Good question! I'm not entirely sure, but if we consider that: 1) Air is a decent insulator compared to earth 2) Most thunderstorms are quite close to the grou

Good question! I'm not entirely sure, but if we consider that: 1) Air is a decent insulator compared to earth 2) Most thunderstorms are quite close to the ground 3) The electric potential is caused by colliding masses of air. I would have to assume a large volume of air gets a bulk charge that is large in a specific direction but not large enough for a strike to the ground. Then you get a breakdown somewhere that connects a lower potential are to a higher potential area, and because there is a bulk charge, the breakdown simply continues after this initial connection. After that, I don't really get how it can continue for 16 seconds, other than massive amounts of charged in the atmosphere, and partially ionized leftover air from the initial blast being conductive enough to trigger more insulation breakdowns. You can use electron beams to charge acrylic which you then trigger to discharge by various means, and it seems to behave in a similar way but on a much smaller scale. Iirc the small lightning bolts inside the acrylic sometimes continue for a surprising amount of time, even though it's tiny, and the flash also (generally) occur entirely within the insulator even it it's quite long and thin.