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St Elmo's Fire
From SKYbrary Wiki
A visible luminous electrical discharge observed around parts of an aircraft when the electrical charge on the aircraft becomes sufficiently intense. Similar in nature to the glow from a neon tube, and often observed as brushlike fiery jets extending from the tip of an aerial, a wing, propeller, windscreen or other part of an aircraft.
St. Elmo's Fire occurs when the atmosphere becomes charged and an electrical potential strong enough to cause a discharge (plasma) is created between an object and the air around it. This can happen to an aircraft flying through heavily charged skies. St Elmo's Fire is usually bluish or violet in colour but can also have a greenish tinge.
Apparently, St. Elmo's Fire can sometimes be heard "singing" on the aircraft's radio, a frying or hissing sound running up and down the musical scale.