Japan is enduring another dangerous summer after temperatures crossed a newly recognized “brutally hot” threshold of 40 degrees Celsius.
The country recorded its first official kokushobi of 2026 on July 21. The Japanese term, translated as “brutally hot,” “cruelly hot” or “severely hot,” now describes any day when the maximum temperature reaches at least 40°C, or 104°F.
Tajimi in Gifu Prefecture recorded 40.3°C, while Toyota in neighboring Aichi Prefecture reached 40.1°C. The Hachiman district of Gujo, also in Gifu, recorded exactly 40°C. The temperatures measured in Toyota and Gujo were the highest ever recorded at their respective observation stations.
The heat extended far beyond those locations. Of Japan’s 914 observation stations, 278 recorded temperatures of 35°C or higher, meeting the country’s existing definition of a moshobi, or extremely hot day. The widespread readings show that the problem was not limited to a few unusual local hotspots.
Why Japan Created the “Kokushobi” Category
Until recently, 35°C represented the highest named category in Japan’s temperature terminology. However, readings above 40°C have become frequent enough that the Japan Meteorological Agency decided a stronger term was necessary.
The agency officially adopted kokushobi in April 2026 after a public naming process. The designation was created to make exceptionally dangerous temperatures easier to communicate and to encourage residents to take the threat seriously.
Japan already classifies days above 25°C as summer days, those above 30°C as midsummer days and those above 35°C as extremely hot days. The new 40°C category reflects an uncomfortable reality: conditions once regarded as rare are becoming a recurring feature of Japanese summers.
The World Economic Forum’s examination of Japan’s heat preparations notes that this was the country’s first new heat-related weather term since 2007. The agency formalized it as worsening heat made the older classifications less capable of communicating the highest level of danger.
A Temperature Reading Does Not Reveal the Full Risk
Air temperature is only one part of the threat. Japan’s summers are often highly humid, which makes it more difficult for perspiration to evaporate and cool the body.
A person can therefore experience severe heat stress even when the recorded air temperature remains below 40°C. Direct sunlight, limited airflow, physical activity and heat radiating from roads or buildings can make the body’s actual thermal burden considerably greater than the thermometer suggests.
Japan uses Wet-Bulb Globe Temperature, or WBGT, to evaluate these combined conditions. The index considers temperature, humidity, radiant heat and wind rather than relying on air temperature alone.
The Ministry of the Environment’s official WBGT guidance classifies a reading of 31 or higher as dangerous and advises that exercise should generally be stopped. At readings between 28 and 31, heavy exercise should be avoided and frequent breaks become essential.
This distinction matters because a day may not technically qualify as kokushobi while still creating life-threatening conditions. The 40°C designation is a powerful public warning, but the heat-stress index provides a more complete picture of what the human body is experiencing.
Heatstroke Alerts Have Spread Across Japan
The Japanese government issued heatstroke alerts across much of the country as the extreme temperatures developed. Residents were urged to use air conditioning, drink fluids regularly and limit prolonged outdoor activity.
A standard heatstroke alert is issued when the predicted WBGT reaches 33 or higher in one of Japan’s 58 designated regions. The Ministry of the Environment and Japan Meteorological Agency issue the alert at 5 p.m. on the previous day and update it at 5 a.m. using the latest forecast.
The government’s Heat Illness Prevention Information service provides current alert maps and WBGT readings across Japan. Its guidance advises residents to keep indoor spaces cool, avoid unnecessary outings, stop exercise and check on elderly people who may not use air conditioning consistently.
The recommendations are not merely precautionary. Heat illness can progress rapidly from weakness, cramps and dizziness to confusion, unconsciousness, organ damage and death.
Thousands Have Already Required Emergency Treatment
Even before temperatures officially crossed the 40°C threshold, Japan’s emergency services were treating thousands of people for heat-related illness.
Between July 6 and July 12, 2026, 4,580 people were taken to hospital because of the heat. Seven later died, according to data cited from the Fire and Disaster Management Agency. More than 100,000 people required hospital transportation for heatstroke during the 2025 summer season.
Older adults face particularly high risk. Japan has one of the world’s oldest populations, and many elderly residents live alone. Age can reduce the body’s ability to regulate temperature and recognize thirst, while heart disease, diabetes and certain medications can increase vulnerability.
Indoor exposure is also a major concern. A person does not need to be working outside to develop heatstroke. Poor ventilation, reluctance to operate air conditioning and buildings that retain heat overnight can create dangerous conditions inside homes.
This is why government warnings emphasize checking on relatives and neighbors rather than concentrating only on outdoor workers and athletes.
Hot Nights Are Making Recovery More Difficult
Extreme daytime temperatures receive the most attention, but unusually warm nights can make a heatwave more dangerous.
The body normally uses cooler overnight conditions to recover from daytime heat exposure. When temperatures remain high after sunset, the cardiovascular system continues working to release heat, sleep quality declines and physical stress accumulates over several days.
Dense cities are particularly vulnerable because concrete, asphalt and buildings absorb solar energy during the day and slowly release it at night. Air-conditioning systems, vehicles and commercial equipment add more heat to the urban environment.
Tokyo, Nagoya, Osaka and other heavily developed areas can therefore remain dangerously warm even after official daytime readings begin to fall. The result is not one isolated hot afternoon but a continuous period of exposure with limited recovery.
Schools, Sports and Workplaces Are Being Forced to Adapt
Japan’s heat is changing ordinary routines across the country.
The national high school baseball tournament, one of Japan’s most popular sporting events, has introduced morning and evening sessions to reduce exposure during the hottest part of the day. The scheduling changes recognize that tradition cannot justify placing players, spectators and staff in avoidable danger.
Schools and sports organizations increasingly use WBGT measurements when deciding whether outdoor activities can continue. Government guidance recommends stopping children’s exercise when the index reaches the danger category.
Workplace culture is changing as well. Tokyo authorities have encouraged more relaxed summer clothing, including polo shirts, trainers and shorts where appropriate. The recommendation expands Japan’s Cool Biz program, which was originally designed to reduce dependence on intensive air conditioning by allowing employees to abandon jackets and ties.
These adjustments may appear minor, but they demonstrate how extreme heat is reshaping institutions built around older climatic expectations.
Agriculture Is Also Feeling the Pressure
High temperatures can affect crop quality, reduce yields and disrupt planting cycles. Rice is a particular concern because excessive heat during grain development can reduce quality and produce discolored or less marketable harvests.
Japanese farmers are increasingly experimenting with heat-tolerant rice varieties. Some producers are also exploring crops that were previously unusual in their regions, including avocados and other plants associated with warmer climates.
The transition is not straightforward. A crop that tolerates heat may still require different soil, water, pest-management and distribution systems. Farmers must also make decisions years in advance while future weather conditions remain uncertain.
Extreme heat can additionally harm livestock, increase irrigation requirements and shorten the safe number of hours available for agricultural labor.
Japan’s Recent Temperature Records Show a Clear Trend
The current heat follows the country’s hottest summer on record.
Japan registered a national record of 41.8°C in Isesaki, Gunma Prefecture, on August 5, 2025. The national summer temperature anomaly reached 2.36°C above the 1991–2020 average, far exceeding the previous records set in 2023 and 2024.
The Japan Meteorological Agency’s analysis of summer temperatures shows that Japan’s average summer temperature has increased at a long-term rate of approximately 1.38°C per century.
Individual heatwaves are influenced by immediate atmospheric conditions, including high-pressure systems, wind patterns and ocean temperatures. However, the long-term warming trend raises the baseline from which these events occur, making exceptional temperatures more likely and intensifying their potential impacts.
The New Threshold Is a Warning, Not Just a Name
Japan’s adoption of kokushobi gives authorities, media organizations and residents a direct way to describe days above 40°C. However, naming the danger cannot solve it by itself.
Reducing the health impact will require accessible cooling, reliable electricity, shaded public areas, heat-resistant housing and stronger protection for outdoor workers. Cities will need more trees, reflective surfaces and designs that release heat rather than trapping it overnight.
Emergency communication must also reach older residents, tourists and people who do not regularly follow official weather services. The danger becomes greatest when someone does not recognize early symptoms or cannot access a cooler environment.
Japan’s first brutally hot days of 2026 mark more than another uncomfortable period of summer weather. They show that the country has crossed a threshold once considered extraordinary and has been forced to give it an official name.
The most important question is no longer whether temperatures will reach 40°C. It is whether communities, workplaces and public services can adapt quickly enough when they do.
