(28)Why did "black rain" fall on a clear day in Hiroshima?

Three kinds of clouds were caused by the atomic bomb

Peace Volunteer Yoshinori Obayashi recalling his experience with the black rain (Hiroshima peace Memorial Museum)

From books and movies, you may already know that the morning of August 6, 1945, was sunny. In a 1953 report on damage caused by the atomic bomb, staff from the local weather station in Hiroshima reported that the weather on that morning was "clear and windless, almost completely calm".

Kiyoko Sumikawa, 73, lived in Yuki town, northwest of Hiroshima city, where she saw black rain falling near her house. She recalls her anxious feeling at the time: "Raindrops as big as the tip of my little finger came falling down, and my white blouse turned black. It rained hard and splashed as high as thirty centimeters. I didn't know about the situation in the city center, so I wondered what on earth had happened and was quite alarmed."

First of all, why does rain fall, ordinarily? Hiroshi Wakisaka, Curator at the Hiroshima City Ebayama Museum of Meteorology, explained that to me. Water vapor in rising air currents is cooled high in the sky and forms ice crystals or water droplets around particles of dust in the air. The longer the rising currents continue, the more vapor is carried upward, and the bigger the drops of water become. As the drops get bigger, they become too heavy and they fall.

So then, why did rain fall on the day of the bombing?

Professor Masaharu Hoshi, a specialist in radiation biology and physics at the Hiroshima University Research Institute for Radiation Biology and Medicine, joined a study group on black rain sponsored by the former Ministry of Health and Welfare. He explained that black rain was a product of three types of clouds produced by the atomic bomb.

First, a cloud of soil and dust was stirred up by the blast and carried upward by rising air currents heated by the fireball. This cloud rose 4000 meters into the sky and formed the column or pillar of the "mushroom cloud."

Next, the cloud spread out high in the sky, forming the cap of the mushroom cloud. As the cloud spread, the temperature and pressure decreased, and water vapor in the air condensed to form droplets.

The third cloud was caused by huge fires that were started by heat from the bomb. The air currents rising above these fires lifted more hot air and water vapor 800 meters into the air. Rain produced by big fires isn't associated only with atomic bombs. Emeritus Professor Hitoshi Koyama of Kansai University, writing about the Osaka air raids, noted that "after a big air raid, black rain almost always fell."

Professor Hoshi points out that "All three types of rain contained radioactivity. We call it 'black rain,' but actually it is thought to have included brown or colorless rain as well."

In 1976, the government decided to offer health checkups, at government expense, to people who lived inside an area where they had determined that heavy black rain had fallen. However, some people who lived outside that area claimed that heavy rain had fallen in their areas as well. In that same year, those people established an association to demand that the government expand the designated area.

The city intends to survey 30,000 citizens who have lived in Hiroshima since 1945 regarding the psychological effects of their exposure to black rain. Based on the results of the survey, the city plans to ask the national government to improve relief measures for these survivors. (Kensuke Murashima, staff writer)