The Japanese have created a new epithet for heatwaves that reach 40°C (104°F) or above, after the country experienced its hottest summer on record in 2025.

The name is kokushobi, which translates as a “cruelly hot”, “brutally hot” or “severely hot” day. The name has been shortened by the Japanese  Meteorological Agency to “Koku”, meaning harsh or cruel and will be used by the media to confirm dangerous weather conditions.

Extreme weather events like heatwaves are becoming more common and more intense around the world, as experienced across Europe last year, fuelled by human activities such as the burning of fossil fuels.