Phoenix mystery flash caught live, everyone confuzzled! -
Click to view
I say it's just a power line insulator cleaning itself. These things happen after a long and dusty dry spell and followed by a humid cooling off. Moisture condenses on the dust particles coating the surfaces of the insulator segments, causing a conducting arc to spontaneously start. The insulators are designed to do this and survive unharmed. All that's heard is a 'zzzzap' and then things go back to normal. If the camera had been color instead, there would be the usual blue-green tint to the light flash.