Could someone explain what a green fire-ball in the sky that dissipates quickly and drops vertically might be?

Question

I've heard that it might be St. Elmo's fire.. what is that? my sister said she saw this object one night. It was spherical in shape that was bright green and dissapeared quickly.

Answer 1

It may be a 'ball lightning' which is a rare phenomenon.It is a glowing mass of energised air in the form of a spheroid.On striking an earthed object, it will disappear.

Answer 2

maybe it was an alien space ship. one minute they're there, the next, they're gone...

Answer 3

What would cause it be green could be a copper (II)  or cuprous compound igniting.