Friday, October 31, 2014

The Ghostly Glow of St. Elmo's Fire

Adapted from Wikimedia Commons

"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
"... sometime I'ld divide,
And burn in many places; on the topmast,
The yards and bowsprit, would I flame distinctly,
Then meet and join."
— William Shakespeare, The Tempest 
"About, about, in reel and rout, The death fires danced at night; The water, like a witch's oils, Burnt green and blue and white."
—Samuel Taylor Coleridge, The Rime of the Ancient Mariner 

The tale of a blue-white ghostly glow appearing on a dark and stormy night is littered throughout our history and story books. What's remarkable is the uniformity of the account: often during a thunderstorm, an eerie blue flame would appear and disappear on the tips of ship masts and yet would not burn.

These sightings are much more than mere ghost stories, although certainly many such tales have originated as a result. As a modern day equivalent, I'd like to think that the Siren in Doctor Who's The Curse of the Black Spot is just such a reference (there's a nerdy tangent if there ever was one).
Credit: Screenshot from BBC's Doctor Who episode, The Curse of the Black Spot

But what exactly are these strange flames? They are in fact a natural phenomenon known as St. Elmo's fire, first referenced by Pliny the Elder (23-79 AD) in his Naturalis Historia.

St. Elmo's fire works the same way that a neon light glows. A strong electric field in the atmosphere can separate protons and electrons, ionizing gas into a plasma. Thunderstorms generate exactly this sort of electric field when the charged clouds overhead repel charges in the Earth and an electric potential develops.

The electric field is strengthened around pointy objects like ship's masts, church steeples, and even the tips of airplane wings. These are all objects which, by their shape and conductive properties, make for good lightning rods during storms.

The reason for this pointy preference comes from the repulsive nature of electrons. If they are free to move, electrons like to get as far away from other electrons as they can and thus cluster in the 'corner' of an object, creating a stronger electric field than elsewhere.

This strong electric field provides the necessary energy to separate electrons and protons, breaking atomic bonds and forming a local plasma around, let's say, a church steeple. Electrons will start to flow off the steeple into the surrounding conductive plasma, colliding with and further exciting air molecules.

Discharge tubes of various elements. Nitrogen and oxygen molecules are shown third and fourth from the left, respectively, glowing violet and white-blue. Credit: Heinrich PniokFAL 

Our atmosphere is composed mostly of nitrogen and oxygen molecules, and when these molecules are excited, they begin to release a characteristic violet or blue light. With such a concentration of energy around our church steeple, air molecules are constantly being excited into a higher energy state before rapidly relaxing back down, releasing a blue photon along the way. Hence the blue 'fire' that does not burn.

This color completely depends on the electron structure of nitrogen and oxygen. If our atmosphere was instead filled with neon gas, apart from the fact that no human life could exist, St. Elmo's fire would appear red instead of blue.

Sighting a St. Elmo's fire during a storm was historically viewed as a good omen among sailors.
"A ghostly flame which danced among our sails and later stayed like candlelights to burn brightly from the mast....When he appears, there can be no danger" 
— Christopher Columbus, Second Voyage 
It is believed the name St. Elmo derives from St. Erasmus, the patron saint of Mediterranean sailors. Since St. Elmo's fire tends to occur towards the end of a thunderstorm, its presence meant the sailors' prayers were about to be answered.
Credit: NOAA Photo Library via Flickr
A good omen for historic sailors perhaps, but these days if you are ever lucky enough to spot this blue glow, you might want to be cautious.

Professor Steven Ackerman of the Atmospheric and Oceanic Sciences department at the University of Wisconsin-Madison says, "St. Elmo's fire can be caused by the same conditions that generate lightning, so while it is not necessarily a precursor to lightning, the conditions are likely ripe for a lightning strike and I certainly would look for other shelter - after enjoying the glow of a little bit..."

Not all ghostly apparitions are mysterious. Have a happy and nerdy Halloween!

By Tamela Maciel, also known as "pendulum"


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