Japan is experiencing extreme heat across multiple regions with temperatures exceeding 40 degrees Celsius this July [1].
These temperature spikes represent a growing public health risk as the country grapples with more frequent and intense heatwaves. The severity of the current weather has prompted the government to formalize new terminology to categorize these dangerous temperature thresholds.
Imura Ryoko said the current conditions are driven by stagnant high-pressure systems and weather patterns following the end of the rainy season [1]. Climate change and the increasing frequency of heatwaves are also cited as primary causes for the spike [3].
In Mie Prefecture, officials recorded a peak temperature of 40.6 degrees Celsius [1]. This extreme heat falls under the newly defined category of "kokushobi," or extreme heat days. The Japan Meteorological Agency officially adopted the term "kokushobi" on July 17 [5].
Under these guidelines, a "manatsubi" is a day with temperatures of 30 degrees Celsius or higher, while a "moshobi" is 35 degrees Celsius or higher [3]. The "kokushobi" designation is reserved for days reaching 40 degrees Celsius or more [3].
Forecasts for the 2026 summer season suggest that between six and 10 locations nationwide will experience these extreme heat days [2]. Some reports suggest that the most intense heat will spread across more regions toward the end of July [4]. This follows a record-breaking trend in 2025, when 30 locations across the country recorded temperatures of 40 degrees Celsius or higher [2].
Imura said the duration of this heat depends on the movement of high-pressure systems and the approach of typhoons, which can sometimes either exacerbate heat through pressure changes or provide temporary relief through rain [1, 2].
“Mie Prefecture recorded a peak temperature of 40.6 degrees Celsius.”
The formalization of the 'kokushobi' term by the Japan Meteorological Agency signals a shift in how the government views extreme heat, moving from rare anomalies to a recurring seasonal threat. By establishing a specific category for 40-degree days, Japan is creating a standardized trigger for emergency health alerts and urban cooling interventions to mitigate heatstroke fatalities.
