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St. Elmo's fire (also St. Elmo's light) is a weather phenomenon in which luminous plasma is created by a coronal discharge from a sharp or pointed object in a strong electric field in the atmosphere (such as those generated by thunderstorms or created by a volcanic eruption).
St. Elmo's fire is named after St. Erasmus of Formiae (also called St. Elmo, one of the two Italian names for St. Erasmus, the other being St. Erasmo), the patron saint of sailors. The phenomenon sometimes appeared on ships at sea during thunderstorms and was regarded by sailors with religious awe for its glowing ball of light, accounting for the name. Because it is a sign of electricity in the air, which can interfere with compass readings, some sailors may have regarded it as an omen of bad luck and stormy weather. Other references indicate that sailors may have actually considered St. Elmo's fire as a good omen (as in, a sign of the presence of their guardian saint).
Physically, St. Elmo's fire is a bright blue or violet glow, appearing like fire in some circumstances, from tall, sharply pointed structures such as lightning rods, masts, spires and chimneys, and on aircraft wings. St. Elmo's fire can also appear on leaves, grass, and even at the tips of cattle horns. Often accompanying the glow is a distinct hissing or buzzing sound. It is sometimes confused with ball lightning.
St. Elmo's fire is a plasma, as are flames and stars. The electric field around the object in question causes ionization of the air molecules, producing a faint glow easily visible in low-light conditions. Roughly 1000 volts per centimeter induces St. Elmo's fire; the number depends greatly on the geometry of the object. Sharp points lower the required voltage because electric fields are more concentrated in areas of high curvature, so discharges are more intense at the ends of pointed objects.
Conditions that can generate St.Elmo's fire are present during thunderstorms, when high voltage differentials are present between clouds and the ground underneath. Air molecules glow owing to the effects of such voltage, producing St. Elmo's fire.
In ancient Greece, the appearance of a single one was called helene (Greek: ἑλένη), meaning torch, and two were called Castor and Pollux. Occasionally, it was associated with the Greek element of fire, as well as with one of Paracelsus's elementals, specifically the salamander, or, alternatively, with a similar creature referred to as an acthnici.
References to St. Elmo's fire can be found in the works of Julius Caesar (De Bello Africo, 47), Pliny the Elder (Naturalis Historia, book 2, par. 101), and Antonio Pigafetta's journal of his voyage with Ferdinand Magellan. St. Elmo's fire, also known as "corposants" or "corpusants" from the Portuguese corpo santo ("holy body"), was a phenomenon described in The Lusiads.
In 15th-century Ming China, Admiral Zheng He and his associates composed the Liujiagang and Changle inscriptions, the two epitaphs of the treasure voyages where they made a reference to St. Elmo's fire as a divine omen of Tianfei, the goddess of sailors and seafarers.
The power of the goddess, having indeed been manifested in previous times, has been abundantly revealed in the present generation. In the midst of the rushing waters it happened that, when there was a hurricane, suddenly a divine lantern was seen shining at the masthead, and as soon as that miraculous light appeared the danger was appeased, so that even in the peril of capsizing one felt reassured and that there was no cause for fear.
— Admiral Zheng He and his associates (Changle inscription) 
Robert Burton wrote of St. Elmo's fire in his Anatomy of Melancholy: "Radzivilius, the Lithunian duke, calls this apparition, Sancti Germani sidus; and saith moreover that he saw the same after in a storm, as he was sailing, 1582, from Alexandria to Rhodes". This refers to the voyage made by Mikołaj Krzysztof "the Orphan" Radziwiłł in 1582-1584.
On the 9th of May, 1605, while on the second voyage of John Davis commanded by Sir Edward Michelborne to the East Indies, an unknown writer aboard the Tigre describes the phenomenon; "In the extremity of our storm appeared to us in the night, upon our maine Top-mast head, a flame about the bigness of a great Candle, which the Portugals call Corpo Sancto, holding it a most divine token that when it appeareth the worst is past. As, thanked be God, we had better weather after it".
On Thursday 20th, I was gratified for a few minutes with the luminous appearance described above [viz., "such flashes of lightning from the west, repeated every two or three minutes, sometimes at shorter intervals, as appeared to illumine the whole heavens"]. It was about nine o'clock, P.M. I had no sooner got on horseback than I observed the tips of both the horse's ears to be quite luminous: the edges of my hat had the same appearance. I was soon deprived of these luminaries by a shower of moist snow which immediately began to fall. The horse's ears soon became wet and lost their luminous appearance; but the edges of my hat, being longer of getting wet, continued to give the luminous appearance somewhat longer.
I could observe an immense number of minute sparks darting towards the horse's ears and the margin of my hat, which produced a very beautiful appearance, and I was sorry to be so soon deprived of it.
The atmosphere in this neighbourhood appeared to be very highly electrified for eight or ten days about this time. Thunder was heard occasionally from 15th to 23d, during which time the weather was very unsteady: frequent showers of hail, snow, rain, &c.
I can find no person in this quarter who remembers to have ever seen the luminous appearance mentioned above, before this season,—or such a quantity of lightning darting across the heavens,—nor who have heard so much thunder at that season of the year.
This country being all stocked with sheep, and the herds having frequent occasion to pay attention to the state of the weather, it is not to be thought that such an appearance can have been at all frequent, and none of them to have observed it.
Everything is in flames, — the sky with lightning, — the water with luminous particles, and even the very masts are pointed with a blue flame.
St. Elmo's fire is reported to have been seen during the Siege of Constantinople by the Ottoman Empire in 1453. It reportedly was seen emitting from the top of the Hippodrome. The Byzantines attributed it to a sign that the Christian God would soon come and destroy the conquering Muslim army. According to George Sphrantzes, it disappeared just days before Constantinople fell, ending the Byzantine Empire.
In Two Years Before the Mast, Richard Henry Dana, Jr. describes seeing a corposant in the Horse latitudes of the northern Atlantic Ocean. However, he may have been talking about ball lightning; as mentioned earlier it is often erroneously identified as St. Elmo's fire: "There, directly over where we had been standing, upon the main top-gallant mast-head, was a ball of light, which the sailors name a corposant (corpus sancti), and which the mate had called out to us to look at. They were all watching it carefully, for sailors have a notion, that if the corposant rises in the rigging, it is a sign of fair weather, but if it comes lower down, there will be a storm".
Many Russian sailors have seen them throughout the years. To them, they are "Saint Nicholas" or "Saint Peter's lights". They were also sometimes called St. Helen's or St. Hermes' fire, perhaps through linguistic confusion.
Nikola Tesla created St. Elmo's Fire in 1899 while testing out a Tesla Coil at his laboratory in Colorado Springs. St. Elmo's fire was seen around the coil and was said to have lit up the wings of butterflies with blue halos as they flew around.
Shortly before the crash of the Luftschiffbau Zeppelin's LZ 129 Hindenburg in 1937, Professor Mark Heald of Princeton saw St. Elmo's Fire flickering along the airship's back a good minute before the fire broke out. Standing outside the main gate to the Naval Air Station, he watched, together with his wife and son, as the airship approached the mast and dropped her bow lines. A minute thereafter, by Mr. Heald's estimation, he first noticed a dim "blue flame" flickering along the backbone girder about one-quarter the length abaft the bow to the tail. There was time for him to remark to his wife, "Oh, heavens, the thing is afire," for her to reply, "Where?" and for him to answer, "Up along the top ridge" - before there was a big burst of flaming hydrogen from a point he estimated to be about one-third the ship's length from the stern.
Accounts of Magellan's first circumnavigation of the globe refer to St. Elmo's fire being seen around the fleet's ships multiple times off the coast of South America. The sailors saw these as favorable omens.
On August 26, 1883, the British warship Charles Ball sailing the Sunda Strait en route to Hong Kong came within 20 km of the exploding Krakatau volcano and witnessed a great deal of static electricity in the atmosphere, generated by the movement of tiny particles of rocks and droplets of water from the volcano's steam, which caused spectacular brush discharges taking place from the masts and rigging of the ship.
Among the phenomena experienced on British Airways Flight 9 on June 24, 1982 were glowing light flashes along the leading edges of the aircraft, which were seen by both passengers and crew. While it shared similarities with St Elmo's fire, the glow experienced was from the impact of ash particles on the leading edges of the aircraft, similar to that seen by operators of sandblasting equipment.
The ill-fated Air France Flight 447 flight from Rio de Janeiro-Galeão (GIG) to Paris-Roissy (CDG) in 2009 is understood to have experienced St. Elmo's fire 23 minutes prior to crashing into the Atlantic Ocean. The tropical storm system that gave rise to the phenomenon was a key factor in the Airbus A330-200's eventual crash.
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One of the earliest references to the phenomenon appears in Alcaeus's Fragment 34a about the Dioscuri, or Castor and Pollux. It is also referenced in Homeric Hymn 33 to the Dioscuri who were from Homeric times associated with it. Whether the Homeric Hymn antedates the Alcaeus fragment is unknown.
Also in The Castaways of the Flying Dutchman by Brian Jacques St. Elmo's Fire is mentioned.
The phenomenon appears to be described first in the Gesta Herwardi, written around 1100 and concerning an event of the 1070s. However, one of the earliest direct references to St. Elmo's fire made in fiction can be found in Ludovico Ariosto's epic poem Orlando furioso (1516). It is located in the 17th canto (19th in the revised edition of 1532) after a storm has punished the ship of Marfisa, Astolfo, Aquilant, Grifon, and others, for three straight days, and is positively associated with hope:
But now St. Elmo's fire appeared, which they had so longed for, it settled at the bows of a fore stay, the masts and yards all being gone, and gave them hope of calmer airs.
In Shakespeare's The Tempest (c. 1623), Act I, Scene II, St. Elmo's fire acquires a more negative association, appearing as evidence of the tempest inflicted by Ariel according to the command of Prospero:
Hast thou, spirit,
Perform'd to point the tempest that I bade thee?
To every article.
I boarded the king's ship; now on the beak,
Now in the waist, the deck, in every cabin,
I flamed amazement: sometime I'ld divide,
And burn in many places; on the topmast,
The yards and bowsprit, would I flame distinctly,
Then meet and join.
— Act I, Scene II , The Tempest
Later in 18th century and 19th century literature associated St. Elmo's fire with bad omen or divine judgment, coinciding with the growing conventions of Romanticism and the Gothic novel. For example, in Ann Radcliffe's The Mysteries of Udolpho (1794), during a thunderstorm above the ramparts of the castle:
"And what is that tapering of light you bear?" said Emily, "see how it darts upwards,—and now it vanishes!"
"This light, lady", said the soldier, "has appeared to-night as you see it, on the point of my lance, ever since I have been on watch; but what it means I cannot tell".
"This is very strange!" said Emily.
"My fellow-guard", continued the man, "has the same flame on his arms; he says he has sometimes seen it before...he says it is an omen, lady, and bodes no good".
"And what harm can it bode?" rejoined Emily.
"He knows not so much as that, lady".
— Vol. III, Ch. IV , The Mysteries of Udolpho
Kurt Vonnegut's book, The Sirens of Titan, also notes the phenomenon affecting Winston Niles Rumfoord's dog, Kazak, the Hound of Space, in conjunction with solar disturbances of the chrono-synclastic infundibulum.
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