Japan names 40C days as severely hot days to warn of heat

Japan’s weather agency formally named days with maximum temperatures of 40C or higher “kokusho-bi,” written , on April 17, 2026, as officials moved to give extreme heat a clearer public warning label. The Japan Meteorological Agency said it chose the term after an online questionnaire ran from February 27 to March 29 and after it weighed expert opinions. Outlets have translated the phrase as “cruelly hot day” or “severely hot day.”

The agency said it would use the new term in its own communications to encourage stronger caution about extreme heat. The change followed Japan’s hottest summer on record in 2024 and came as temperatures above 40C became more frequent. In a country where heat-related illness has become an increasingly urgent public health concern, giving 40C days a formal name turns an already dangerous number into a clearer trigger for warnings, precautions and planning.

JMA’s move also sits inside a broader climate framework that already treats the recent past as the reference point for its forecasts. The agency’s climate outlook products use the 1991 to 2020 period to define climatological thresholds, and it routinely issues long-range forecasts. That matters because the 40C label is not just a vocabulary change. It reflects the reality that older expectations for summer heat no longer capture what people across Japan, including in Tokyo and other urban areas, are now facing more often.

The naming comes as meteorological agencies around the world are under pressure to update the language they use for dangerous weather. On April 21, 2026, JMA said National Oceanic and Atmospheric Administration National Weather Service Director Kenneth Graham visited Tokyo and met JMA Director-General NOMURA Ryoichi. Against that backdrop, Japan’s new term signals how weather agencies are trying to make escalating heat more legible to the public, and how 40C is shifting from an exceptional benchmark to a category governments now feel obliged to name.