Section 1.1: Historical Context

“It was a stunning sight, that electric glow dancing around the ship’s masts, casting a gentle light over the vessel. It resembled a magical scene, and we all stood in awe.”

— Jules Verne (1828–1905), From: Twenty Thousand Leagues Under the Sea

“The St. Elmo’s fire was a breathtaking vision, yet it served as a warning of the dangers that lay ahead.”

— Alfred Lansing (1921–1975), From: Endurance: Shackleton’s Incredible Voyage

My father, a U.S. Air Force pilot shortly after the Korean War, flew various aircraft including B25 Mitchell Bombers and C-119 Flying Boxcars. His passion for flying was infectious, and my siblings and I eagerly listened to his tales. One story that particularly intrigued me was about the bluish glow that sometimes appeared on his aircraft’s wings during thunderstorms, a phenomenon he referred to as “St. Elmo’s Fire.”

The Origins of the Name

St. Elmo’s Fire was named after Saint Erasmus of Formia (circa 300 AD), the patron saint of sailors. Interestingly, he is also the patron saint of abdominal pain, likely due to the horrific nature of his demise.

This phenomenon has been documented for over two millennia. Pliny the Elder (circa 23–79 AD) wrote in his Naturalis Historia:

“Stars also manifest at sea and on land. I have witnessed a star-like radiance clinging to soldiers’ javelins during the night watch, and on voyages, stars have illuminated parts of the ship, producing sounds reminiscent of birds calling to one another.”

Numerous accounts of Saint Erasmus’ trials make it difficult to ascertain exactly how he became the patron saint of sailors. Tales tell of his sermons to seafarers, his survival of storms, and his close encounters with lightning while preaching, culminating in his gruesome execution by disembowelment. Regardless, sailors have long viewed the appearance of St. Elmo’s Fire on ship masts as a sign of divine protection.

Section 1.2: The Science Behind the Phenomenon

Although commonly referred to as “fire,” St. Elmo’s Fire is not a flame. Rather, it is a form of plasma, considered the fourth state of matter alongside solid, liquid, and gas. This phenomenon occurs when air molecules are ionized in the presence of a strong electric field.

Thunderstorms generate significant electric fields. When objects such as masts, spires, or the edges of airplane wings are present, they concentrate the local electric field. The resultant field strength can cause air molecules to ionize, making them electrically conductive. As these ionized molecules recombine, they emit photons that create the bluish-green or violet glow characteristic of excited nitrogen and oxygen molecules.

The principles governing this phenomenon can be explained through Coulomb’s Law:

Coulomb’s Law, named after Charles-Augustin de Coulomb (1736–1804), states that the force (F) between two charges, q₁ and q₂, is directly proportional to the product of the charges' magnitudes and inversely proportional to the square of the distance (r) between them. Here, k denotes Coulomb’s constant.

In the context of an equipotential spherical surface, Coulomb’s law can also be represented as 1/(4πε₀), where ε₀ signifies the permittivity of free space.

This law is comparable to Newton’s law of gravitation—both describe forces that decrease with the square of the distance. However, while gravitational force is solely attractive, Coulomb’s law can be either attractive or repulsive depending on the charges' polarity.

In the context of St. Elmo’s Fire, we focus on the electric field strength (E) produced by a charge (q) at a specific location. The electric field is defined as the force exerted per unit charge on a small positive test charge (q₀) placed at that point:

By applying Coulomb’s law to determine the force (F), we arrive at another representation:

During a storm, electric charge tends to gather at a ship’s mast or the edge of an airplane wing. The amplification of this effect by pointed tips or edges is evident in the r² term in the denominator of the equation, as these shapes feature a smaller radius of curvature, yielding a greater overall value.

When the electric field strength (E) surpasses the breakdown voltage of air, approximately 3 x 10³ Volts/meter, the ionized air begins to emit a glow.

If you're curious, there's an excellent applet available that enables you to visualize various charge and field configurations.