Did you see the quick movement of the cloud between the dark cloud on the lower half of the sky and the lighter clouds above it? Bad Astronomy has the details on what is happening in this video.
Joel had sent the video to Walter Lyons, a meteorologist from WeatherVideoHD, who was able to identify this phenomenon. Here’s his reply:
The answer lies in this: ice crystals, especially long needles, tend to become aligned with the ambient electric field.
So what you are seeing is sunlight reflecting off ice crystal faces that are constantly being oriented by the developing electric field just above the [cumulonimbus] top. Then there is a discharge in the cloud, and the field collapses momentarily, and the crystals begin to realign again. Then this just keeps happening over and over.
Aha! I hadn’t thought of that. The outer surface of ice crystals can hold a static electric charge, similar to what happens when you rub a glass rod with a cloth, or rub a balloon on your hair and stick it to the wall. When placed in an electric field, the charges feel a force on them, and align themselves along the field. So all the ice crystals above that cloud top are aligned one way in the field. Then the field snaps (maybe due to lightning releasing the energy) and then reforms. The ice crystals change their orientation suddenly when that happens.
So why does it look like the entire shape is changing? That’s because ice crystals can act like little prisms, bending light when it passes through them (or they can act like mirrors, with light reflecting off their flat surfaces). When they float in the air you get all sorts of astounding and beautiful formations like sun dogs, halos, sun pillars, and more. These all depend on the angle between you, the Sun, and the orientation of the crystal in between.