Japan has officially designated days with temperatures of 40C (104F) or higher with a new term: 'Kokushobi'. The name, translating to 'cruelly hot' or 'severely hot', won a public vote.
This marks the first time the Japan Meteorological Agency (JMA) has introduced a new weather forecast term since 2007, when 'mōshobi' (extremely hot day) was created for temperatures over 35C.
Summer 2025 was Japan’s hottest on record, with average temperatures 2.36C above normal. Temperatures surpassed 40C on nine days between June and August, reaching a national high of 41.8C in Isesaki. This contrasts with 2024, which saw only four days above 40C.
Scientists attribute the extreme heat to the climate crisis and warming ocean waters around Japan. These conditions also contribute to heavier rainfall and more intense typhoons.
'Kokushobi' was chosen by over 203,000 participants in an online survey. The JMA currently categorizes days of 25C and above as 'natsubi' (summer day), 30C and above as 'manatsubi' (midsummer day), and 35C and above as 'mōshobi' (extremely hot day).