Causes of St. Elmo's Fire: The Fire That's Not a Fire
Like lightning, St. Elmo's Fire is plasma, or ionized air that emits a glow. But while lightening is the movement of electricity from a charged cloud to the ground, St. Elmo's Fire is simply sparking, something like a shot of electrons into the air. It's a corona discharge, and it occurs when there is a significant imbalance in electrical charge, causing molecules to tear apart, sometimes resulting in a slight hissing sound.
The first step in generating St. Elmo's Fire is a thunderstorm. As you can learn in How Lightning Works, a thunderstorm creates an electrically charged atmosphere. There is a charge difference between the storm clouds and the ground, and this difference creates voltage, or electrical pressure. In between the clouds and the ground, the atoms in the air undergo changes; most important to our discussion, electrons move farther away from protons, creating an environment that allows electrons to move around freely. In other words, the air becomes a good conductor.
Once the air is conducive to the movement of electrons, those electrons continue to increase the distance between their positively charged counterpart, protons. This is ionization, and plasma is simply ionized air. The phenomenon that causes St. Elmo's Fire is a dramatic difference in charge between the air and a charged object, like the mast of a ship, the tip of an airplane wing or the 30-foot steeple of a church -- things we often think of as potential lightning rods.
When the voltage gets high enough, usually around 30,000 volts per centimeter of space, the charged object will discharge its electrical energy [source: Scientific American]. The reason why St. Elmo's Fire occurs most often on pointed objects is that a tapered surface will discharge at a lower voltage level. The tip of a steeple, mast or airplane wing presents something like a condensed surface charge.
When the air molecules tear apart, they emit light. In the case of St. Elmo's Fire, the discharge is continuous -- sometimes lasting several minutes -- and creates a constant glow. The glow is blue because different gasses glow different colors when they become plasmas. Earth's atmosphere has nitrogen and oxygen in it, and this particular combination happens to glow blue.
St. Elmo's Fire is exactly what's happening in neon tubes -- essentially a continuous spark. If Earth's atmosphere were made up of neon, St. Elmo's Fire would glow orange instead of blue. A neon tube is simply St. Elmo's Fire contained in glass. St. Elmo's Fire also behaves something like a plasma globe. One pilot described the phenomenon occurring on the windshield of her small plane while flying through a storm cloud; when she touched the inside of the windshield, blue streaks reached toward the tips of her fingers [source: USA Today].
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