What Happened
- Japan's Meteorological Agency (JMA) announced the creation of a new temperature classification term — "kokusho-bi" (酷暑日), meaning "cruelly hot day" — for days when the temperature reaches 40°C or above.
- The term was selected through a public consultation process involving 478,000 respondents; "kokusho-bi" received the most votes, ahead of alternative suggestions like "cho-mosho-bi" (super-extremely hot day), "sauna day", "boiling day", and "stay-at-home day."
- Japan recorded its hottest summer since records began in 1989 during 2025, making a new terminology category practically necessary and scientifically justified.
- The JMA stated the new term "will more effectively call for vigilance against extremely high temperatures" and strengthen public awareness of heat-related health risks.
- The decision reflects the increasing frequency of extreme heat events globally, driven by anthropogenic climate change.
- Japan's summer 2026 forecast projects temperatures higher than 2025, with increased rainfall and more intense typhoons.
Static Topic Bridges
Heatwaves and Extreme Heat: Definition and Classification
A heatwave is defined by meteorological agencies as a prolonged period of excessively hot weather relative to normal conditions for that region. The World Meteorological Organization (WMO) does not impose a universal definition — each country sets thresholds based on local climate norms. Japan's existing classification system now spans five tiers: summer day (25°C+), midsummer day (30°C+), extreme midsummer day (35°C+), and the newly added "cruelly hot day" (40°C+).
- Japan's temperature classification tiers: 25°C+ (summer day), 30°C+ (midsummer day), 35°C+ (extreme midsummer day/mousho-bi), 40°C+ (kokusho-bi/cruelly hot day)
- WMO: no single universal definition; heatwaves defined relative to regional climate norms
- India's National Disaster Management Authority (NDMA): defines heatwave as ≥40°C (plains) or ≥30°C (hilly regions), with heat wave warning when temperature is ≥45°C or 4.5°C above normal
- Heat-related mortality is one of the most significant climate-driven health risks globally
Connection to this news: Japan's new "kokusho-bi" classification formalises a temperature tier that previously had no official designation, creating a clearer public communication tool for extreme heat risk — a model relevant for countries like India where heat action plans are actively being developed.
Climate Change and Rising Global Temperatures
Scientific consensus, as documented in IPCC (Intergovernmental Panel on Climate Change) assessment reports, establishes that human-caused greenhouse gas emissions are the dominant driver of the observed warming since pre-industrial times. The IPCC Sixth Assessment Report (AR6, 2021–2022) found that global surface temperature has already increased by approximately 1.1°C above pre-industrial levels, and extreme heat events are projected to become more frequent, more intense, and longer-lasting.
- IPCC established: 1988 (by UNEP and WMO); publishes assessment reports roughly every 6–7 years
- IPCC AR6 (2021–2022): confirmed 1.1°C warming above pre-industrial; extreme heat events will intensify at every level of global warming
- Paris Agreement temperature goal: limit warming to 1.5°C above pre-industrial levels
- Wet bulb temperature: a combined heat-humidity measure; >35°C wet bulb is considered unsurvivable for humans without cooling
- Urban Heat Island (UHI) effect: cities experience higher temperatures than surrounding rural areas due to heat-absorbing built surfaces
Connection to this news: Japan's record 2025 summer — and the perceived need for a new heat classification — is a direct empirical manifestation of the IPCC's projections about intensifying extreme heat events as global temperatures rise.
Heat Action Plans and Disaster Risk Reduction
Heat Action Plans (HAPs) are policy frameworks developed by governments to reduce mortality and morbidity from extreme heat. They typically include early warning systems, public cooling centres, health advisories, employer obligations, and urban planning standards. India has been a leader in HAP development in the Global South.
- India's first HAP: Ahmedabad (2013), following a deadly 2010 heatwave that killed 1,344 people; credited with significantly reducing heat-related deaths
- National Disaster Management Authority (NDMA): issues guidelines on heatwave preparedness; defines thresholds for warnings
- Sendai Framework for Disaster Risk Reduction (2015–2030): global policy framework; calls for multi-hazard early warning systems covering extreme heat
- WHO's Heat Health Action Plan guidance: recommends integrating heat into national health emergency systems
Connection to this news: Japan's creation of a new public-facing terminology for extreme heat mirrors the evidence-based communication strategy behind India's Heat Action Plans — both recognise that clear, resonant language drives protective behaviour during dangerous heat events.
Key Facts & Data
- New Japanese term: "kokusho-bi" (酷暑日) — "cruelly hot day"; threshold: 40°C and above
- Public consultation: 478,000 respondents; "kokusho-bi" received most votes
- Japan's hottest summer on record: 2025 (records begin 1989)
- Japan's existing tiers: 25°C+ (summer day), 30°C+ (midsummer day), 35°C+ (extreme midsummer day)
- IPCC AR6: global warming of ~1.1°C above pre-industrial confirmed; extreme heat events intensifying
- Paris Agreement goal: limit warming to 1.5°C above pre-industrial
- India heatwave threshold (NDMA): ≥40°C plains; heatwave warning at ≥45°C or 4.5°C above normal
- Ahmedabad Heat Action Plan: launched 2013; first HAP in South Asia
- Sendai Framework: 2015–2030; multi-hazard early warning systems including extreme heat