What Happened
Weather · Japan · Terminology · ExplainerFirst kokushobi 21 Jul 2026 · season to 26 Aug 2026
Weather terminology · explainer, updated for the 2026 season
Japan's First Kokushobi, and Its First Season
On 21 July 2026, three cities in central Japan crossed 40 °C, logging Japan's first official kokushobi (酷暑日) — the ≥40 °C heat-day category the Japan Meteorological Agency named by public vote on 17 April 2026, after 40 °C days stopped being exceptional. Its first season is now the busiest for kokushobi since comparable records began in 2010, with 36 station-days of ≥40 °C by 26 August — yet the 41.8 °C national record, set the previous summer, still stands.
By the Numbers definition, vote, and the season so far
- ≥40 °Cthe kokushobi threshold — the new top rung of the heat ladder
- 202,954votes that named 酷暑日 — 42.4% of 478,296 cast, 3.1× the runner-up
- 40.3 °CTajimi, Gifu — the day's highest, on the first kokushobi
- 36station-days of ≥40 °C by 26 Aug 2026 — already past 2025's full season
The JMA Heat Ladder daily maximum, unless noted
| Term | Reading | English gloss | Daily max |
|---|---|---|---|
| 夏日 | natsubi | summer day | ≥ 25 °C |
| 真夏日 | manatsubi | midsummer day | ≥ 30 °C |
| 猛暑日 | mōshobi | extremely hot day | ≥ 35 °C |
| 酷暑日 | kokushobi | severe / "cruelly hot" day (new) | ≥ 40 °C |
| 熱帯夜 | nettaiya | tropical night | night ≥ 25 °C |
Timeline
How It Got Here 2007 → 2026
- 2007猛暑日 (≥35 °C) added to JMA's heat-day scale — the last new rung before kokushobi.
- 5 Aug 2025National record set: 41.8 °C at Isesaki, Gunma.
- 27 Feb – 29 Mar 2026JMA's public online survey to name the ≥40 °C tier; 478,296 votes cast.
- 17 Apr 2026JMA announces the name: 酷暑日 (kokushobi), with 42.4% of the vote.
- 21 Jul 2026First official kokushobi — Tajimi, Toyota and Gujō (Hachiman) top 40 °C.
- 21–25 Jul 2026Japan's first-ever run of five consecutive nationwide kokushobi days (Tokai); Kyushu records its first-ever kokushobi.
- 23 Jul 2026Season's hottest readings: Sakuma, Shizuoka reaches 41.1 °C; Toyota, Aichi reaches 40.8 °C — both enter the all-time top 20.
- 3 Aug 2026JMA confirms 2026 is already the busiest kokushobi season since comparable records began in 2010.
- 26 Aug 2026Running season total reaches 36 station-days of ≥40 °C — the last complete day in this record.
By late July 2026, Japan was recording heat unlike any season since the kokushobi era began. The Tokai region strung together five consecutive ≥40 °C days for the first time nationwide, and Kyushu crossed the line for the first time ever. By 3 August, JMA's own count already exceeded 2025's full-season total; by 26 August — five days short of the season's official close — the running tally stood at 36 station-days of ≥40 °C, against 2025's finished total of 30. What did not happen: no station came within a full degree of the 41.8 °C set at Isesaki the previous summer. The season set a record for how often the line was crossed, not for how far past it the mercury went.
- 36station-days ≥40 °C · through 26 Aug 2026
- 41.8 °Cnational record · still stands, 5 Aug 2025
The Argument
What Actually Changed only the top band split
| Daily max | Label before Apr 2026 | Label from summer 2026 |
|---|---|---|
| 30 – 34.9 °C | 真夏日 | 真夏日 — unchanged |
| 35 – 39.9 °C | 猛暑日 | 猛暑日 — unchanged |
| ≥ 40 °C | 猛暑日 | 酷暑日 — the new tier |
Named by Vote definition · naming
Definition · JMA
酷暑日 · Kokushobi
- Coined by the Japan Meteorological Agency for days reaching 40 °C.
- Announced 17 April 2026; its first season, and the first new rung on the scale since 猛暑日 in 2007.
- 酷 (koku) means "cruel / harsh" — glossed "severe" or "cruelly hot" day.
- 酷暑 alone is an old word for heat; 酷暑日 is the new formal category.
Naming · public poll
Named by Vote
- 478,296 people voted in JMA's own online survey.
- 酷暑日 won with 202,954 votes — 42.4% of the total, over three times the runner-up.
- Runner-up: 超猛暑日 (chōmōshobi), 65,896 votes.
- The winning name already had public currency: the Japan Weather Association had used it informally since 2022.
Risk Communication, Not New Physics the season tested the label, and it held
The case for kokushobi was always that a five-degree band — 35 up to a possible 41 °C — had been carried by one word, 猛暑日, flattening the gap between uncomfortable and life-threatening. The 2026 season is the first real test of whether a name changes anything, and by the measure that matters it holds: the season that broke every frequency record for kokushobi did not touch the absolute record set the year before. Kokushobi is doing exactly what a risk-communication label is for — marking a threshold crossed more often, not implying a hotter kind of heat now exists.
- +20%more ≥40 °C station-days than all of 2025, reached three weeks before this season closed
- 41.8 °Cthe national ceiling · unmoved since 2025
What Others Add
Nationwide Snapshot first kokushobi, 21 Jul 2026
- 914JMA observation points nationwide
- 734reached ≥ 30 °C — JMA's own count for the day
- 278reached ≥ 35 °C (猛暑日), counted apart from kokushobi
- 3crossed 40 °C — the first kokushobi
The Terminology Trap pin the number to the word
The label matters — but it can also mislead. Nagoya reached 39.7 °C on the very day of the first kokushobi, just short of the new line and so still 猛暑日. And the English glosses overlap: 猛暑日 (35 °C) is usually "extremely hot day", while 酷暑日 (40 °C) is "severe" or "cruelly hot" — yet some outlets call the 40 °C tier "extreme heat" too. Note also that 酷暑 on its own is an old, general word for intense heat; only 酷暑日 is the new ≥40 °C category. Always attach the number to the word.
- 35 °C猛暑日 · "extremely hot day"
- 40 °C酷暑日 · the new severe tier
The Human Cost the record summer this scale answers to
- 100,510heat-related hospital transports, May–Sep 2025 — a record since Japan's Fire and Disaster Management Agency began counting in 2008
- 1,803workplace heatstroke casualties in 2025 (19 deaths) — a record since Japan's health ministry began counting in 2005
Conclusion
Where It Lands one season, and what it settles
Bottom line
Kokushobi gives Japan a distinct ≥40 °C label a forecast can escalate to and a headline can single out — restoring a warning that a single 35–41 °C top tier had lost. Its first season, the busiest for kokushobi since comparable records began in 2010, is the label doing exactly that: a record for how often the line was crossed, not for how hot it got.
The stakes
The heat is already lethal: 100,510 heat-related hospital transports and 1,803 workplace heatstroke casualties, both records, in 2025 — the summer that also set the 41.8 °C national mark kokushobi's first season did not beat. The label exists for exactly this: a threshold the public can recognise, in the season when recognising it matters most.