St. Elmo’s fire

Everything is in flames—the sky with lightning—the water with luminous particles, and even the very masts are pointed with a blue flame…’ wrote Charles Darwin while aboard the Beagle. He was describing of a fascinating natural phenomenon that sailors have seen for thousands of years—a sudden glow atop a ship’s mast near the end of a thunderstorm, which many sailors believed to be a sign of salvation from St. Elmo. ‘St. Elmo’s Fire’ is a weather phenomenon that behaves a bit like lightning because it’s plasma—i.e. ionised air that emits a glow—and it’s similarly created in thunderstorms, when the air is electrically charged and there’s a significant charge imbalance in the air. However, lightning is a movement of electricity between clouds and ground, while St. Elmo’s Fire is a spark between the air and a charged object, such as the mast of a ship, a church steeple, or an aeroplane wing. These charged, pointed objects discharge electrical energy when the voltage in the air gets high enough, and the imbalance between the discharge and the air causes atoms of gas molecules (the nitrogen and oxygen of our atmosphere) to tear apart. Negatively-charged electrons move away from positively-charged protons, creating ionised air that emits light. Since the discharge usually lasts several minutes, it creates a constant blue glow—different gases glow different colours when they become plasmas, and nitrogen and oxygen glow blue. Interestingly, St. Elmo’s Fire behaves somewhat like a plasma globe: a pilot once reported that the phenomenon occurred on the windshield of her plane while flying through a storm cloud, and when she touched the windshield, blue plasma streaked out to meet the tips of her fingers.

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