What is St. Elmo's fire?

St. Elmo's fire is a weather phenomenon in which a strong electric field in the air ionises gas molecules around sharp points such as ship masts, church spires and aircraft wings. The ionised air glows with a blue or violet light, a form of plasma called a corona discharge. It is often accompanied by a hissing or buzzing sound. Sailors named it after St. Erasmus of Formiae, also called St. Elmo, the patron saint of sailors, and often saw it as a good omen. Aircraft pilots still sometimes see it on windscreens and wingtips when flying through charged storm clouds.