Japan Travel Safety Kit · Guides

What is Japan's Heat Stroke Alert, and what counts as an 'intense heat day'?

Japan's Heat Stroke Alert is issued by the Ministry of the Environment and JMA when the WBGT heat-stress index is forecast to reach 33 or higher, announced at 5:00 pm the previous day and again at 5:00 am, for each of 58 regions. JMA calls a day with a maximum of 35 °C or more an intense heat day (mōshobi); 30 °C or more is a mid-summer day, 25 °C or more a summer day, and a night that stays at 25 °C or more is a tropical night.

The terms

TermJMA definition
Summer day (natsubi)Daily maximum of 25 °C or more
Mid-summer day (manatsubi)Daily maximum of 30 °C or more
Intense heat day (mōshobi)Daily maximum of 35 °C or more
Tropical night (nettaiya)Night-time minimum of 25 °C or more

The alert

The Heat Stroke Alert will be announced when the WBGT index is 33 or higher. It is announced at 5:00 pm the previous day, and again at 5:00 am the following day, by region (58 regions).

The one rule

On an alert day, move sightseeing to morning and evening, drink before you are thirsty, and use air-conditioned spaces (stations, department stores, convenience stores). Watch children and older travelers.

The live warnings, in your language

Japan Disaster Guide is a free iPhone app that delivers JMA warnings for the places you save, after JMA confirms them, in 7 languages, with 115,000+ evacuation sites on a map.

Free on the App Store See the Japan Travel Safety Kit

The full procedures, the 119 script, shelter life, the 171 steps, 21 pointing cards and the worksheets are in the Japan Travel Safety Manual, a 66-page PDF made to read on a phone.

Get the manual ($9.99 PDF)
Sources (read 2026-09-09; quotations are verbatim)
  1. Ministry of the Environment, Heat Stroke Alert (English). https://www.wbgt.env.go.jp/en/sp/alert.php
  2. Ministry of the Environment, press release: Heat Stroke Alert starts throughout Japan. https://www.env.go.jp/en/headline/2512.html
  3. Japan Meteorological Agency, 予報用語 気温 (Japanese). https://www.jma.go.jp/jma/kishou/know/yougo_hp/kion.html

Japan Disaster Guide is an independent app. It is not affiliated with the Japan Meteorological Agency or any government body. In an emergency, follow the instructions of local officials.

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