Fat Man was the codename of the atomic bomb dropped on Nagasaki on August 9, 1945.
The weapon was a plutonium implosion-type bomb carried by the B-29 bomber Bockscar. It exploded at approximately 11:02 a.m. local time over the Nagasaki area, three days after Little Boy was used against Hiroshima.
Nagasaki was the secondary target because clouds and smoke obscured Kokura, the primary target. A brief opening in the cloud cover allowed bombardier Kermit Beahan to make a visual drop. The explosion devastated much of the Urakami Valley and caused tens of thousands of immediate deaths, with additional deaths from injuries and radiation effects.
Fat Man was more complex than Little Boy because it used an implosion design to compress a plutonium core. The two bombs used different nuclear materials and different engineering approaches.