I thought that the flash should have been a lot brighter. Instead all the damaage seemed to come from the shock wave. But that may be just the higher yield H-bombs. And I think I recall it being an air burst as well because of Nagasaki’s hilly terrain.
All nuclear detonations are air burst, usually at around 1,500 feet. Unlike a conventional demolition bomb witch has a detonator cap on the nose of the bomb a nuclear weapon has an electronically triggered detonation, a kind of ‘’shotgun’’ effect where the positive charged hydrogen atoms collide with the negative charged atoms and all hell breaks loose. You can’t have something that sophisticated and delicate smashing into the ground at around two hundred miles an hour.
Ground burst bombs have been in inventory for various special purpose roles, and of course there are the deep penetrating bunker busters.