Japan's weather authority has introduced a new official category for days when temperatures reach 40 degrees Celsius or above, calling them "kokusho-bi" — meaning "cruelly hot" — as such extreme heat events grow more frequent across the country. The Japan Meteorological Agency (JMA) announced the new designation on Friday, saying it would help officials more effectively alert the public to the dangers of extreme temperatures.

The term was chosen through an unusually large public consultation: the JMA surveyed citizens and received nearly 478,000 responses before settling on "kokusho-bi" as the most popular option. Runners-up included "cho-mosho-bi" (super-extremely hot day), as well as more colloquial suggestions such as "sauna day", "stay-at-home day", and "boiling day".

The new label slots into an existing tiered system the JMA uses to classify hot days. Days reaching 25°C or above are already called "summer days", those hitting 30°C are "midsummer days", and temperatures of 35°C and above are classified as "extremely hot days". The 40°C threshold now gets its own distinct — and deliberately alarming — name.

The move comes after Japan endured its hottest summer since records began in 1989 last year, a trend consistent with the broader global pattern of rising temperatures driven by climate change. Days that once seemed exceptional are becoming a regular feature of the Japanese summer, placing significant strain on public health, infrastructure, and outdoor workers.

By giving the most dangerous heat tier a vivid, emotionally resonant name, authorities are betting that language itself can be a tool for public safety — nudging people to take precautions before stepping out into conditions that can be life-threatening, particularly for the elderly and those working outdoors.