VISIBLE sonic boom

Originally posted by lisa

But this effect apparently occurs only at, or very close to, the speed of sound, so it's the "visual counterpart" of a sonic boom. I suppose the phrase "Visible sonic boom" just appealed to me.

Yah, but I was really interested in it when I read it like a week ago, and it's bugging the hell out of me 'cause I can't remember what it actually is/is caused by, and I can't find the source of information either.