The power of a dramatic cloud

On Friday, we found an "It's a bird; it's a plane; no...."-type situation in a store parking lot. People were standing in the cold, looking up, pointing and muttering odd things about crop circles and alien invasions.
We had no camera, but Alec was able to get this photo with his iPhone, which doesn't quite do justice to how dramatic it looked to the naked eye.

 After the phenomenon dissipated, we hurried home and pulled out our cloud books. We found an image like this one in Richard Hamblyn's Extraordinary Clouds, which identified it as a fallstreak hole "created by the sudden freezing of an isolated patch of supercooled cloud, which falls away to leave a visible gap behind." The dramatic red lighting was due to the sunset.
It's rare, in our urban environment, to find a group of strangers transfixed by a natural phenomenon. This fabulous cloud accomplished that, temporarily suspending the pressure to complete errands and get home. What a lovely way to end a work week.