which it occurs, a current of electrons will result (Ohm's Law).) During fair weather, the electrical field strength of the atmosphere is about 1 volt per centimetre. In the initial stages of cumulonimbus (thunderstorm) formation, however, the field increases to 5 volts per centimetre, and just before a lightning flash, reaches ten thousand volts per centimetre. Thus, the atmospheric electrical field is only strong enough, under normal circumstances, to produce St. Elmo's Fire during thundery weather. When the storm is particularly heavily charged, leaves, blades of grass and even the horns of cattle may glow at their tips. In fact, the glow of St. Elmo's Fire has often been observed on sharp objects in the vicinity of tornadoes.
When objects rise above the surface, they retain, to some degree, the electrical potential of the ground (that is, they are grounded). For example, a 10-meter high object, if it were a perfect conductor, would have the same electrical potential at its top as its base. If the atmospheric electrical field decreased at a rate of 100 volts per meter (1 volt per centimetre), the top of the perfectly conducting object would be at a potential 1000 volts higher than the air surrounding it. This would cause a weak electrical current to flow from the object to the air.
Electrical current flows away from a point of higher potential to one of lesser potential, which is a normal, usually invisible process. However, when the electrical potential field becomes sufficiently strong, electrons torn from molecules of the higher potential surface may acquire enough energy between initial escape and collision with another molecule to avoid capture. Instead, the collision will tear off another electron, ionizing the molecule. When the collisions between free electrons, ionized molecules (ions), and molecules of the air become frequent, enough energy is available to excite air molecules into luminosity. If the region of collisions is confined to a small volume, such as around a pointed object, the luminosity, under low ambient light, becomes visible as a blue or bluish white glow.
Near any point conductor projecting into the atmosphere such as a ship's mast, the lines of electrical force are deflected from their normal position and tend to concentrate around the tip. Near this point, the electrical field potential strength will be considerably higher than that of the undisturbed atmosphere. The extent of this concentration is dependent upon the geometry of the object -- the sharper the point, the stronger the surrounding field. (It is for this reason that lightning rods work.) Thus, ionization may occur around sharpe points under atmospheric conditions that would not ordinarily give ionization. When an isolated point is raised to 30 to 40 metres, such as the mast of a large ship, coronas can occur in electrical fields of around 200 volts per centimetre.
St. Elmo's Fire also forms on aircraft flying through heavily charged skies, often as a precursor to a lightning strike. The glow can be seen concentrated on wing tips, antennae, the tail, nose and propeller blades when the potential difference is large enough. St. Elmo's Fire can be heard "singing" on the craft's radio, a frying or hissing sound running up and down the musical scale, according to some pilots.