St. Elmo's fire

St. Elmo's fire (also St. Elmo's light) is a weather phenomenon in which luminous plasma is created by a coronal discharge from a sharp or pointed object in a strong electric field in the atmosphere (such as those generated by thunderstorms or created by a volcanic eruption).

St. Elmo's fire is named after St. Erasmus of Formia (also called St. Elmo, one of the two Italian names for St. Erasmus, the other being St. Erasmo), the patron saint of sailors. The phenomenon sometimes appeared on ships at sea during thunderstorms and was regarded by sailors with religious awe for its glowing ball of light, accounting for the name. Sailors may have considered St. Elmo's fire as a good omen (as in, a sign of the presence of their patron saint).

St. Elmo's fire is a bright blue or violet glow, appearing like fire in some circumstances, from tall, sharply pointed structures such as lightning rods, masts, spires and chimneys, and on aircraft wings or nose cones. St. Elmo's fire can also appear on leaves and grass, and even at the tips of cattle horns. Often accompanying the glow is a distinct hissing or buzzing sound. It is sometimes confused with ball lightning.

In 1751, Benjamin Franklin hypothesized that a pointed iron rod would light up at the tip during a lightning storm, similar in appearance to St. Elmo's fire.

St. Elmo's fire is a form of matter called plasma, which is also produced in stars, high temperature flame, and by lightning. The electric field around the object in question causes ionization of the air molecules, producing a faint glow easily visible in low-light conditions. Conditions that can generate St. Elmo's fire are present during thunderstorms, when high voltage differentials are present between clouds and the ground underneath. A local electric field of approximately 100 kV/m is required to induce a discharge in air. The magnitude of the electric field depends greatly on the geometry (shape and size) of the object. Sharp points lower the necessary voltage because electric fields are more concentrated in areas of high curvature, so discharges preferably occur and are more intense at the ends of pointed objects.

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