Japan recorded its first officially designated "extreme heat day" of the year on Tuesday after temperatures exceeded 40°C in central regions [1].
This milestone marks the first time the Japan Meteorological Agency (JMA) has applied the specific "酷暑日" (kokushobi) designation in 2026. The agency only formally defined the term in April 2024, making this the first occurrence of the phenomenon under the new naming convention during the current calendar year [1], [2].
The heat peaked before 2 p.m. on July 21 [1]. In Gifu Prefecture, the city of Tajimi reached a high of 40.3°C [4], while Gujo Hachiman recorded 40.0°C [1]. In Aichi Prefecture, Toyota city saw temperatures hit 40.1°C [1]. These readings represent the highest temperatures recorded across the country for the day [3].
Meteorologists said the spike was due to a strong summer high-pressure ridge. This weather system produced intense sunshine and stagnant air, conditions that trapped heat across the central plains.
The extreme temperatures prompted the JMA to issue heatstroke alerts for 42 prefectures [1]. Local authorities said residents should stay hydrated and utilize air conditioning to avoid heat-related illnesses as the pressure ridge continues to influence the region.
The designation of a "kokushobi" is based on a strict threshold of 40°C. While Japan has experienced such temperatures in previous years, the formalization of the term allows the JMA to track and categorize extreme heat events with greater precision than the broader "extremely hot day" category, which typically triggers at 35°C [2].
“Japan recorded its first officially designated "extreme heat day" of the year on Tuesday.”
The application of the 'kokushobi' label reflects the JMA's effort to modernize weather tracking in response to rising global temperatures. By distinguishing between 'extremely hot' (35°C+) and 'extreme heat' (40°C+), Japanese officials can better communicate the severity of heatwaves to the public and improve the precision of emergency health responses in high-risk urban and industrial zones.
