Japan is experiencing a severe heat wave with temperatures reaching or exceeding 40 °C [1, 2].

The extreme weather has prompted the Japan Meteorological Agency to implement a new classification for heat levels to better protect residents and tourists from life-threatening conditions. This effort aims to reduce the incidence of heat stroke during the peak of summer.

Officials introduced the term “kokushobi,” which translates to “cruelly hot day,” to describe days when temperatures hit approximately 40 °C [1, 3]. The agency first introduced the term in April 2024 [4] to create a more urgent warning system for the public.

Recent data shows that temperatures have soared over 40 °C [2], marking the first official “cruelly hot day” of the current period [2]. While some reports indicate temperatures were near 40 °C [1], other records confirm the threshold was surpassed.

The heat wave has affected major cities across the country [3]. In response, the government issued heat-stroke warnings in more than 40 prefectures [5]. These alerts encourage citizens to stay hydrated, and seek air-conditioned environments to avoid medical emergencies.

The use of “kokushobi” represents a shift in how the Japanese government communicates weather risks. By labeling these days as “cruelly hot,” the agency seeks to emphasize the physical danger associated with this specific temperature bracket, a move necessitated by the increasing frequency of extreme thermal events in the region [3].

“Japan is experiencing a severe heat wave with temperatures reaching or exceeding 40 °C”

The introduction of the 'kokushobi' terminology indicates that previous heat warnings were insufficient for the escalating intensity of Japanese summers. By creating a specific linguistic category for temperatures at or above 40 °C, the Japan Meteorological Agency is attempting to trigger a higher level of public vigilance and institutional readiness to prevent mass heat-related casualties.