In Japan, 30°C is not simply a hot day. It has a name.
So does 35°C.
And as of 2026, 40°C has one too.
In April, the Japan Meteorological Agency officially introduced kokushobi (酷暑日), a term for a day when the maximum temperature reaches 40°C or higher. It joins a vocabulary that people in Japan have heard in weather forecasts for years: natsubi, manatsubi, and mōshobi.
For people who live here, these words are so ordinary that they barely register as something unusual. A weather presenter says that tomorrow will be a mōshobi, and you immediately have a rough idea of what the day will be like. Seen from outside Japan, though, there is something slightly curious about how carefully we have divided up different levels of summer heat.
Japan has a name for each level of heat
The official definitions are straightforward:
- 25°C or higher — natsubi (夏日), literally a “summer day”
- 30°C or higher — manatsubi (真夏日), a “mid-summer day”
- 35°C or higher — mōshobi (猛暑日), a day of intense heat
- 40°C or higher — kokushobi (酷暑日), a day of severe heat
The categories are based on the daily maximum temperature.
This vocabulary has become part of everyday summer life in Japan. At 29°C, people simply complain that it is hot. Once the forecast reaches the thirties, words such as manatsubi start appearing regularly on television and weather apps. At 35°C, it becomes a mōshobi, and the tone of the forecast changes as well. There are warnings about heatstroke, reminders to use air conditioning and repeated advice not to spend too long outside.
For years, mōshobi was the highest category most people needed to know. Increasingly, however, Japanese summers have gone beyond it.
Even “mōshobi” is a relatively new word
Mōshobi has not always been part of the official weather vocabulary. The Japan Meteorological Agency adopted the term in 2007 for days with a maximum temperature of 35°C or higher.
There was some discussion at the time about what such days should be called, and one of the proposed names was kokushobi — the same word that would eventually be used for 40°C days almost two decades later.
Looking back, that history feels significant. In 2007, 35°C was considered extreme enough to deserve a new category. By the mid-2020s, temperatures above 40°C were being recorded often enough that the top of the scale needed to be extended again.
The summer of 2025 was especially hot. Japan recorded its highest national summer average temperature for the third consecutive year, and temperatures of 40°C or more were observed at an unusually large number of locations.
Against that background, the Meteorological Agency decided to give 40°C days an official name.
Nearly half a million people voted on the name
Rather than simply choosing a term internally, the agency asked the public for opinions. The survey received 478,296 responses.
Kokushobi was the clear winner, receiving 202,954 votes. Other candidates included chōmōshobi, gokushobi, enshobi and resshobi — various combinations of Japanese words and kanji suggesting extreme, fierce or scorching heat.
Some of the free-form suggestions were more memorable. People proposed names that could roughly be translated as “scorching day,” “sauna day,” “boiling day” and “stay-at-home day.”
In the end, kokushobi was chosen because it had the strongest public support and was considered relatively easy to understand. The word itself was not completely new. The Japan Weather Association had already been using it independently for temperatures of 40°C or higher since 2022.
On April 17, 2026, it became an official meteorological term.
It did not take long for the new category to become relevant. During the summer, several observation points again recorded temperatures above 40°C.
These words become part of daily life
Japan has many words connected to the seasons. Some sound almost poetic in translation: the rainy season, the first strong wind of spring, autumn rain, tropical nights.
Kokushobi is not particularly poetic. Forty degrees Celsius is difficult to romanticize.
But weather terms like these do more than describe a number. Over time, they become part of the way people think about the day ahead. Hearing that tomorrow will be 36°C gives you the temperature. Hearing that tomorrow will be a mōshobi carries a little more meaning because the word has become associated with a certain kind of summer day.
You may decide not to go out in the afternoon. Schools may keep children indoors. Someone who never used a parasol ten years ago may now carry one to work. Bottles of tea disappear from the refrigerator faster, and air conditioners stay on through the night.
These are small things, but this is how weather terminology becomes part of everyday life.
The addition of kokushobi creates another threshold. A 40°C day is no longer simply an unusually hot mōshobi. It now has its own category, and the Meteorological Agency intends to use the term when stronger warnings about exceptional heat are needed.
A new word that says something about Japan’s summers
There is a strange feeling about seeing kokushobi become official.
New words often appear because society has invented something new: a technology, a habit, a type of work or a new way of communicating. This word was added for a different reason. The existing vocabulary for hot weather was no longer quite enough.
Japan already had natsubi for 25°C, manatsubi for 30°C and mōshobi for 35°C. In 2026, the list gained one more step at 40°C.
For visitors to Japan, kokushobi may simply be another unfamiliar Japanese word heard on a summer weather forecast. For people who live here, it will probably become ordinary very quickly. That may be the more striking part of the story.
A temperature that once would have sounded exceptional now has a standard name.
