St Elmo's Fire

St Elmo's Fire

St Elmo's Fire is a blue or reddish glow, sometimes with the appearance of a flame, accompanying a powerful electrical discharge from a pointed conducting object in an intense electric field. It is caused by collision ionization (removal of electrons from their molecules), followed by recombination of air molecules with their electrons and the resulting emission of light. This process is similar to that which occurs in the bulb of a neon sign. It usually occurs beneath or in the vicinity of THUNDERSTORMS, which generally carry a negative electrical charge at their base and so draw upwards a positive charge (in the form of positive ions) from pointed objects on the Earth's surface. If they are not "grounded," these objects can acquire a strong negative charge.

St Elmo's fire is most readily seen at night proceeding from such pointed objects as the masts and rigging of ships, aircraft propellers, flagpoles and church steeples, and even from cattle horns and from the fingers and hairs of mountaineers. At times it may have the appearance of a glow, halo or aura, while on other occasions the glow may be accompanied by visible streamers of light. While remaining attached to a conductor of electricity, it may move along it and can last for many minutes. Frequently a sound described as hissing, fizzing or crackling accompanies it. Because natural point discharges of electricity from an aircraft are intermittent, they cause radio "static" and can interfere with communications and navigation. On modern aircraft, this problem is overcome by installing cables tipped with wire brushes on the trailing edges of the wings. These "pig tails" allow the electrical charge collected by the aircraft to drain off continuously in the form of a quiet, glowing discharge.

There is speculation that Moses observed St Elmo's fire in the burning bush on Mount Sinai; Shakespeare refers to it in The Tempest; and it was reported by early explorers of Canada. Sailors viewed it as a good omen, presaging the imminent end of bad weather. However, it can also be the precursor of a LIGHTNING stroke, which may be "attracted" to the pointed object from which the discharge occurs. The name St Elmo is a corruption of St Erasmus, the patron saint of Mediterranean sailors. English sailors call it the corposant or cormazant, from Spanish or Italian corpo santo, "holy body" or "saint's body."