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News Navigator: What is a 'typical year' for the rainy season in Japan?

Elementary school students are seen going home in the rain in the city of Kagoshima on May 11, 2021, after it was announced that the rainy season had likely begun in the area. (Mainichi/Junko Adachi)

The Mainichi Shimbun answers some common questions readers may have about how the start of rainy season and other weather-related phenomena are compared with a "typical year."

    Question: Is it true that this year's rainy season began on the earliest dates on record in some parts of Japan?

    Answer: Yes, it is. The rainy season has already started in the western and central parts of Japan, and these dates were the earliest on record in the regions of Shikoku and Kinki, both of which were 21 days earlier than usual.

    Q: How is it decided if the rainy season begins "earlier" or "later" than a typical year?

    A: The start of the season is compared with a "normal value," which is the average over the past 30 years. In fact, the normal value for weather information was revised in May for the first time in 10 years. Regarding the rainy season's start, up until now the average date over 30 years between 1981 and 2010 was used, but over the next decade, the average date over 30 years between 1991 and 2020 will be used.

    Q: What about other weather-related phenomena?

    A: There are normal values for temperatures, rainfall, hours of sunshine, snowfall, humidity, the number of typhoons that form, or hit or approach Japan, the start of the cherry blossom season, etc. And of course, the end of the rainy season, too.

    Q: Wow, so many. Are these revised normal values different from previous ones?

    A: Average temperatures across Japan rose by 0.1 to 0.5 degrees Celsius compared to previous normal values. The number of "extremely hot days," in which the highest temperature marks 35 C or above, increased in many parts of Japan, while the numbers of "winter days," in which the lowest temperature of the day drops below 0 C, decreased. The effects of global warming and urbanization are seen as the causes of these changes. Furthermore, the amount of rainfall during the rainy season has increased by as much as 10 to 30% mainly in western Japan.

    Q: So we have to be careful, right?

    A: Yes. For example, there will be cases in which rainfall that used to be "higher than normal" under the previous standards will be "about average" according to the new normal values. Instead of lowering your guard because the amount of rainfall is expected to be the same as an average year, we need to keep an eye on the latest weather information.

    (Japanese original by Azusa Yamazaki, Kyushu News Department)

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