Nagasaki: Life After Nuclear War by Susan Southard:
The five-ton plutonium bomb plunged toward the city at 614 miles per hour. Forty-seven seconds later, a powerful implosion forced its plutonium core to compress from the size of a grapefruit to the size of a tennis ball, generating a nearly instantaneous chain reaction of nuclear fission. With colossal force and energy, the bomb detonated a third of a mile above the Urakami Valley and its thirty thousand residents and workers, a mile and a half north of the intended target. At 11:02 a.m., a superbrilliant flash lit up the sky—visible from as far away as Omura Naval Hospital more than ten miles over the mountains—followed by a thunderous explosion equal to the power of twenty-one thousand tons of TNT. The entire city convulsed.