Unlike Hiroshima, Nagasaki lies in a series of narrow valleys bordered by mountains in the east and west. The bomb exploded about 500 m. (1,625 ft.) above the ground and directly beneath it (the hypocentre) was a suburb of schools, factories, and private houses. The radius of destruction for reinforced concrete buildings was 750 m. (2,437 ft.), greater than at Hiroshima where the blast caused by the bomb was more vertical. But because of the topography, and despite the Nagasaki bomb being more powerful, only about sq. km. ( sq. mi.) of Nagasaki was reduced to ashes compared with 13 sq. km. (5 sq. mi.) of Hiroshima. Of the 51,000 buildings in the city % were completely destroyed or burt, with % escaping any damage.
The decision to use nuclear weapons on Japan was made after an analysis showed that more than a million people—combatants of both nations and civilians—would die if Japan was invaded by Allied forces. The nuclear bombings, while shocking and unprecedented, actually paled in comparison with the Operation Meetinghouse firebombing of Tokyo on March 9/10, 1945, which killed 100,000 civilians and destroyed 16 square miles in a single night. Had not for the nuclear weapons, the . would have resorted to firebombing Hiroshima and Nagasaki which would have caused equal amount of damage, but did not produce the shock value that nuclear weapons have.  In addition, doing nothing or waiting it out for surrender was not an option because hundreds of thousands of combatants, civilians, and prisoners of war were dying each month in Japanese-occupied lands.  
In the early stages of the explosion, temperatures of tens of millions of degrees were produced. The light emitted is roughly ten times the brightness of the sun. During the explosion, various types of radiations such as gamma rays and alpha and beta particles eminate from the explosion. These radiative particles give the atomic bomb its greatest deadliness. They may last years or even centuries in dangerous amounts. Gamma radiation and neutrons caused thousands of cases of radiation sickness in Japan. First the blood was affected, and then the blood making organs were impaired including the bone marrow, the spleen and the lymph nodes. When radiation was severe, the organs of the body became necrotic within a few days, marking the victim for certain death within a short period of time.