A weather phenomenon.
A bright blue or violet glow, appearing like fire sometimes, from tall, sharply pointed structures such as masts. Often accompanying the glow is a hissing or buzzing sound.
Some sailors saw the glow as an unlucky sign; it interfered with compass readings because of the electricity in the air. Others took it as a sign of the presence of St. Erasmus's (Elmo's) protection, the patron saint of sailors.
What must it be like to be at sea today? Stormy gale warnings forecast for this weekend.