Cause
St. Elmo's fire is a mixture of gas and plasma, as are flames in general and stars. The electric field around the object in question causes ionization of the air molecules, producing a faint glow easily visible in low-light conditions. Roughly 1000 volts per centimeter induces St. Elmo's fire; the number depends greatly on the geometry of the object. Sharp points lower the required voltage because electric fields are more concentrated in areas of high curvature, so discharges are more intense at the ends of pointed objects.
Conditions that can generate St.Elmo's fire are present during thunderstorms, when high voltage levels are present between clouds and the ground underneath. Air molecules glow due to the effects of such voltage, producing St. Elmo's fire.
The nitrogen and oxygen in the Earth's atmosphere causes St. Elmo's fire to fluoresce with blue or violet light; this is similar to the mechanism that causes neon lights to glow.
Read more about this topic: St. Elmo's Fire