Abstract
In 2008, Hiroshima city conducted a self-writing questionnaire survey for 31598 inhabitants of Hiroshima city and its suburb who might encounter with "Black Rain", and investigated rainy intensity, color, start time, end time and the location. The subjects were restricted to those who met "Black Rain" on 6 August 1945 with having exact information on the exposed location, and then 1565 subjects were available for statistical analyses. The individual's locations were classified to representative spots such as the investigation local government office and school. For each spot, the answer results were summarized with a ratio or a mean value. Using nonparametric smoothing method based on local linear regression model, we estimated spatial-time distributions of rain area, rainy color and rain intensity. It was revealed that "Black Rain" began to rain at around western suburb of Hiroshima city at about 9 am then became heaviest at about 10 am with spreading toward north-west direction, and disappeared at about 3pm on 6 August 1945. As a result, the circumference of the domain where rain time after a linear adjustment was estimated to be less than one hour was wider than the Uda rainy area. It was suggested that the rain area contains almost whole area of current Hiroshima city except the East and northeast side.
