The good news is, whether it's the Bomber responsible for dropping the Atomic Bomb or the Fighter Aircraft, their speed is very fast.
The Atomic Bomb takes about 36 seconds to reach the estimated explosion point at 550 meters high from an altitude of ten thousand meters. With a parachute to slow the descent, this time can be increased to around 45 seconds.
This also means that Shier and his group's window for escape is within these 45 seconds. If they can't get out of the Atomic Bomb's kill radius within 45 seconds, they'll be forced to endure the blast wave's impact.
However, the good news is, given the current speed of the Fighter Aircraft and Bombers, 45 seconds is enough for them to escape to a distance of about 7.5 kilometers away.
