Japan’s Meteorological Agency has given exceptionally hot days a name with very little interest in being reassuring: kokushobi, 酷暑日. Roughly, a severely or cruelly hot day. Pick whichever English adjective makes you sweat most.
In its April 17 announcement, the JMA adopted the term for days when the maximum temperature reaches at least 40°C. The purpose is to communicate the severity of the heat more effectively. It is a new official label, not a promise that the thermometer will behave better now that it has been formally introduced.
The Public Picked the Cruelly Hot One
The agency’s online survey received 478,296 responses between February 27 and March 29. Kokushobi attracted 202,954 selections, ahead of chōmōshobi, 超猛暑日, with 65,896. The published results also list suggestions resembling “sauna day” and “boiling day.” Public consultation briefly acquired the atmosphere of a group chat with broken air conditioning.
The JMA considered both the responses and expert opinions. It was a naming consultation, not a binding referendum on climate policy. The agency cited familiarity and suitability in Japanese among the reasons for its choice.
A Brief Tour of the Heat Vocabulary
The existing thresholds in the JMA’s explanation of the consultation are:
- Natsubi, 夏日: a daily maximum of at least 25°C.
- Manatsubi, 真夏日: a daily maximum of at least 30°C.
- Mōshobi, 猛暑日: a daily maximum of at least 35°C.
Kokushobi adds a name at 40°C. These thresholds overlap: a day reaching 40°C has also exceeded 35°C. They describe the daily maximum, not the temperature remaining above the threshold for every hour.
The word itself did not emerge from a government laboratory this April. The Japan Weather Association says it has used kokushobi for 40°C days since 2022. That organisation and the government’s Japan Meteorological Agency are distinct. The change is the JMA’s adoption of the term.
Why the Extra Name Feels Less Than Comforting
Japan’s previous summer provides the backdrop. In its analysis of summer 2025, the JMA reported a national summer mean temperature anomaly of +2.36°C relative to the 1991–2020 average. It was the hottest summer in the series beginning in 1898. Isesaki recorded a national daily maximum of 41.8°C on August 5.
A seasonal average anomaly and one station’s daily maximum measure different things, but both help explain the attention to extreme heat. There is no need to turn them into an unsupported count of “heat events” or a prediction about how many will occur this summer.
The unsettling part is ordinary language catching up with an experience people already recognise. “Very hot” covers a lot of ground. A more specific term can make a forecast easier to communicate without making the conditions any more welcome.
A Name Is Not an Automatic Alert
Crossing from 39.9°C to 40°C changes which temperature label applies. It does not, by itself, trigger a different national warning, close every school or activate a new hospital staffing rule.
Japan’s heatstroke alerts use forecasts of WBGT, the wet-bulb globe temperature index, which accounts for factors including humidity and solar radiation as well as air temperature. It is a separate measure from the number on a conventional thermometer.
The practical point is to follow local alerts and guidance rather than wait for a day to qualify for the newest name. A memorable category helps communication; it does not replace the information behind the warning.
The Cat’s Unofficial Forecast
Cats, naturally, appear to maintain their own highly informal temperature vocabulary: the loaf, the side-flop, the full sprawl and the bathroom-tile splat. This is household comedy, not a calibrated warning system. The cat has declined to submit its methodology.
Kokushobi feels like the human equivalent of finally giving the tile-splat mood a line in the dictionary. The sensation was already familiar; now the forecast has a more specific word for it.
There is no announced 45°C tier hiding in the runner-up’s envelope, and no guarantee that English will adopt the word next. For now, Japan has a label that makes the situation unusually plain. Cruelly hot. Even the cat might approve of the wording, provided someone else handles the paperwork.
