Japan Just Created a New Word for 40C Days

Japan’s Meteorological Agency said on April 17 that days with highs of 40C or above will now officially be called kokushobi, a new forecast term often rendered as a cruelly or severely hot day, according to the agency announcement and SoraNews24’s report. The move gives Japan’s forecasters a formal label for the most extreme summer heat just months after the country logged its hottest summer on record.

Key developments

• The new term applies to days when the maximum temperature reaches 40C or higher. It extends the agency’s existing heat vocabulary, which already includes natsubi for 25C-plus days, manatsubi for 30C-plus days, and moshobi for 35C-plus days, as laid out in the JMA’s February survey notice.
• JMA said it chose kokushobi after a public survey conducted from Feb. 27 to March 29 and consultations with outside experts. In the PDF attached to the April 17 release, the agency said the term won 202,954 votes out of 478,296 total responses, far ahead of runner-up chomoshobi at 65,896.
• The agency said it will use kokushobi in information it publishes going forward, arguing that the new wording should help it more effectively call for caution during exceptionally dangerous heat.
• The timing reflects a broader shift in Japan’s summers. In its climate monitoring report, JMA said summer 2025 was the hottest since records began in 1898. The agency also said the national all-time high of 41.8C was set in Isesaki, Gunma, on Aug. 5, 2025, reinforcing why a new 40C category is no longer theoretical.

What to watch

The next test is practical use. JMA has now formalized the term, but this summer will show how quickly kokushobi moves from an official label into everyday forecasts, headlines and local heat messaging. If forecasters start using it regularly, the word could become part of Japan’s standard summer vocabulary almost immediately.

The bigger question is frequency. JMA launched the naming process because 40C readings have been appearing often enough to warrant their own category, and because Japan has just come through multiple years of exceptional summer heat. If another severe summer unfolds in 2026, kokushobi may stop sounding like a new coinage and start sounding like a routine warning.

Sources

• Japan Meteorological Agency: New name adopted for days with highs of 40C or above
• Japan Meteorological Agency PDF: Survey results for the new 40C heat term
• Japan Meteorological Agency: February notice launching the public survey
• SoraNews24: Japanese government creates new word for cruelly hot summer days

Photo by Roméo A. on Unsplash