Japan has introduced a new official term to describe dangerously high temperatures, reflecting growing concern over the country’s increasingly severe summers. The designation, “kokushobi,” will be used for days when temperatures exceed 40°C, marking a significant shift in how authorities communicate extreme heat risks, according to reports from The Independent, Phys.org, and The Straits Times.
The term, which translates roughly as “cruelly hot” or “severely hot day,” has been adopted by the Japan Meteorological Agency (JMA) as part of an expanded heat classification system. Previously, Japan categorised hot weather using a tiered structure: “summer days” above 25°C, “midsummer days” above 30°C, and “extremely hot days” above 35°C. The addition of “kokushobi” introduces a new threshold that reflects the growing frequency of temperatures surpassing 40°C, a level once considered rare but now increasingly common.
The decision to introduce the new term follows a period of unprecedented heat across the country. According to The Independent, Japan experienced its hottest summer on record in 2025, with temperatures significantly exceeding historical averages. In recent years, extreme heat events have become more frequent, with a large proportion of all recorded days above 40°C occurring within the past few years alone.
Phys.org similarly reports that the new classification is intended to better alert the public to the dangers posed by extreme heat. By clearly identifying the most severe conditions, the JMA hopes to encourage stronger precautions and improve public awareness of heat-related risks. Officials believe that more precise terminology will make warnings more effective, particularly as heatwaves intensify due to climate change.
The process of selecting the term itself also reflects public engagement with the issue. According to both Phys.org and The Straits Times, the JMA conducted a nationwide survey that drew nearly half a million responses. Participants were asked to choose from several possible names, including alternatives such as “boiling day” and “stay-at-home day.” Ultimately, “kokushobi” was selected as the most widely supported option, signalling a shared recognition of the severity of the problem.
The introduction of the new category underscores a broader shift in Japan’s climate patterns. Scientists have warned that the country is warming at a faster rate than the global average, contributing to longer and more intense summers. Rising sea temperatures around Japan are also believed to play a role, trapping heat and prolonging periods of high temperatures well into the autumn months.
These changes are already having tangible impacts on daily life. Heatstroke cases and hospitalisations tend to rise sharply during periods of extreme heat, particularly among elderly populations. Past heatwaves have resulted in significant numbers of fatalities, highlighting the importance of timely warnings and effective public communication. The new “kokushobi” designation is therefore seen as not just a linguistic update, but a practical tool aimed at protecting public health.
The increasing frequency of extreme heat events has also raised concerns about the long-term implications for Japan’s climate and society. Experts cited by The Independent suggest that the country’s traditional four-season climate may be gradually shifting, with spring and autumn becoming shorter as summers extend. This trend mirrors broader patterns observed across parts of Asia, where climate change is altering seasonal cycles and intensifying weather extremes.
At the same time, the move highlights how governments are adapting their communication strategies in response to climate change. By refining the language used in weather forecasts, authorities aim to convey urgency more effectively and encourage behavioural changes, such as staying indoors during peak heat or increasing hydration. The JMA has indicated that the new term will be integrated into forecasts and alerts beginning this summer, ensuring that the public becomes familiar with its meaning and implications.
While the introduction of “kokushobi” may appear symbolic, it reflects a deeper reality: extreme heat is no longer an occasional anomaly in Japan but an increasingly common feature of its climate. The Straits Times notes that the growing number of days exceeding 40°C was a key factor in prompting the change, underscoring the need for updated terminology that matches current conditions.
As Japan prepares for another potentially record-breaking summer, the success of the new classification will likely depend on how effectively it influences public behaviour and awareness. With climate trends pointing toward continued warming, measures such as this may become increasingly important in helping societies adapt to a hotter and more unpredictable future.