What Temperature Counts as a “Kokushobi” Extreme Heat Day in Japan?

If you follow Japanese weather reports, you may have encountered the word kokushobi and wondered whether it simply means a very hot day or marks a specific temperature. Older explanations can make the answer more confusing because they often say the term was not officially defined by Japan’s national weather agency.

A kokushobi is a day when the daily maximum temperature reaches at least 40.0°C, or 104°F. A reading of exactly 40.0°C qualifies, while 39.9°C does not meet the definition. Since April 17, 2026, kokushobi has also been an official term used by the Japan Meteorological Agency (JMA).

This guide explains the threshold, how kokushobi differs from the older category mōshobi, why the term’s status changed in 2026, and why a heatstroke alert can be issued even when the forecast remains below 40°C. Knowing what the label means is useful, but using it to make safer plans is more important.

A kokushobi starts at a daily maximum of 40.0°C

The JMA’s official temperature terminology defines a kokushobi as a day with a daily maximum temperature of 40°C or higher. The deciding figure is not the day’s average temperature or the temperature at the moment you check an app. It is the highest air temperature observed during that day.

For example, a relatively mild morning does not prevent the day from becoming a kokushobi if the temperature later reaches 40.0°C. Conversely, a maximum of 39.9°C remains below the category’s boundary. That tenth of a degree changes the label, although it does not make 39.9°C safe.

Japan’s named warm-day categories are:

• Natsubi, or summer day: a daily maximum of at least 25°C
• Manatsubi, or midsummer day: a daily maximum of at least 30°C
• Mōshobi, or extremely hot day: a daily maximum of at least 35°C
• Kokushobi, an even more extreme heat day: a daily maximum of at least 40°C

The kokushobi threshold is five degrees higher than the mōshobi threshold. Because a temperature of 40°C is also at least 35°C, locations counted as kokushobi are included in the JMA’s broader count of mōshobi locations.

The term became official in 2026

The word kokushobi did not suddenly appear in 2026. In an August 2, 2022 announcement, the Japan Weather Association said it would independently use kokushobi for days reaching at least 40°C. It selected the name after surveying 130 weather forecasters who belonged to the association. At that time, the organization clearly noted that kokushobi was its own term, not an official JMA definition.

As temperatures above 40°C became a recurring concern, the JMA considered creating a national name for the category. It collected responses through a public online survey and consulted experts. On April 17, 2026, the JMA announced that it had selected kokushobi and would use the term in information issued by the agency.

This history explains why two apparently conflicting descriptions exist online. Saying that kokushobi was an independent Japan Weather Association term was accurate in 2022. It is outdated if presented as the current status after April 17, 2026. The JMA and the Japan Weather Association now use the same name and the same daily-maximum threshold of 40°C.

An official name has a practical advantage: it distinguishes temperatures of 40°C or higher from the already severe mōshobi category in a short, recognizable way. That can make warnings about exceptional heat easier to communicate. Its limitation is equally important: one air-temperature threshold cannot describe every person’s heat-illness risk.

Mōshobi and kokushobi are five degrees apart

A mōshobi begins at a daily maximum of 35°C, while a kokushobi begins at 40°C. The difference is five degrees, but both categories describe conditions that call for heat precautions. “Not a kokushobi” should never be interpreted as “safe.” A forecast in the high 30s can still be dangerous.

The maximum temperature reported for a forecast area also cannot represent every environment within it. Conditions near sunlit pavement, inside a poorly ventilated room, on an athletic field, or in a workplace with radiant heat may differ substantially from those at an official observation site. Age, health conditions, sleep, acclimatization, clothing, and physical activity can further change how heat affects a person.

The kokushobi label therefore has a focused purpose: it tells you that a day’s maximum air temperature reached an exceptional level. To assess heat stress more fully, you also need information about humidity, solar and radiant heat, and wind.

Heatstroke alerts use a different measure

Kokushobi is a temperature-based name for a day. Japan’s Heatstroke Alert and Heatstroke Special Alert are not triggered by air temperature alone.

The Ministry of the Environment explains that the heat index used in Japan, called WBGT, accounts for three major parts of the heat exchange between the body and its surroundings: humidity, radiant heat such as sunlight and reflected heat, and air temperature. Two places with the same air temperature can impose different levels of heat stress when their humidity, sun exposure, or wind conditions differ.

Under the ministry’s 2026 alert guidance, a Heatstroke Alert is issued when the day’s maximum WBGT is forecast to reach at least 33 at any information point in the relevant forecast area. In principle, a Heatstroke Special Alert is issued when the maximum WBGT is forecast to reach at least 35 at all applicable information points in a prefecture.

This means an alert may be issued even when the maximum air temperature is expected to remain below 40°C. It also means that the word kokushobi alone does not tell you whether an alert has been issued. For safer decisions, check the maximum-temperature forecast, local WBGT information, and the current alert status separately.

Take precautions before the temperature reaches 40°C

Do not use 40°C as the point at which heat precautions begin. Japan’s Ministry of Health, Labour and Welfare advises people to avoid heat and replace fluids frequently. When a Heatstroke Alert is active, avoiding unnecessary outings is also an important protective step.

Before outdoor work, exercise, or travel, check hourly WBGT forecasts as well as the expected maximum temperature. Consider whether the activity can be moved to a cooler time, shortened, postponed, or brought indoors. When you must be outside, use a parasol or hat and take frequent breaks in shade or an air-conditioned space.

Heat illness can also occur indoors. Use air conditioning or other appropriate cooling, reduce direct sunlight with curtains or shades, and monitor indoor conditions. Drink fluids regularly rather than waiting until you feel thirsty. People who have been told to limit fluids or salt because of a medical condition should follow their clinician’s instructions instead of applying general advice without adjustment.

Children, older adults, people with disabilities, and anyone who is unwell may need help taking protective action. Family members, neighbors, and caregivers can check whether cooling is being used, fluids are available, and the person is behaving as usual. Early contact is more useful than waiting for the day to receive a kokushobi label.

Summary

A kokushobi is a day with a daily maximum temperature of at least 40.0°C. Exactly 40.0°C qualifies; 39.9°C does not. A mōshobi begins at 35°C, placing the two category boundaries five degrees apart.

The Japan Weather Association introduced kokushobi as an independent term in 2022. On April 17, 2026, the Japan Meteorological Agency adopted it as an official term with the same 40°C threshold. Older statements that it is not an official JMA term no longer describe the current situation.

Kokushobi is a day category based on air temperature, not a heat-health alert. Japan’s alerts use forecast WBGT, which also reflects humidity and radiant heat. Check the weather forecast, WBGT, and alert status, then adjust travel or exercise and start cooling, hydration, rest, and check-ins before the temperature approaches 40°C.

The new name is most useful not as a reason to wait for a record-breaking number, but as a reminder to make safer choices well before the heat crosses that line.