Saint Elmo's Fire
The Fire of Saint Elmo, Santelmo, or the Holy Body of Saint Elmo, is an igneous meteor consisting of a discharge of electroluminescent corona effect caused by the ionization of the air within the strong electric field that causes electrical storms.
Although it is called “fire”, it is actually a low-density, relatively low-temperature plasma caused by a huge atmospheric electrical potential difference that exceeds the dielectric breakdown value of air, around 3 MV/m. Physically, it is a brilliant bluish-white glow, which in some circumstances has the appearance of fire, often in double or triple jets from tall, pointed structures such as masts, poles, spars, pinnacles, and chimneys.
Saint Elmo's fire is named after Saint Erasmus of Formia (also known as Saint Elmo), the patron saint of sailors. The phenomenon, which can warn of impending lightning, was viewed by sailors in awe and was sometimes considered a good omen.
Causes
St. Elmo's fire is a form of plasma. The electric field around the affected object causes ionization of the air molecules, producing a faint glow easily visible in low light conditions. Conditions that can generate St. Elmo's fire are present during thunderstorms, when there are high voltage differentials between the clouds and the ground below. A local electric field of approximately 100 kV/m is required to initiate a discharge in moist air. The magnitude of the electric field is highly dependent on the geometry (shape and size) of the object. Sharp points lower the necessary voltage because electric fields are more concentrated in areas of high curvature, so discharges occur preferentially and are more intense at the ends of sharp objects.
Nitrogen and oxygen in Earth's atmosphere cause St. Elmo's fire to fluoresce with blue or violet light; this is similar to the mechanism that makes neon lights glow, albeit with a different color due to the different gas involved.
In 1751, Benjamin Franklin hypothesized that a pointed iron rod would ignite at the tip during a thunderstorm, similar in appearance to St. Elmo's fire.
Recently, experiments conducted at MIT have shown that St. Elmo's fire behaves differently on airborne objects compared to grounded structures.
History and significance
Saint Elmo's fire is frequently observed on the masts of ships during electrical storms at sea, where in such circumstances the compass was also altered, to the greater uneasiness of the crew. This phenomenon takes its name from Saint Erasmus of Formia (Sanct' Elmo), patron saint of sailors, who had observed the phenomenon since ancient times and believed that its appearance was a bad omen; although other sailors associated it with a form of protection from their employer, as in the case of Spanish sailors during the conquest of America. When there were as many fires as there were masts, in addition to Saint Elmo it was believed that Saint Nicholas and Saint Catherine. As British explorer George Dixon comments in his 1789 book A voyage Round the World, English sailors instead sometimes attributed this phenomenon to a pixie they called Davy Jones. Spaniards would refer to San Pedro González Telmo.
It also occurs in airplanes and airships. In the latter it was very dangerous since many of them were loaded with hydrogen, a highly flammable gas, and could catch fire, as happened in 1937 with the Hindenburg airship.
It is said that St. Elmo's fire can also appear on the tips of cattle's horns during thunderstorms and on sharp objects in a tornado, but it is not the same phenomenon as ball lightning, although it can be related.
In ancient Greece, the appearance of a single St. Elmo's fire was called "Helena", from its original meaning of "torch", and when there were two of them they were called "Castor and Pollux" the Dioscuri or twin sons of Zeus, patron saints of sailors who calmed storms at their request.
Benjamin Franklin correctly observed in 1749 that it is electrical in nature.
Quotes
Christopher Columbus encountered the fire of San Telmo on October 26, 1493, in the context of his second voyage to America, and this fact was written up by his son:
On the same Saturday night the fire of San Telmo was seen, with seven lit candles on top of the gavia. With a lot of rain and terrible thunders. I mean, you saw the lights that the sailors claim to be the body of San Telmo, and they sing to him many litanies and prayers, having by the way that in the storms where nothing appears can endanger.Hernando Colón, Second trip of Columbus
Antonio Pigafetta in the chronicle he wrote during the Magellan-Elcano expedition entitled Relation of the first trip around the world, mentions that they observed the phenomenon on October 3, 1519 sailing along the coast of Africa, at the height of the equator. He recounts that the light flamed for two hours, until it disappeared, projecting a strong flash.
References to Saint Elmo's fire are found in the works of Julius Caesar (De bello Africo 47), Pliny the Elder (Naturalis historia ii.101) and Herman Melville.
- Look up!" said Starbuck suddenly. The fire of San Telmo on top of the big stick!
In fact, the arms of the gates were surrounded by a light fire, and the triple needles of the pararrhoea looked with three tongues of fire. The whole masts seemed to burn.
- Fire of San Telmo, have mercy on us!" said Stubb.Herman Melville, Moby Dick
Another reference:
General silence. The wind is quiet. Nature does not breathe. She looks dead. Throughout the mast, the fires of San Telmo are weakly sparked; the candle falls into heavy folds.Julio Verne, Travel to the center of the Earth
The green lights of the fire of Santelmo surrounded every stick, mast and notebook of the ship, illuminating The wandering Dutch with a phantomish and overcoating green shine.Brian Jacques, "The Dutch Errant Shipwrecks"
Charles Darwin observed the effect while aboard the Beagle one night while it was anchored in the Río de la Plata and described the episode in a letter to J. S. Henslow:
Everything was on fire, in the sky there were rays and in the water luminous particles (bioluminescence), and even the masts themselves were crowned with a flame, the ancient Greeks interpret the fire of san telmo as the presence of the semi-gods.Charles Darwin, Letter to J. S. Henslow