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Documenting Hiroshima of 1945: Early October, traces of black rain found here and there

by Minami Yamashita, Staff Writer

In early October 1945, a group that included Yukio Miyazaki from Japan’s Institute of Physical and Chemical Research (Riken) was looking in the western areas of Hiroshima City for traces of “black rain,” which had fallen soon after the atomic bombing of the city. The survey was part of the work carried out by a special task force formed to investigate the damages caused by the atomic bombing. In the area of Koi-cho (in Hiroshima’s present-day Nishi Ward), located around 3.5 kilometers from the hypocenter, the survey team took a photograph of a pair of trousers that had been turned black by the rain.

The black rain formed after dirt and soot containing radioactive materials swirled up into the sky from the ground by the atomic bombing and mixed with moisture in the air. The rain is thought to have fallen primarily between 9 a.m. and 3 p.m. on August 6. According to the Record of the Hiroshima A-bomb War Disaster, published in 1971, the rain fell in torrents in the city’s western area (in the direction of Koi and Takasu) and the northern part of the city (toward Kabe).

Shigeaki Mori, 87, a resident of Hiroshima’s Nishi Ward, experienced the atomic bombing in Koi-cho in an area around 2.5 kilometers from the hypocenter, when he was a third-year student at Koi National School (present-day Koi Elementary School). Mr. Mori was exposed to the flash from the bombing on a bridge crossing the Yahata River, which runs through that part of the city, and was blown into the river by the bomb’s blast. Shortly after that, black rain fell hard, raising the river’s water level. “The black rain fell as if striking me, and it was painful. The national uniform I wore had turned pitch-black so, thinking it was dirty, I took it off and threw it away,” said Mr. Mori.

At the beginning of September, Riken’s Fumio Yamasaki and other staff confirmed that there were high levels of radiation in a sediment sample collected from a rain gutter in the district of Furue (in Hiroshima’s present-day Nishi Ward). However, still unclear is the extent of damage from the black rain. Between August and December 1945, Michitaka Uda and other technicians at the Hiroshima District Meteorological Observatory (present-day Hiroshima Local Meteorological Observatory) conducted a survey made up of interviews with local people and compiled the information into a report on the areas in which the rain had fallen. However, Isao Kita, one of Mr. Uda’s colleagues who had also participated in the survey, later spoke about how the area was only “tentative” and about how the information had “taken on a life of its own.”

Based on the rainfall information reported by Mr. Uda, Japan’s national government defined an oval-shaped area that measured 19 kilometers long and 11 kilometers wide, stretching northwest of the hypocenter, as being the “heavy rain area” and, in 1976, designated that area as being eligible for relief. In 2022, those exposed to the black rain outside of that designated area won a class-action lawsuit, which led to an expansion of the number of people considered eligible for relief. Nevertheless, residents who claim they were exposed to the black rain but are still not designated as such continue their efforts to pursue lawsuits against the government.

(Originally published on October 3, 2024)

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