Japanese Description of Explosion.
Survivors of the atomic-bomb attack stated that the detonation seemed like a vast combustion of magnesium filling the entire sky. These persons reported that the flame contained various colors such as greenish-white and yellowish-red. Reports of the duration of the flash varied from instantaneous to 2 to 4 seconds. At the same time an overpowering heat wave emanated from the source of the flash.
The entire city of Hiroshima was darkened by a dense pall of smoke and dust, which limited visibility to a few feet. From a distance, a mushroom-shaped cloud was seen expanding and covering the entire city. It then rose, reaching a height of 23,000 feet 4 minutes after the explosion. The column began to disintegrate after 8 minutes, the top of the mushroom separating itself from the column and remaining intact. The color of the column was gray and white while the ball on top was white tinged with crimson.
In the center of the city a violent blast of air immediately followed the flash, knocking down trees and poles, stripping branches off trees, tearing sheets of galvanized metal off buildings, derailing streetcars, and squashing or knocking over houses.
A slight interval between the flash and the blast was noted by persons who were removed from the center of the city. Few persons in the downtown area heard any noise of explosion. They were aware only of a blinding flash and the overpowering heat and wind. Outside the city, however, a loud rumbling noise was heard.
Fires soon broke out throughout the city and developed into an engulfing inferno in the central area of destruction. Some of the fires were caused by direct ignition of thatched roofs, curtains, trees, and the like but the majority resulted from secondary effects.
Directly after the explosion survivors reported finding in some places a “substance” which emitted a weak, bluish-white fluorescent light. This substance, upon contact, burned through or ignited combustible objects. When it fell upon clothing it burned through to the flesh, producing water blisters which gradually diffused and became extremely painful.
Following the explosion, strong, changeable winds arose, attaining velocities of 25 to 35 miles an hour. Whirlwinds were reported at a few points.
While the fires were spreading, there was a light rainfall throughout the central part of the city, with occasional heavy showers in the northwestern section.
Barnett, Erin, and Philomena Mariana, eds., Hiroshima: Ground Zero 1945. New York: International Center of Photography, 2011. p. 10
United States Government, The United States Strategic Bombing Survey: Physical Damage Division, The Effects of the Atomic Bomb on Hiroshima, Japan, Volume II, May 1947, p.10 (2011.23.2)
United States Government, The United States Strategic Bombing Survey: Physical Damage Division, The Effects of the Atomic Bomb on Hiroshima, Japan, Volume II, May 1947, p.34 (2011.23.2)