Tokyo: Japan has introduced a new term, ‘kokushobi’, to describe days when temperatures reach 40°C (104°F) or higher, following its hottest summer on record last year. The term has been widely translated as ‘cruelly hot,’ ‘brutally hot,’ or ‘severely hot’ by both Japanese and international media.
The name was selected as the most popular choice in a nationwide online survey conducted between February and March, which gathered around 478,000 responses. Participants were asked to choose from 13 possible names for extremely high-temperature days, with ‘super extremely hot day’ ranking second.
The terminology was formally introduced by the Japan Meteorological Agency, which used the word ‘koku,’ meaning harsh or cruel, to reflect the intensity of such heat conditions. Japan already categorises hot days with terms for temperatures exceeding 25°C, 30°C, and 35°C.
The introduction of ‘kokushobi’ follows a period of unprecedented heat across the country. The summer of 2025 was the hottest recorded since measurements began in 1898, with national average temperatures rising 2.36°C above normal levels.
During that summer, temperatures exceeded 40°C on nine occasions between June and August. A new national record of 41.8°C was registered in the city of Isesaki.
The total number of extremely hot days also surpassed previous records. Tokyo experienced 25 days with temperatures above 35°C, significantly higher than its average of 4.5 days. Similarly, Kyoto recorded 52 such days, compared to its typical average of 18.5.
Experts attribute the rising frequency and severity of extreme weather events, such as heatwaves, droughts, wildfires, and floods, to human-driven climate change, particularly the continued burning of fossil fuels.
Looking ahead, the Japan Meteorological Agency has forecast a high likelihood of above-average temperatures across Japan from June to August, raising concerns over more frequent ‘kokushobi’ days of extreme heat.
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