Japan has logged its first official kokushobi — literally "brutally hot days" — since the Japan Meteorological Agency introduced the new designation in April for temperatures reaching or exceeding 40 degrees Celsius. On Tuesday, the central Japanese cities of Gujo and Tajimi in Gifu prefecture, and Toyota in the adjacent Aichi prefecture, all hit 40°C, with Gujo and Toyota recording all-time highs at their respective weather stations. Authorities forecast a second consecutive kokushobi day on Wednesday, with temperatures expected to top 40°C in Kumagaya, north of Tokyo, and Nagoya in central Japan.

The scale of the crisis is evident in the numbers. Tokyo's fire department reported that 287 people — ranging in age from three to 103 — were taken to hospital with suspected heatstroke on Tuesday alone, the highest single-day figure recorded in the capital this year. Nationwide, more than 4,500 people required hospital treatment in just one week in early July, including seven who died. Two further heat-related deaths were reported this week in Niigata and Kanagawa prefectures. Across the country, heatstroke deaths surpassed 2,000 for the first time in 2024, and more than 100,000 people required hospitalisation during the 2025 summer season to date. Authorities are urging residents to use air conditioning freely, hydrate regularly and limit time outdoors.

The kokushobi category sits above the existing tier of moshobi, or "extremely hot days," which applies when temperatures exceed 35°C — a threshold crossed at 278 of Japan's 914 weather stations on Tuesday alone. Days above 30°C are called manatsubi, or "true summer days," while those above 25°C are known as natsubi, or "summer days." The new terminology reflects a stark shift: temperatures once considered freakish are increasingly routine. Average summer temperatures in Japan have risen by about 1.3°C over the past century, and both 2023 and 2024 were record-breaking years, before 2025 surpassed them again. Last summer saw a new all-time Japanese high of 41.8°C recorded at Isesaki, in Gunma prefecture north of Tokyo.

The heat is reshaping daily life across Japan in tangible ways. The koshien national high school baseball tournament — one of the country's most beloved sporting events, traditionally held in August — has been split into morning and evening sessions since 2024 to reduce the risk of heatstroke for players and spectators. Tokyo Governor Yuriko Koike has extended the city's "Cool Biz" workplace dress code to allow polo shirts, T-shirts and trainers in offices, aiming to cut air conditioning use. Agriculture is also under pressure, with rice yields and quality declining, prompting some farmers to switch to heat-tolerant varieties or even crops like avocado, not previously grown at scale in Japan. A study by the National Institute for Environmental Studies and Waseda University found that without drastic reductions in greenhouse gas emissions, outdoor school sports could become unplayable by the 2060s.