Japan announces new name for days over 40C after hottest summer ever
Japan now has a special name for days that are 40C or hotter. ‘Kokushobi’ translates as cruelly hot, brutally hot or severely hot. The name won a public vote, with ‘chōmōshobi’, meaning super extremely hot...
Japan now has a special name for days that are 40C or hotter.
‘Kokushobi’ translates as cruelly hot, brutally hot or severely hot. The name won a public vote, with ‘chōmōshobi’, meaning super extremely hot day, coming in second place.
It is the first time the Japan Meteorological Agency (JMA) has been forced to introduce a new term for weather forecasts since 2007, when ‘mōshobi’ (extremely hot day) was introduced for days over 35C.
Japan’s hottest summer ever
Summer 2025 was Japan’s hottest summer since records began in 1898, with average nationwide temperatures 2.36C above average.
Temperatures topped 40C on nine days between June and August, with the mercury hitting 41.8C in the city of Isesaki on 5 August - the highest on record nationwide.
For comparison, 2024 saw a total of four days of 40C heat, with the highest temperature of 41C recorded in the city of Sano.
The JMA has already predicted a hotter-than-average summer this year, with high temperatures expected across the country.
Scientists attribute much of this to the climate crisis, particularly the warming of waters around the Japanese archipelago which locks the country into higher temperatures well into autumn. Warmer oceans also fuel heavier rainfall and more intense typhoons.
Kokushobi joins list of Japanese names for hot days
In an online survey conducted earlier this year, 203,000 people chose ‘kokushobi’ as their favourite word for days over 40C. This was more than triple the votes for ‘chōmōshobi’, meaning super extremely hot day, which came in second place.
Alongside the survey results, the decision was made based on expert opinion that the name is socially familiar and appropriate in the Japanese language, according to the Japan Times.
‘Kokushobi’ uses the Japanese character koku (酷) which means harsh or cruel.
According to news outlet Asahi Shimbun, other suggestions included ‘gekiatsubi’ meaning a brutally intense, explosive kind of heat; ‘shakunetsubi’, evoking a blistering heat that seems to burn the skin; and ‘futtobi’, a day so hot it feels as though the world itself is boiling.
Currently, the JMA categorises days with temperatures reaching 25C or higher as ‘natsubi’ meaning summer day. ‘Manatsubi’, meaning midsummer day, for days over 30C, and ‘mōshobi’, meaning extremely hot day, for days over 35C.
Extreme heat causes a range of health risks, from sunburn and heat stroke to death. It also affects the economy as outdoor industries are unable to operate and schools often have to shut.
Japan follows the global trend as the climate crisis creates deadly heat
2025 was the third hottest year on record both globally and in Europe, according to Copernicus.
The past three years - 2024, 2023 and 2025 - were the hottest ever recorded globally.
A Copernicus report emphasised that there are two main reasons why the years 2023 to 2025 were exceptionally warm. The first is the build-up of greenhouse gases in the atmosphere, from continued emissions and reduced uptake of carbon dioxide by natural sinks like forests.
Second is sea-surface temperatures reaching exceptionally high levels across the ocean, associated with an El Niño event and other ocean variability factors, amplified by climate change.
While fossil fuels continue to be burned at alarming rates, temperatures will continue to rise, putting millions of lives, homes and businesses at risk.
During the current energy crisis, caused by the effective closure of the Hormuz Strait, renewable energy sources like wind and solar have continued to prove their worth.
Governments are key to the clean energy transition, by doing things like cutting subsidies for fossil fuel companies. But citizens can also play their part by installing solar at home and reducing their energy consumption.
