Often in nuclear test footage there’s a great big mushroom cloud. As I don;t need to explain but will anyway because why not, the cloud is basically dirt and smoke that has gotten sucked up into the fireball; and since the fireball is *very* low density compared to the cooler surrounding air, it ascends like a balloon and drags the dirt and smoke and dust and ash along with it.
But then there was the “Wrangell” test from Operation Hardtack II, Oct 22, 1958. This was a dinky 0.115 kiloton airbust, suspended from a balloon at 1500 feet above the desert floor. As you can see in the videos, the fireball never comes anywhere near the surface. The ground isn’t cratered, probably isn’t substantially disturbed by the relatively small burst (dust is kicked up over a distance of perhaps a mile or two, but that dust isn’t sucked up to the fireball). And yet as the fireball cools, a whole lot of smoke is left floating in the air. I *assume* that this is the balloon and the bomb itself, converted into vapor and reacted with the air, but there sure does seem to be a lot of it. Could some of it be the air itself, perhaps nitric oxide compounds created by the high temperatures?