Today, as you will have noted earlier this morning when you opened up your breviary, is the feast of St Erasmus, the patron saint of sailors in the Mediterranean. This worthy cleric was an Italian bishop who ministered to his flock in the reign of the Emperor Domitian, and since his activities came to be regarded by the latter as subversive, Erasmus was broken on the wheel in AD 304 and thus became a martyr.
Of particular interest to meteorologists, however, is the fact that in certain places his name has been affectionately shortened to "St Ermo", and a local variant, "St Elmo", became the eponym for the phenomenon we call St Elmo's fire. St Elmo's fire, or Corpo Santo as it is sometimes called, is a luminous brush-like glow with flame-like streamers, which in stormy weather was often seen to play around the masts and rigging of the old sailing ships.
It is electrical in origin. It occurs in thundery conditions when the electrical tension between the clouds and the earth below is such that there is a strong tendency for an electric current to flow from one to the other; an electric charge "dribbling" upwards in these circumstances causes the surrounding air to incandesce. The phenomenon was well known even before the days of St Erasmus. The Roman writer Pliny, for example, writing in the first century AD, described it as "resembling lightning, leaping to and fro and shifting its place, just as birds do when they fly about from bough to bough". But for sailors, St Elmo's fire became an influential portent of the future. If only one stream of light appeared, it was believed to be a manifestation of Helen of Troy - she "whose beauty summoned Greece to arms, and drew a thousand ships to Tenedos". Considering the trouble Helen caused on that occasion, mariners came to the reasonable conclusion that her reappearance boded ill, and so a single flame of St Elmo's fire foretold storm and tempests on the way. But if two flames appeared, the signs were good. The twin flames were believed by sailors to be Castor and Pollux, the semi-divine twin brothers of the aforementioned Helen. These two had been given a protective role over ships at sea, and their sudden appearance, therefore, was a benign prognosis of calmer conditions on the way.
Legend has it that Christopher Columbus, on his voyage to the New World, raised the spirits of his crew during a violent tempest by pointing to the "holy fire" as a sign that their troubles were about to end.