What's the mechanism behind things like sparks, lightning, St. Elmo's fire, etc.? I've heard two different explanations, both of which seem reasonable: 1) Stray electrons are accelerated by the potential difference created between eg the thundercloud and the ground. These electrons attain enough energy to ionize air molecules, creating more stray electrons which ionize more air molecules, and so on, creating an avalanche of electrons. 2) The potential difference is great enough to ionize the air (without needing say stray electrons), creating free charges which can then be accelerated by the potential difference to create a current. Which one is right?