The electrons in metal are the worker ants of electricity: ubiquitous, able to work together to carry great loads, and free to roam in any direction. Since they’re unbound to any single atom or molecule, they can swim through the metal and move charge from one place to another. Air, on the other hand, lacks these mighty swimmers. All its electrons are held tight to their parent molecules. If you want to get air to conduct electricity like a metal, you have to pull those electrons away—and pull real hard.
That, in effect, is what the 500,000 volts in this switchyard are doing. When the circuit breaks at the beginning of the clip, the electrical field between the contacts is so strong that it yanks electrons free from the nitrogen and oxygen in the air. These electrons flow uninhibited between terminals as if they were in a metal and allow the air—now acting as a plasma, not a gas—to conduct electricity. It’s the same thing that happens in lightning, except lightning is one quick burst of energy from cloud to ground. Here, we’ve got a power plant spitting out energy to spare. Electricity tears the air apart so that it can flow through the cracks.
Unsurprisingly, all this activity heats the air pretty quickly. That’s why the arc—the area of lowest resistance, where the electrons can be freed from their host molecules—moves up. Hot air rises, after all. —Michael Moyer