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What Is St. Elmo's Fire? (No, Not The Song)

St. Elmo's Fire Jerry.Raia

St. Elmo's fire is a kind of electric spark called a "glow discharge". It can happen on airplanes, but even if you haven't seen it at 35,000 feet, you've seen it with your feet on the ground. That's because St. Elmo's fire is almost exactly the same as what happens on the inside of a fluorescent lightbulb, neon sign or one of those "Eye of the Storm" plasma globes.

Who Saw It First?

St. Elmo's fire was first described by sailers on ships at sea. The phenomenon often occurred during thunderstorms, and resulted in glowing flashes of light moving around the ship. At the time sailers didn't know what it was, but they knew it was problematic because it interfered with their compass readings, making navigation difficult.

What Does It Look Like?

St. Elmo's fire is a bright blue or violet glowing electrical discharge. It often has the same shape as fire, hence the name. Check out this video of St. Elmo's fire appearing on the windscreen of a 737.

What Causes St. Elmo's Fire?

St. Elmo's Fire is made up of something called plasma, which is the same stuff produced in lightning, high-temp flames, and the sun. St. Elmo's fire typically appears on metal, pointed objects, which makes jets a perfect place for it to occur.

Here's what happens: when a highly charged electric field surrounds an object, the difference in voltage between the field and object begins to ionize the air. In the video above, the rain clouds are highly charged, and the 737 isn't.

What happens next? If the voltage difference is great enough, the voltage starts to rush through the air and into the airplane. The voltage starts tearing air molecules apart and turing them into a glowing mixture of protons and electrons called plasma. What's the result? What you see in the video - glowing, blue flashes that look like flames.



Want to see more St. Elmo's fire? Check out these videos shot from airline cockpits.


Colin Cutler

Colin is a Boldmethod co-founder, pilot and graphic artist. He's been a flight instructor at the University of North Dakota, an airline pilot on the CRJ-200, and has directed development of numerous commercial and military training systems. You can reach him at colin@boldmethod.com.

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