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Cocktail of high humidity, vapor blown into Japan caused 2018 torrential rains: research

(Mainichi)

OSAKA -- Torrential rains that lashed western Japan in July 2018 were caused by large amounts of water vapor in skies up to 10,000 meters above a wide area of Japan spanning Kyushu in the southwest to Honshu's central regions, according to research by a team at Kyoto University.

Humidity levels were at 80%, creating weather conditions comparable with tropical regions, according to a paper by the Disaster Prevention Research Institute, Kyoto University.

Factors including high atmospheric pressure reportedly projected westward from the Pacific Ocean conspired to create a mass of water vapor in skies over southern sea regions and from South East Asia to the East China Sea. Seasonal winds then carried the conditions into the air above Japan.

Tetsuya Takemi, an associate professor in meteorology at the research institute analyzed information from the Japan Meteorological Agency including humidity, water vapor quantities, air pressure and temperature in 5-kilometer-square sections around Japan from the three days commencing midnight July 4.

By doing so, they found that the atmosphere from Kyushu to the center of Japan had been very insecure, with their numbers showing that conditions were ripe for the formation of cumulonimbus clouds. Concentrations of water vapor were particularly high in the seas off of Shikoku and Kyushu.

The amount of water vapor calculated to have been present in the atmosphere over 10 kilometers above sea level in each section within those regions was at least 20% more than the average numerical amount recorded in a typical July.

With humidity levels also exceeding 80% at altitudes between 3,000 and 9,600 meters, the team said, "During ordinary periods of rainfall, humidity is around 60%. The conditions (at the time of the disaster) were similar to those in tropical regions, as high levels of humidity in the air enabled cumulonimbus clouds to form continuously over a wide area."

Takemi said, "If we can become able to more accurately estimate amounts of water vapor and rates of humidity, we could forecast torrential rains."

The team's findings were published in the electronic edition of the Journal of the Meteorological Society of Japan on Sept. 19.

(Japanese original by Ryo Watanabe, Osaka Science & Environment News Department)

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