Hiroshima professors infer changes in area of "black rainfall" over time
Mar. 1, 2010
by Uzaemonnaotsuka Tokai, Staff Writer
A group led by Professor Megu Otaki at Hiroshima University's Research Institute for Radiation Biology and Medicine (RIRBM), which has been charged with analyzing the survey conducted by Hiroshima Prefecture and Hiroshima City on the "black rain" in the aftermath of the atomic bombing, has compiled its conclusions about the rainfall. The group believes that the black rain began to fall in the western part of the city 45 minutes after the bombing, at about 9 a.m. on August 6, 1945, covered the largest area at about 10 a.m., then stopped at about 3 p.m.
In 2008, Hiroshima Prefecture and Hiroshima City sent survey forms to 36,614 people in and around the city, mainly A-bomb survivors (hibakusha) and those exposed to the black rain. Among 27,147 respondents, 1,844 people exposed to the black rain were able to specify the area they encountered the black rainfall.
The team led by Professor Otaki divided the city and the surrounding areas into 94 sections and grouped the 1,844 respondents according to where they experienced the black rain. The team has recognized a section with more than 10 witnesses as a "black rainfall area" and thoroughly examined the descriptions of the starting times of the black rain, the rainfall duration, and the heaviness of the rain.
As a result, the group has concluded that 1) the black rain began to fall at about 9 a.m. mainly in former Furuta Town (now, Takasu in Nishi Ward), 2) the rain covered the largest area at about 10 a.m., 3) the area of the rainfall gradually became smaller, moving northwest, and 4) the rain stopped at about 3 p.m. in former Kake Town (now, Akiota town).
On January 25 this year, Professor Otaki reported on the progress of their analysis at a meeting of the Hiroshima City Investigative Research Study Group on the Actual Situation of the Atomic Bomb Damage. Afterward, in consideration of the fact that the survey is based on memories of over 60 years ago, Professor Otaki revised the criterion for the black rainfall area from an area with "more than five witnesses" to an area with "more than ten witnesses," the drew the final conclusions.
Professor Otaki said, "This is the first time the results of the analysis of the black rainfall area over time have been compiled. This data will be able to serve as evidence in order to seek the expansion of the Health Examination Special Designated Area."
Keywords
Black rain
The black rain, which is believed to have contained radioactive fallout, fell in the aftermath of the atomic bombing. In 1976, the Japanese government defined an oval area stretching northwest from the hypocenter for about 19 kilometers from north to south and for about 11 kilometers from east to west as the "heavy rain area" and designated the area as the Health Examination Special Designated Area. Concerning the area stretching northwest from the hypocenter for about 29 kilometers from north to south and for about 15 kilometers from east to west, which includes the above-mentioned "heavy rain area," the Japanese government defined the other area of the oval as the "light rain area" and did not include it in the "heavy rain area." Meanwhile, according to the analysis by the group led by Professor Otaki at RIRBM, on the basis of the survey by Hiroshima Prefecture and Hiroshima City in 2008, the "heavy rain area" was located a little more to the northwest and roughly six times larger than the "heavy rain area" currently designated by the government.
(Originally published on February 24, 2010)
Related articles
Radioactive materials contained in black rain detected outside the Health Examination Special Designated Area (Feb. 3, 2010)
Survey raises possibility of expanding area of black rainfall (Jan. 28, 2010)
Professor at Hiroshima University seeks soil under floors to discern area of "black rainfall" (Jan. 13, 2010)
A group led by Professor Megu Otaki at Hiroshima University's Research Institute for Radiation Biology and Medicine (RIRBM), which has been charged with analyzing the survey conducted by Hiroshima Prefecture and Hiroshima City on the "black rain" in the aftermath of the atomic bombing, has compiled its conclusions about the rainfall. The group believes that the black rain began to fall in the western part of the city 45 minutes after the bombing, at about 9 a.m. on August 6, 1945, covered the largest area at about 10 a.m., then stopped at about 3 p.m.
In 2008, Hiroshima Prefecture and Hiroshima City sent survey forms to 36,614 people in and around the city, mainly A-bomb survivors (hibakusha) and those exposed to the black rain. Among 27,147 respondents, 1,844 people exposed to the black rain were able to specify the area they encountered the black rainfall.
The team led by Professor Otaki divided the city and the surrounding areas into 94 sections and grouped the 1,844 respondents according to where they experienced the black rain. The team has recognized a section with more than 10 witnesses as a "black rainfall area" and thoroughly examined the descriptions of the starting times of the black rain, the rainfall duration, and the heaviness of the rain.
As a result, the group has concluded that 1) the black rain began to fall at about 9 a.m. mainly in former Furuta Town (now, Takasu in Nishi Ward), 2) the rain covered the largest area at about 10 a.m., 3) the area of the rainfall gradually became smaller, moving northwest, and 4) the rain stopped at about 3 p.m. in former Kake Town (now, Akiota town).
On January 25 this year, Professor Otaki reported on the progress of their analysis at a meeting of the Hiroshima City Investigative Research Study Group on the Actual Situation of the Atomic Bomb Damage. Afterward, in consideration of the fact that the survey is based on memories of over 60 years ago, Professor Otaki revised the criterion for the black rainfall area from an area with "more than five witnesses" to an area with "more than ten witnesses," the drew the final conclusions.
Professor Otaki said, "This is the first time the results of the analysis of the black rainfall area over time have been compiled. This data will be able to serve as evidence in order to seek the expansion of the Health Examination Special Designated Area."
Keywords
Black rain
The black rain, which is believed to have contained radioactive fallout, fell in the aftermath of the atomic bombing. In 1976, the Japanese government defined an oval area stretching northwest from the hypocenter for about 19 kilometers from north to south and for about 11 kilometers from east to west as the "heavy rain area" and designated the area as the Health Examination Special Designated Area. Concerning the area stretching northwest from the hypocenter for about 29 kilometers from north to south and for about 15 kilometers from east to west, which includes the above-mentioned "heavy rain area," the Japanese government defined the other area of the oval as the "light rain area" and did not include it in the "heavy rain area." Meanwhile, according to the analysis by the group led by Professor Otaki at RIRBM, on the basis of the survey by Hiroshima Prefecture and Hiroshima City in 2008, the "heavy rain area" was located a little more to the northwest and roughly six times larger than the "heavy rain area" currently designated by the government.
(Originally published on February 24, 2010)
Related articles
Radioactive materials contained in black rain detected outside the Health Examination Special Designated Area (Feb. 3, 2010)
Survey raises possibility of expanding area of black rainfall (Jan. 28, 2010)
Professor at Hiroshima University seeks soil under floors to discern area of "black rainfall" (Jan. 13, 2010)
