Unless somebody has actually been to a blast and done that I would take their pontifications with a large accretion of salt.
In Operation Orange in 1958,was a 3.8 megaton explosion that was launched by a Redstone rocket that occured 250,000 feet However, due to a programming failure it burst directly over the island at the desired elevation making the island the effective ground-zero. It caused temporary blindnes on the island. Wehrner Von Braun was present at that test.