Tree Branch vs Power Lines

IIRC it's the sap that conducts the electricity.

What interests me is that it doesn't arc until the wood combusts. Presumably the flame itself has very low resistance and started the arc.

It isn't so much the wood conducting electricity as the moisture in the wood. The whistling sound is the moisture escaping (as steam) as it's heated by the current until it reaches that critical point when a full blown arc appears across the lines.

I find it unlikely that someone was out in their back yard filming their cat or whatever at just the moment that this happened

looks cool but don't do it again

May I know exactly what happened there? It looked like the branch was sitting between the live and neutral lines, then.... what?

I had that happen in my backyard once. Was a much bigger piece of wood. Made a hell of a racket and lit up the entire house at 3am. It was raining like crazy or else the falling fireball would have set the back 40 on fire.