Which phenomenon was once seen on ship masts and church steeples, and now is occasionally seen on the tips of airplane wings?
And the answer: St. Elmo’s fire.
Named after the patron saint of sailors, St. Elmo's fire is a weather phenomenon in which luminous plasma is created on the pointed tip of a ship or plane. It occurs primarily during thunderstorms, or other conditions that create a slightly more electrified atmosphere.
St. Elmo's fire appears during thunderstorms, when the ground below the storm is electrically charged. If there's high voltage in the air between the cloud and the ground, the voltage tears apart the air molecules and the gas begins to glow. It takes about 30,000 volts per cubic centimeter to start a St. Elmo's fire (although sharp points like ship masts can trigger it at somewhat lower voltage levels).
Interestingly, this continuous electric spark is one we see frequently in our daily lives. The composition of St. Elmo’s fire is almost exactly the same as the charge found inside fluorescent tubes, mercury vapor streetlights, old orange-display calculators, and in "eye of the storm" plasma globes.
The color of this phenomena depends on the gas content of our atmosphere. Because our air is comprised of nitrogen and oxygen, it glows blue and violet in high voltage fields. If we lived on a planet of neon gas, St. Elmo's fire would appear red/orange.
Learn more about this rare phenomena here.