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St. Elmo's Fire

St. Elmo's fire is a persistent blue glow that occasionally appears near pointy objects during storms. The name is something of a misnomer, as the electric phenomenon has more in common with lightning or the northern lights than it does with flame.

St Elmo’s Fire on ship
St Elmo’s Fire on ship | Image credit : Mystic Sciences

Captains of the seas and skies know St. Elmo's fire best, as the ethereal light has long been sighted clinging to the masts of ships and more recently the wings of planes. Mariners have noted the spectacle for thousands of years, but only in the last century and a half have scientists learned enough about the structure of matter to understand why the phenomenon takes place. It's not gods or saints that kindle the enigmatic fire, but one of the five states of matter: plasma.

St. Elmo's fire seen on the coils
St. Elmo's fire seen on the coils | (Image credit: Getty Images)

Reports of blue lights dimly flickering from the rigs of ships date back to antiquity, when the Greeks and Romans interpreted the sight as visitations from the demigod twins Castor and Pollux. Considered saviors of those in danger, the twins' apparition would have come as a hopeful sign to sailors weathering a storm.

The phenomenon later got its modern name from St. Erasmus, or St. Elmo for short, who lived in the third century. St. Elmo gained fame as the patron saint of sailors and intestinal distress, after he was reportedly killed by disemboweling. Sailors prayed to him in moments of distress and continued to interpret the glow of St. Elmo's fire dancing and hissing on the tips of their boats as a favorable omen.

What causes St. Elmo's fire ?

A scientific understanding of St. Elmo's fire became possible only after British chemist and physicist William Crookes produced what he called "radiant matter" through his work with vacuum tubes in 1879. The discovery of the electron came two decades later, revealing that the world was made of more than neutral atoms. Finding that atoms contained smaller, charged particles proved essential to understanding why Crookes's matter shined, launching the whole new field of plasma physics.

Plasma occurs when excess energy breaks up atoms in a neutral gas to create a charged gas. One way to create plasma is with heat. For example, heating solid ice breaks molecular crystals into liquid water, and boiling liquid water liberates water molecules to rise as a gaseous vapor. Continue to dump energy into the vapor (by heating it past 21,000 degrees Fahrenheit, or 12,000 degrees Celsius, for instance), and the atoms in the water molecules get roughed up, losing their electrons and becoming charged ions. This point represents the transition from a gas, a cloud of neutral particles, to a plasma, a cloud containing many charged particles.

Electricity can tear up gas molecules and make a plasma more easily than heat can, which is the key to St. Elmo's fire. During a storm, friction builds up extra electrons in certain parts of clouds, generating powerful electric fields that reach the ground. A strong enough field can theoretically break air down into a plasma anywhere, but in practice, sharp points (such as the mast of a ship) tend to concentrate the field, stripping electrons from atoms to leave behind charged ions in especially high numbers near sharp places.

St. Elmo’s fire can be also produced during a volcanic eruption.