“After Biak the enemy withdrew to deep caverns.Rooting them out became a bloody business which reached itsultimate horrors in the last months of the war.You think of the lives which would have been lost in an invasion of Japan’s home islands – a staggering number of Americans but millions more of Japanese..."

The bombing of Hiroshima

The bomb was armed in flight 31,000 feet (9,400 m) above the city, then dropped at approximately 08:15 (JST) August 6, 1945...... After falling for 44.4 seconds, the time and barometric triggers started the firing mechanism. The detonation happened at an altitude of 1,968 feet (600 m). With a yield of 13 to 18 kilotons, it was less powerful than "Fat Man", which was dropped on Nagasaki (21–23 kt).

The official yield estimate of "Little Boy" was about 16 kilotons of TNT equivalent in explosive force, i.e. 6.3 × 1013 joules = 63 TJ (tera-joules).[30] However, the damage and the number of victims at Hiroshima were much higher, as Hiroshima was on flat terrain, while the hypocenter of Nagasaki lay in a small valley.

According to figures published in 1945, 66,000 people were killed as a direct result of the Hiroshima blast, and 69,000 were injured to varying degrees.

The Blast
The blast from a nuclear bomb is the result of X-ray-heated air (the fireball) sending a shock/pressure wave in all directions at a velocity greater than the speed of sound (aka, the "Mach-Stem"),[17] analogous to thunder generated by lightning. Most knowledge about nuclear weapon urban blast destruction originates from studies of Little Boy at Hiroshima. Data from the explosion at Nagasaki offers less insight, since hilly terrain deflected the blast and generated a more complicated pattern of destruction.
Frame house in 1953 nuclear test, 5 psi overpressure.

At Hiroshima, severe structural damage to buildings extended about 1 mile (1.6 km) in radius from ground zero, making a circle of destruction 2 miles (3.2 km) in diameter. The blast sent out a hyper-intensified shock wave which travelled at (slightly above) the speed of sound, turning buildings into shrapnel. There was little or no structural damage outside of this one-mile (1.6 km) radius. At one mile (1.6 km), the force of the blast wave was 5 psi, with enough duration to implode houses and reduce them to kindling