St. Elmo's fire?
Although referred to as "fire", St. Elmo's fire is, in fact, 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. Approximately 100–3000 kV per meter is required to induce St. Elmo's fire; however, this number is greatly dependent on the geometry of the object in question. Sharp points tend to require lower voltage levels to produce the same result because electric fields are more concentrated in areas of high curvature, thus discharges are more intense at the end of pointed objects.
St. Elmo's fire and normal sparks both can appear when high electrical voltage affects a gas. St. Elmo's fire is seen during thunderstorms when the ground below the storm is electrically charged, and there is high voltage in the air between the cloud and the ground. The voltage tears apart the air molecules and the gas begins to glow.
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.