St. Elmo's fire

[Roshi]

https://en.wikipedia.org/wiki/St._Elmo%27s_fire

St. Elmo's fire is a bright blue or violet glow, appearing like fire in some circumstances, from tall, sharply pointed structures such as lightning rods, masts, spires and chimneys, and on aircraft wings or nose cones.

Cause

St. Elmo's fire is a form of plasma. The electric field around the object in question causes ionization of the air molecules, producing a faint glow easily visible in low-light conditions. Conditions that can generate St. Elmo's fire are present during thunderstorms, when high voltage differentials are present between clouds and the ground underneath. A local electric field of approximately 100 kV/m is required to induce a discharge in air. The magnitude of the electric field depends greatly on the geometry (shape and size) of the object. Sharp points lower the necessary voltage because electric fields are more concentrated in areas of high curvature, so discharges preferably occur and are more intense at the ends of pointed objects.