SEED Science

Lightning: The Sky Out of Balance
St. Elmo’s Fire

If you’ve seen neon or fluorescent lights, then you’ve seen artificial versions of St. Elmo’s fire. All of these glowing lights are plasmas, or ionized gases. Toward the end of a thunderstorm, St. Elmo’s fire appears as eerie blue “flames” at the tips of ship’s masts, airplane wings, flagpoles, steeples, street lamps, and other tall, pointy objects.

St. Elmo’s fire

Photo courtesy of Captain James Ashby, Hong Kong Observatory.

When Captain James Ashby’s jet flew through a cumulonimbus cloud while descending to the Phnom Penh International Airport in Cambodia, St. Elmo’s fire radiated from the front of the plane.

Despite the name and appearance, no fire is involved and nothing burns. The glow isn’t lightning, either—which, as you’ll recall, is a spark of charged particles zipping through the sky. St. Elmo’s fire is a different electrical phenomenon—a sustained spark called a corona discharge. Unlike lightning, it doesn’t travel anywhere.

St. Elmo’s fire

Photo courtesy of Wikimedia Commons

Sailors have reported seeing blue “flames” shooting up like torches at the tips of masts since ancient times. They named the eerie glow after the patron saint of sailors—St. Erasmus, which was corrupted into St. Elmo. The sailors regarded St. Elmo’s fire as a good omen since it meant a thunderstorm was winding down.

What creates the eerie blue glow? By definition, ions are electrically imbalanced because they have lost or gained electrons. The imbalance rips apart molecules in the air (such as O2, an oxygen molecule), sometimes with a soft hissing sound. This violent act releases electromagnetic energy, which we perceive as a glow. St. Elmo’s fire glows for several minutes before dying out. In any plasma, the type of gas determines the color—orange for neon and blue for nitrogen and oxygen molecules, the main gases in the atmosphere.

St. Elmo’s fire usually forms at the tips of tall objects because the reduced surface area requires less voltage for the oppositely charged particles to attract each other. Though the plasma is stationary, there is a form of lightning that can’t seem to sit still.

United States Air Force photo by Senior Airman Joshua Strang.

The aurora borealis, popularly called the northern lights, consists of glowing curtains of red and green that appear above the North Magnetic Pole. Like St. Elmo’s fire, they are plasmas. The plasmas form when the solar wind, a stream of charged particles from the sun, ionizes atoms in the air. Earth’s magnetic poles—north and south—attract these ionized particles. The phenomenon is called the aurora Australis at the South Pole.

Aurora borealis