Final answer:
St. Elmo's fire is a luminous glow or halo that appears on pointed objects, such as ship masts, during a thunderstorm when the electric potential near the ground increases. It is caused by a positive charge current that moves up the pointed object due to the strong electric field and ionization of air molecules.
Step-by-step explanation:
The phenomenon described in the question is known as St. Elmo's fire. It is a luminous glow or halo that appears on pointed objects, such as ship masts, during a thunderstorm when the electric potential near the ground increases. St. Elmo's fire is caused by a positive charge current that moves up the pointed object due to the strong electric field and ionization of air molecules.
During a thunderstorm, the electric field around the pointed object, such as a lightning rod, becomes very large. When the field reaches a certain value, the free ions in the air are accelerated to high energies, resulting in ionization of molecules. The resulting free electrons then flow through the rod to Earth, neutralizing some of the positive charge, and producing the luminous halo known as St. Elmo's fire.
St. Elmo's fire is named after St. Erasmus of Formiae, the patron saint of sailors, as it was often observed on ship masts and rigging during storms at sea.