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As the air raid "all-clear" alarm sounded that morning, damage was extremely heavy. It's estimated that the death toll of the A=bomb from August 6 to the end of December 1945 amounted to 140,000.

The yield of the A-bomb was equivalent to about 13 kilotons of TNT in explosive power (1.3x100, 000,000,000,000 calories of energy). Three factors worked simultaneously: thermal radiation, blast pressure, and radiation.

1. Thermal Radiation.

The temperature of the fireball that developed in the air was estimated at 300,000 degrees Centigrade, 1/10,000 of a second after the detonation; 3/10 of a second later a maximum diameter of 280 meters (about 920 feet) and a surface temperature of the fireball was about 5,000 degrees Centigrade.

The intense thermal radiation released by this fireball caused burns on the bodies of those who were within 3.5 kilometers (about 2 miles) of the hypocenter. Within 1 kilometer (about 6/10 of a mile) of the hypocenter, clothes and wooden houses were burned and the surface of granite stones cracked in the heat. Glass-like bubbles formed on the surface of roof tiles within 600 meters (about 1,965 feet) of the hypocenter.

2. Blast Pressure.

 The blast pressure at ground zero was estimated at approximately 1.3 kilometers (about 8/10 of a mile) from the hypocenter the maximum blast pressure and velocity were calculated at about 7 tons per square meter and 20 meters per second (about 390 feet per second) respectively.

The effect of the blast on human bodies was tremendous. Some victims were blown several meters off the ground. The blast stripped off clothing, tore off burned skin, and in some victims caused the rupture of internal organs. Glass fragments penetrated human bodies. People who got caught under crumbled houses could not escape and burned to death. Wooden buildings within a 2.3 kilometers (about 1.4 mile) radius were almost totally obliterated and those within 3.2 kilometers (about 2 miles) were half destroyed. The concrete buildings around the hypocenter were completely destroyed. Ceilings caved in and doors and windows shattered.

 3. Radiation.

Numerous physical disturbances were caused by the radiation of gamma rays and neutrons emitted within one minute of the bombing. The radiation is what made the A-bomb different from conventional weapons. Its effects extended as far as 2.3 kilometers (about 1.4 miles) from the hypocenter. The area within a 1-kilometer (about 6/10 of a mile) radius from the hypocenter was severely affected by large doses of radioactivity.

Residual radiation was present on the ground long after the explosion. Anyone who entered a 1 kilometer (about 6/10 of a mile) radius of the hypocenter thirty minutes after the explosion to aid victims or search for people who were still alive were affected by the exposure to gamma rays within 100 hours of the bombing. A "Black Rain" containing large doses of radioactivity fell on the city for ninety minutes, causing severe damage to humans, animals, and vegetation.