I'm going to add something here, I might be wrong, but I might also be right.
I believe the shockwaves caused showing the "smokey bubble" has a lot to do with humidity as well. Can anyone approve of this?
Vietnam is especially humid and I believe the shockwave could have caused moist air to vapourize and create this effect a lot easier. Also like aircraft wingtip contrails are dependant on the weather conditions
Just me trying to be all scientific here
EDIT:
LOL, Letum beat me to it by 8 minutes :p Man I'm slow
