St Elmos Fire

What is St Elmo's Fire?

St. Elmo's fire is a weather phenomenon in which luminous plasma is created by a corona 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 Formia (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. Sailors may have considered St. Elmo's fire as a good omen (as a sign of the presence of their patron saint)

Characteristics

St. Elmo's fire is a bright blue or violet glow, appearing like fire in some circumstances, from tall, sharply pointed structures such as masts, spires, and chimneys, and on aircraft wings or nose cones. St. Elmo's fire can also appear on leaves and grass, and even at the tips of cattle horns.[3] Often accompanying the glow is a distinct hissing or buzzing sound. It is sometimes confused with ball lightning. In the picture from the cockpit to the right, it is the background blue-violet glow and not the spark discharge across the cockpit window. St Elmo's fire is akin to corona discharge from a negative electrode and not the spark, when the air finally breaks down and conducts.

Cause

St. Elmo's fire is a form of plasma. The electric field around the object in question 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 high voltage differentials are present between clouds and the ground underneath. A local electric field of approximately 100 kV/m is required to begin a discharge in moist air. The magnitude of the electric field depends greatly 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 preferentially occur and are more intense at the ends of pointed objects.

The nitrogen and oxygen in the Earth's atmosphere cause St. Elmo's fire to fluoresce with blue or violet light; this is similar to the mechanism that causes neon lights to glow, albeit at a different colour due to the different gas involved.[4]

In 1751, Benjamin Franklin hypothesized that a pointed iron rod would light up at the tip during a lightning storm, similar in appearance to St. Elmo's fire.[5][6]

Notable Observations

Julius Caesar

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), Alcaeus frag. 34, 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[17] ("holy body"), was a phenomenon described in The Lusiads. Earlier, Xenophanes of Colophon had alluded to the phenomenon.[18]

Charles Darwin

Charles Darwin noted the effect while aboard the Beagle. He wrote of the episode in a letter to J. S. Henslow that one night when the Beagle was anchored in the estuary of the Río de la Plata:

Everything is in flames, — the sky with lightning, — the water with luminous particles, and even the very masts are pointed with a blue flame.
— Charles Darwin, 1832[24]

Nikola Tesla

Nikola Tesla created St. Elmo's fire in 1899 while testing a Tesla coil at his laboratory in Colorado Springs, Colorado, United States. 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.[26]

St Elmo's Fire