(23) What color was the mushroom cloud?

The cloud gradually changed colors

The mushroom cloud is widely thought to have been white or grey in color. This is probably because photographs of the cloud are in black-and-white.

Mr. Tagashira explains his picture of the mushroom cloud.

In the "Hiroshima Atomic Bomb Damage Report, Volume 2," the cloud is described as "A mass of smoke, produced by the explosion, rising thousands of meters into the air, swirling in strange, lovely colors like a rainbow, then forming a mushroom shape and blanketing the sky over the city."

It may be true, then, that the cloud was tinted with color. I then checked the database at Peace Memorial Museum to examine drawings that survivors have made of the bombing and found some pictures that portray the cloud as orange or violet. On the other hand, other drawings show the cloud as white or grey.

I also spoke with two people who witnessed the mushroom cloud that day.

Kazunori Tagashira, 80, was a teacher in Etajima, about 13 kilometers south of the hypocenter. After the blast, he saw the cloud from a window of his school. "It was a pale color, with shades of violet, red, blue, and green," he recalled. "Those colors appeared in different parts of the cloud."

He watched the cloud for just a few seconds, but during this time, its colors changed as the smoke expanded.

Hiroyuki Miyakawa, 78, saw the cloud as he was fleeing from his home, located 2.3 kilometers from the hypocenter. "There were flames of orange, green, yellow, and purple," he said. "I thought it was toxic, so I was desperate to escape. Soon after, the color changed to grey."

How, in fact, was the mushroom cloud produced? I asked Kiyoshi Shizuma, 58, a professor of Hiroshima University's Graduate School of Engineering and an expert on nuclear engineering, to explain.

The expansion of the fireball produces a drop in the pressure and temperature of the surrounding air, turning vapor into water droplets and the fireball into fleecy, white clouds. Because of this lower atmospheric pressure, the air and vapor around the fireball flow into it and grow. The air between the hot earth and the fireball becomes lighter and this brings about an ascending current which shapes the cloud into a mushroom form.

Professor Shizuma went on to explain the changing colors of the cloud.

"As the fireball expands, the temperature at its center falls so the color of the fireball gradually changes to red and yellow, colors that appear through the cloud produced around the ball."

But why would some people say they saw purple or green?

Professor Shizuma pointed out the possibility that some metals such as steel and uranium, used to manufacture the atomic bomb, may have reacted with the heat and radiation, emitting a pink or green light, while gamma rays contacting the air could have produced a violet-colored light.

Finally, "When the heat dissipates and the reaction of metal and air has ended, the cloud turns white. Still, it might look darker when mixed with dirt and sand from the ground."

In this way, the mushroom cloud ultimately loses it hues and becomes white or grey. (Toshiko Bajo)