The term 'kokushobi' translates as cruelly hot, brutally hot, or severely hot, reflecting the severity of the conditions. This is the first time the Japan Meteorological Agency has introduced a new term for weather forecasts since 2007, when it added 'mōshobi' for days over 35°C, according to sources. The adoption of 'kokushobi' underscores the increasing frequency of extreme temperatures in Japan.
Summer 2025 was Japan's hottest summer since records began in 1898, with average nationwide temperatures 2.36°C above average. Temperatures topped 40°C on nine days between June and August 2025, and the highest temperature was 41.8°C in the city of Isesaki on 5 August, the highest on record nationwide. In 2024, there were a total of four days of 40°C heat, with the highest temperature of 41°C recorded in the city of Sano, indicating a trend of rising extremes.
The name 'kokushobi' won a public vote, with 'chōmōshobi' coming in second place. In an online survey conducted earlier this year, 203,000 people chose 'kokushobi' as their favourite word for days over 40°C, and it received more than triple the votes for 'chōmōshobi'. According to the Japan Times, the decision to adopt 'kokushobi' was based on expert opinion that the name is socially familiar and appropriate in the Japanese language. According to Asahi Shimbun, other suggestions for the term included 'gekiatsubi', 'shakunetsubi', and 'futtobi'.
Scientists attribute much of the heat to the climate crisis, particularly the warming of waters around the Japanese archipelago. Warmer oceans also fuel heavier rainfall and more intense typhoons. 2025 was the third hottest year on record both globally and in Europe, according to Copernicus, and a Copernicus report emphasised that the years 2023 to 2025 were exceptionally warm due to the build-up of greenhouse gases in the atmosphere and reduced uptake.
Extreme heat causes health risks from sunburn and heat stroke to death, and affects the economy as outdoor industries are unable to operate and schools often have to shut.
The JMA has predicted a hotter-than-average summer this year, with high temperatures expected across the country.