August 25, 2026. Japan is not done with summer. The Japan Meteorological Agency has Kumamoto, Kurume in Fukuoka, and Hita in Oita near 40°C today. The Environment Ministry and JMA have Heat Stroke Alerts over 41 prefectures, from Iwate down to Kagoshima. Okinawa and Amami get a typhoon instead. The alert is about whether your body can cool. Midday UV is a separate meter.

What’s forecast

Asahi, citing JMA this morning, put Tuesday roughly as:

• 40°C: Kumamoto City, Kurume, Hita. Asahi flags the call for quake-hit Kumamoto—districts still recovering from the late-July earthquake, where outdoor cleanup sits in that air.
• 39°C: Kumagaya in Saitama, Kyoto, Saga.
• 38–36°C: Maebashi near 38°C, Nagoya near 37°C, Sendai and Osaka near 36°C.
• Tokyo: central city near 35°C.
• How wide: tenki.jp says yesterday logged 211 stations at 35°C or above (a mōshobi, or extremely hot day). Today’s count is expected to jump under the same high.

The MoE/JMA Heat Stroke Alert map, updated 5:00 a.m., shows 41 regions on alert and none on the rarer Special Heat Stroke Alert. The product fires when forecast WBGT—wet-bulb globe temperature, a heat-illness index from air temperature, humidity, and solar load—hits 33 or higher somewhere in the prefecture. Guidance is blunt: stay inside with the air conditioner, skip exercise, drink before you are thirsty.

That is a heat-stress rule. It is not a UV reading.

Typhoon vs sun

The country is split.

Typhoon 18 is a very strong storm. tenki.jp had it near 940 hPa on Monday afternoon, with center winds around 40 m/s and gusts near 55 m/s. Today it closes on the Daito Islands, then Okinawa and Amami through Wednesday, with gusts locally near 60 m/s and waves around 11 m off Okinawa and 10 m off Amami. Hokkaido and northern Tohoku sit under a separate rain threat.

Kyushu through Kanto, and southern Tohoku, sit under the high. Sun. Heat. The Japan Weather Association’s Tuesday UV ranks for Kyushu are Very Strong in Fukuoka, Oita, Nagasaki, Saga, Kumamoto, Miyazaki, and Kagoshima. Naze in Amami is only Strong—the typhoon’s cloud deck is already doing work there.

A storm over Okinawa does not shade Tokyo. Thick typhoon overcast that hides the disc does cut UV. The bright, still-sunny half of the map does not get that discount.

Heat vs UV

WBGT folds sunlight in as one input to heat illness. The alert’s to-do list is cooling and water. It does not publish today’s UV index, and it does not tell you when unprotected skin burns fastest.

UV still tracks the sun’s angle. Peak burn pace sits near solar noon, often around 11:30 a.m.–12:30 p.m. JST. Air can keep climbing into the afternoon after that. A late morning that already feels punishing in Kumamoto can be the day’s strongest UV. A 3 p.m. walk, when 40°C is nearer, is worse for heat strain and still a protection window.

JMA’s own August climatology (1997–2008 noon averages) puts parts of Kyushu through the Nansei Islands near UV 8–9—the agency’s Very Strong band, where the advice is to limit time outside. Hokkaido and Tohoku still average 4 or higher at noon in that same map. WHO-style protection starts at 3. Late August does not reset that.

Two traps today:

1. “残暑”—lingering summer. The calendar says the season is ending. The high says otherwise. A 35°C Tokyo afternoon is still a High-to-Very-High UV day on a clear August sun.
2. Afternoon storms in Kanto. tenki.jp has a chance of sudden thunder over inland Kanto-Koshin and mountain slopes. Thick rain that hides the disc cuts UV. Broken cloud that still throws a sharp shadow can raise it.

Check the Heat Stroke Alert for cooling. Check today’s UV peak for burn pace. You need both. For why those clocks split, see UV peaks before the heat.

Through tonight

If you are on the sunny side of Japan today:

1. Treat late morning through mid-afternoon as protected time—shirt, hat, sunscreen—even if you already followed the alert and cut exercise.
2. Do not read “stay indoors” as a UV all-clear for the hours you still go out. Commutes, queues, school runs, and quake-cleanup in Kumamoto sit under Very Strong UV on a clear Kyushu day.
3. Move long walks and outdoor work earlier or later than the heat peak; still cover skin if the live index is High or above.
4. On the typhoon side, treat wind, surge, and rain as the emergency. UV drops under thick storm cloud. It does not drop on Honshu because Okinawa is under a warning.
5. Read live UV as well as temperature on the homepage before you commit to midday sun in a new prefecture.

Forecasts shift. Defer to the latest JMA bulletin for heat, rain, and Typhoon 18, and to the MoE Heat Stroke Alert map for your prefecture.

Sources and limits

This brief summarizes publicly reported JMA and MoE products as of August 25, 2026 from Asahi Shimbun, tenki.jp (Japan Weather Association), the MoE Heat Stroke Alert map (5:00 a.m. update), and JMA’s published August UV climatology. Numbers will move.

UV readings on UV Index Today are estimates for everyday sun-safety planning, not medical advice. For SPF and related Q&A, see the Guide. For your location’s current numbers, open the live UV index.