Japan's First Kokushobi

On 21 July 2026, three cities in central Japan crossed 40 °C, logging the country's first official kokushobi (酷暑日) — the ≥40 °C heat-day category the Japan Meteorological Agency added on 17 April 2026 and let the public name. It is a new top rung on Japan's heat ladder, created because 40 °C days, once exceptional, now recur every year: a risk-communication label, not a new kind of weather.

By the Numbers scale & scope

≥40 °Cthe kokushobi threshold — the new top rung of the heat ladder
40.3 °CTajimi, Gifu — the day's highest, on the first kokushobi
202,954public votes that chose the name 酷暑日, of 478,296 cast
since 201840 °C+ days every year — why a new tier was needed

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
熱帶夜	nettaiya	tropical night	night ≥ 25 °C

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.

Watch the words

Glosses collide: 猛暑日 = 35 °C and 酷暑日 = 40 °C are both loosely called an "extreme heat day". Always pin the label to its number.

起承 · Context · Content 1 / 2

起 — A New Word for 40 °C what it is · how it was named

Definition · JMA
酷暑日 · Kokushobi
daily max ≥ 40 °C
- Coined by the Japan Meteorological Agency for days reaching 40 °C.
- Announced 17 April 2026; in use from summer 2026 — this is its first season.
- 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
~1 month from late Feb 2026
- 478,296 people voted in a public online survey.
- 酷暑日 won with 202,954 votes — over triple the runner-up.
- Runner-up: 超猛暑日 (chōmōshobi), 65,896 votes.
- Public naming is the point — a label people recognise and react to.

承 — Why the Ladder Needed a New Rung prose left · records right

For years the top of the ladder was 猛暑日 (35 °C). But 40 °C days — once a once-in-a-decade shock — have now hit every year since 2018, culminating in a national record of 41.8 °C at Isesaki on 5 August 2025. Folding a record 41 °C into the same word as an ordinary 35 °C day flattened a five-degree band that spans the difference between uncomfortable and life-threatening. The JMA needed a rung above 猛暑日 that a forecast could climb to.

41.8 °Cnational record · Isesaki, 5 Aug 2025
2018→40 °C+ every year since

How It Got Here 2018 → 2026

2018→40 °C+ days begin recurring in Japan every year.
5 Aug 2025National record 41.8 °C set at Isesaki, Gunma.
late Feb 2026A ~1-month public online survey opens to name the new tier.
17 Apr 2026JMA announces 酷暑日 (≥40 °C); in use from summer 2026.
21 Jul 2026First official kokushobi — Tajimi, Toyoda and Gujo top 40 °C.

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

轉合 · Pivot & Resolution · Content 2 / 2

轉 — The First Kokushobi · 21 Jul 2026 central Japan, ≥ 40 °C

City	Prefecture	Max temp
Tajimi	Gifu	40.3 °C
Toyoda	Aichi	40.1 °C
Gujo	Gifu	40.0 °C

Nationwide Snapshot 21 Jul 2026, 3 pm

914JMA observation points reporting
731reached ≥ 30 °C nationwide
272qualified as 猛暑日 (≥ 35 °C), counted apart from kokushobi
3cities crossed 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 and Kumagaya 38.6 °C, 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

合 — Where it lands

Kokushobi is risk communication, not new physics. The JMA already had the numbers; what it lacked was a word the public would act on. A distinct ≥40 °C label — named by nearly half a million citizens — is a warning people recognise and take shelter under.

The stakes

The heat is already lethal: ~100,000 heat-related hospital transports in the prior record summer, and a record 1,803 workplace heatstroke casualties in 2025. The JMA paired the first kokushobi with a public heat-illness advisory.