<P> During the night of August 5--6, Japanese early warning radar detected the approach of numerous American aircraft headed for the southern part of Japan . Radar detected 65 bombers headed for Saga, 102 bound for Maebashi, 261 en route to Nishinomiya, 111 headed for Ube and 66 bound for Imabari . An alert was given and radio broadcasting stopped in many cities, among them Hiroshima . The all - clear was sounded in Hiroshima at 00: 05 . About an hour before the bombing, the air raid alert was sounded again, as Straight Flush flew over the city . It broadcast a short message which was picked up by Enola Gay . It read: "Cloud cover less than 3 / 10th at all altitudes . Advice: bomb primary ." The all - clear was sounded over Hiroshima again at 07: 09 . </P> <P> At 08: 09, Tibbets started his bomb run and handed control over to his bombardier, Major Thomas Ferebee . The release at 08: 15 (Hiroshima time) went as planned, and the Little Boy containing about 64 kg (141 lb) of uranium - 235 took 44.4 seconds to fall from the aircraft flying at about 31,000 feet (9,400 m) to a detonation height of about 1,900 feet (580 m) above the city . Enola Gay traveled 11.5 mi (18.5 km) before it felt the shock waves from the blast . </P> <P> Due to crosswind, the bomb missed the aiming point, the Aioi Bridge, by approximately 800 ft (240 m) and detonated directly over Shima Surgical Clinic . It released the equivalent energy of 16 kilotons of TNT (67 TJ), ± 2 kt . The weapon was considered very inefficient, with only 1.7% of its material fissioning . The radius of total destruction was about 1 mile (1.6 km), with resulting fires across 4.4 square miles (11 km). </P> <P> Enola Gay stayed over the target area for two minutes and was ten miles away when the bomb detonated . Only Tibbets, Parsons, and Ferebee knew of the nature of the weapon; the others on the bomber were only told to expect a blinding flash and given black goggles . "It was hard to believe what we saw", Tibbets told reporters, while Parsons said "the whole thing was tremendous and awe - inspiring...the men aboard with me gasped' My God"'. He and Tibbets compared the shockwave to "a close burst of ack - ack fire". </P>

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