The atomic bombing of Nagasaki took place on August 9, 1945, three days after the bombing of Hiroshima. After the bombing of Hiroshima, Japan failed to surrender and the United States decided to hit another city in order to force them to surrender. The primary target was actually the city of Kokura, with Nagasaki as a secondary target if Kokura didn't work out. Nagasaki was a valuable target since it was a large sea port and produced weapons and ships for the Japanese military. Most of the buildings in Nagasaki were made of wood, which could not possibly withstand a nuclear blast.
The number of people in the city was slightly reduced since several conventional bombs were dropped on the city on August 1, 1945. After the bombing, a number of people had evacuated to rural areas. However, a large number of people remained in the city, including some British POWs.
On the morning of August 9, 1945, a B-29 Superfortress named Bockscar headed for Japan. It was manned by Major Charles W. Sweeney and his crew, carrying a nuclear fission bomb named "Fat Man". The plane was preceded by two B-29's scouting the weather ahead of it and accompanied by another B-29, "The Great Artiste" with instrumentation for observing the blast. The bombing mission proceeded thirty minutes behind schedule and by the time they reached Kokura a cloud had obscured the city, preventing visual targeting.
The B-29s circled the city for a while before giving up and heading to Nagasaki, the secondary target. At approximately 7:50 AM Japan time, an air raid siren was sounded in Nagasaki, but cleared at 8:30 AM. The two B-29 bombers were initially spotted at 10:53 AM, but the Japanese assumed the planes were merely doing surveillance and did not sound an alarm.
At 11:00 AM, "The Great Artiste" dropped instruments attached to three parachutes, along with a letter to Professor Ryokichi Sagane. sagane was a nuclear physicist at the University of Tokyo and the letter was from three of his colleagues at Berkeley warning him about the dangers of nuclear bombs. At 11:01 AM, Bockscar's bombardier, Captain Kermit Beahan, visually sighted the target and ordered the bomb drop.
"Fat Man" was released from the plane, carrying its 14 lbs. of Plutonium-239, plummeting towards the city below. It was exploded when it was approximately 1540 feet above the ground between Mitsubishi Steel and Arms Works and Mitsubishi-Urakami Ordnance Works. The explosion's magnitude was equivalent to 21 kilotons of TNT and generated heat estimated at 3900 degrees Celsius, with winds of up to 624 miles per hour.
The city was destroyed by the incredible force of the bomb, leveling houses and killing about 40,000 people instantly over a one mile radius. The radiation effects contributed to further casualties over the next year or so, totaling about 80,000 casualties in all. Although the Nagasaki bombing was not as devastating as the Hiroshima bombing, it had its intended effect and encouraged the Japanese to surrender six days later.