Is there any escaping Japan’s summer 酷暑 (kokusho, extreme heat)?
In April, the 気象庁 (kishōchō, Japan Meteorological Agency) released a bulletin titled 最高気温が40度以上の日の名称を「酷暑日」に決定 (Saikō kion ga yonjū-do ijō no hi no meishō o “kokusho-bi” ni kettei, The name “intense heat day” has been officially adopted for days when the maximum temperature reaches 40 degrees Celsius or higher).
The kanji that comprise 酷暑 are 酷 (hido[i]/koku) and 暑 (atsu[i]/sho), the later meaning “hot” and the former meaning “severe,” “harsh” or “cruel.” This has led some overseas news outlets to translate 酷暑日 (kokusho-bi) as a “cruelly hot day.”
No less cruel is a 猛暑日 (mōshobi, very hot day). The 気象庁 defines a 猛暑日 as a day when the 最高気温 (saikō kion, maximum temperature) reaches 35度 (sanjūgo-do, 35 C) or higher. In 2025, Tokyo recorded 25 猛暑日 and 69 additional 真夏日 (manatsubi, midsummer days) — when the temperature is 30度 or higher. Nearby, Kofu, in Yamanashi Prefecture, experienced 53 猛暑日 and 73 真夏日.
Another feature of the intense Japanese summer 天気 (tenki, weather) is the 熱帯夜 (nettaiya, tropical night). On these nights, the 最低気温 (saitei kion, minimum temperature) remains 25°C or higher. In 2025, Tokyo recorded 55 such nights, while Nagoya, Osaka and Kobe recorded 73, 81 and 82, respectively.
With all this heat, you are more likely to hear warnings about 熱波 (neppa, heat waves) and 熱中症 (netchūshō, heatstroke), including 熱中症警戒アラート (netchūshō keikai arāto, heatstroke alerts).
Where does this extreme heat come from? A number of atmospheric and oceanic 現象 (genshō, phenomena) can contribute to Japan’s increasingly extreme summers. Among them are ラニーニャ現象 (Ranīnya genshō, La Niña), 正のインド洋ダイポールモード現象 (sei no Indo-yō daipōru mōdo genshō, positive Indian Ocean Dipole) and the 二重高気圧 (nijū kōkiatsu, double high-pressure system). The latter, in particular, can trap hot air over Japan and has contributed to causing 酷暑日 in recent summers.
Many of these climate systems affect the entire 日本列島 (Nihon rettō, Japanese archipelago), meaning there can be surprisingly little variation in summer temperatures from south to north when the heat waves arrive. On July 21 to 22, during the peak of a 二重高気圧, Kagoshima and Sendai had the same daily highs of 35.6 and 34.7度 — despite the cities being roughly 1,200 kilometers apart as the crow flies, about the distance from Atlanta to New York or Rome to Berlin.
And it’s not just the 熱 (netsu, heat) that causes problems, it’s the 湿度 (shitsudo, humidity). The 体感温度 (taikan ondo, perceived or “feels-like” temperature) can be quite different from the actual 気温 (kion, air temperature) because humidity affects how hot the body feels. Japan also uses the 暑さ指数 (atsusa shisū, heat stress index), a measure that takes temperature, humidity and radiant heat into account. This means that certain regions along Japan’s coast, where 気温 is lower but 湿度 is higher, can actually feel just as hot as a drier inland city. The aforementioned Kofu averages summer highs of 33.0度 compared with 30.2度 in seaside Atami, Shizuoka Prefecture, but the difference in humidity means Kofu’s 暑さ指数 is only one degree higher than Atami’s, rather than three.
All of these factors mean that Japan’s most extreme summer heat is found neither in the far south nor in the driest or wettest parts of the country. Instead, some of the hottest places are 盆地 (bonchi, basins) such as Kofu and Tajimi in Gifu Prefecture. Surrounded by mountains, they are shielded from ocean breezes and trap heat. When wind does reach them, it can arrive already superheated after passing over big cities such as Tokyo and Nagoya.
じゃあ、どうやったらこの暑さから逃げられるのでしょうか？ (Jā, dō yattara kono atsusa kara nigerareru no deshō ka?, So how can we escape from this heat?)
Moving from the 盆地 to the 海辺 (umibe, seaside) lowered the 暑さ指数 (atsusa shisū, heat stress index) by about one degree. Heading north to 東北 (Tōhoku, the Tohoku region [where Sendai is located]) lowers it by about one more degree. How far north do we need to go?
In recent years, very far north indeed. Even in Aomori and Sapporo, summer temperatures can soar, with the heat index reaching uncomfortable levels. To find truly cooler conditions, you’d practically have to go all the way to Wakkanai, Japan’s northernmost city.
全然逃げられないのではないか？！ (Zenzen nigerarenai no dewa nai ka?!, Maybe we can’t escape at all?!)
Not by fleeing north. There is a true way, a better way: 上へ (ue e, upward).
標高が高ければ高いほど、空気は冷たくなる (Hyōkō ga takakereba takai hodo, kūki wa tsumetaku naru, the higher the elevation, the cooler the air). This useful construction, ～ば～ほど, means “the more A, the more B.” Another heat-related example would be, 気温が高ければ高いほど、眠りにくくなる (Kion ga takakereba takai hodo, nemurinikuku naru, The hotter the temperature, the harder it is to sleep) or 外に長くいればいるほど、汗をかく (Soto ni ireba iru hodo, ase o kaku, The longer I stay outside, the more I sweat).
Even in Japan, the 標高 (hyōkō, elevation) effect is striking. 標高600メートルの松本市 (Hyōkō roppyaku mētoru no Matsumoto-shi, Matsumoto City at 600 meters elevation) has recorded an 暑さ指数 (atsusa shisū, heat stress index) of 34 in recent summers. At 標高1,000メートルの軽井沢 (hyōkō sen mētoru no Karuizawa, Karuizawa at 1,000 meters elevation), the figure is 29. And at 標高1,400メートルの奥日光 (hyōkō sen yonhyaku mētoru no Oku-Nikkō, Inner Nikko at 1,400 meters elevation), it falls to 24.
If you really want to escape the summer heat, head for the mountains. That may be one reason 山登り (yamanobori, mountain climbing) and ハイキング (haikingu, hiking) are such popular summer activities in Japan. Just be prepared: The heat may be less cruel on the mountain, but the trails can be.