Japan's weather agency has introduced the term “kokusho-bi” to describe days when maximum temperatures reach 40 °C (104 °F) or higher [1].
The adoption of this new terminology reflects the increasing frequency of extreme heat waves and the need for clearer public warnings. As temperatures climb to dangerous levels, the government seeks to standardize how it communicates severe thermal risks to the population.
The Japan Meteorological Agency said the term was announced in April 2026 [2]. The phrase “kokusho-bi” is translated into English as “cruelly hot,” “brutally hot,” or “severely hot” [3]. This specific designation is triggered only when the mercury hits the 40 °C [1] threshold.
Officials said the name was introduced because heat-wave days are becoming more frequent across the archipelago [3]. The agency's move comes as forecasts indicate an exceptionally hot summer is expected for the region [2].
By creating a distinct category for these extreme events, the agency aims to highlight the difference between a standard hot day and one that poses a severe risk to human health. The term provides a linguistic marker for the highest tier of heat warnings, a necessary step as the climate shifts.
This change follows a pattern of intensifying weather patterns in East Asia. The agency's decision to formalize “kokusho-bi” acknowledges that temperatures previously considered rare are now becoming a recurring reality for Japanese cities and rural areas alike [2].
“Japan's weather agency has introduced the term “kokusho-bi” to describe days when maximum temperatures reach 40 °C (104 °F) or higher.”
The creation of a specific linguistic category for 40 °C days indicates that Japan is adjusting its public safety infrastructure to a new climatic baseline. By labeling these days as 'cruelly hot,' the government is not only updating its meteorological records but also signaling a heightened state of emergency for public health officials and citizens to prevent heat-related mortality.
