What do people get wrong about reading a Japanese weather forecast?
This answer is a draft. It stays unpublished until legal review and source verification are complete.
Japanese weather forecasts are precise in ways that surprise newcomers — a "probability of precipitation" that is not what it seems, a laundry index, a pollen level, an "umbrella needed" icon, and heat-stroke alerts — and they shape daily behaviour: whether to hang laundry outside, carry an umbrella, or send a child to school in a rain cape. Learning to read them is a domestic skill.
The numbers and what they mean
| Item | Japanese | What it actually means | Household threshold |
|---|---|---|---|
| Probability of precipitation | kōsui kakuritsu (降水確率) | Chance that ≥1 mm of rain (or snow) falls somewhere in the area during the 6-hour period. Issued in 10% steps. Not duration, not amount | ≥30%: folding umbrella. ≥50%: laundry inside. ≥70%: indoor plans |
| Rain radar / nowcast | amagumo rēdā / naukyasuto (雨雲レーダー・ナウキャスト) | JMA's 5-minute-updated radar with 1-hour (and 15-hour) forecast of rain intensity | The tool for "can I make it to the station before it hits" |
| Laundry index | sentaku shisū (洗濯指数) (Japan Weather Association, tenki.jp) | 0–100 scale combining humidity, sun, wind, rain chance: ≥70 "dries well", 30–60 "part-dry", <30 "indoor" | Decides morning laundry for millions |
| Heat-stroke alert | necchūshō keikai arāto (熱中症警戒アラート) | Issued when the WBGT (暑さ指数) is forecast at 33+ | Cancel outdoor activity; check on the elderly (see the heatstroke article) |
| UV index | UV shisū (UV指数) | 1–11+; 8+ is very strong | Sunscreen, hat |
| Pollen | kafun (花粉) | 5 levels from "low" (少ない, sukunai) to "very high" (非常に多い, hijō ni ōi) | Windows shut, laundry in |
| Discomfort / "sensible temperature" | taikan ondo / fukai shisū (体感温度・不快指数) | Includes wind and humidity | — |
| Snow | kōsetsuryō / sekisetsu (降雪量・積雪) | cm of new snow / snow depth | Trains and roads |
The warning hierarchy (what closes schools)
| Level | Japanese | Meaning | Typical response |
|---|---|---|---|
| Advisory | chūihō (注意報) | Possible damage (heavy rain, strong wind, thunder, dry air, frost, dense fog) | Carry on, be careful |
| Warning | keihō (警報) | Serious damage likely (heavy rain, flood, storm, heavy snow, high waves, storm surge) | Many schools close or delay if a warning (esp. heavy rain/storm) is in force at 07:00; companies allow late arrival |
| Emergency warning | tokubetsu keihō (特別警報) | Once-in-decades danger; evacuate | Follow the municipality's evacuation levels |
| Evacuation levels | keikai reberu (警戒レベル) 1–5 | 3: elderly evacuate; 4: everyone evacuate; 5: disaster under way | Set by the municipality via area mail (エリアメール) |
- Where to read: JMA (気象庁) site/app — the official source, sparse design; tenki.jp (Japan Weather Association) — indices, pollen, laundry; Weathernews — crowdsourced rain reports and the "5-minute rain" alerts; Yahoo! Weather (Yahoo! 天気) — the most-used app, with the rain radar and push alerts; NHK for warnings; NERV (特務機関NERV) — the disaster-alert app popular for its speed.
- The 6-hour blocks: forecasts split the day into 0–6, 6–12, 12–18, 18–24; "afternoon 60%" is not "evening 60%" — check the block you need.
- Typhoon reading: the forecast circle (予報円) is where the centre may be, not the storm's size; the storm area (暴風域, red) and strong-wind area (強風域, yellow) are what matter; "landfall" (上陸, jōriku) timing decides train stoppages (計画運休, planned suspensions) announced a day ahead.
- Snow in Tokyo: a 5 cm forecast stops trains and empties convenience-store bread shelves; treat any Tokyo snow forecast as a transport day off.
- Area mail (エリアメール / 緊急速報): the loud alarm on all phones for earthquakes (緊急地震速報), tsunami, evacuation, missile (J-Alert). Not switchable off for the earthquake/tsunami tier on Japanese SIMs; foreign SIMs may not receive it — install NERV or the Yahoo! disaster-alert app (Yahoo! 防災).
- Seasonal words to know: rainy season start/end (梅雨入り/明け, tsuyu-iri/ake), extremely hot day (猛暑日, mōshobi; 35°C+), midsummer day (真夏日, manatsubi; 30°C+), tropical night (熱帯夜, nettaiya; night above 25°C), Kogarashi No. 1 (木枯らし1号, kogarashi ichigō; first cold north wind), start of pollen dispersal (花粉飛散開始, kafun hisan kaishi).
[Verify] JMA definitions (precipitation probability, warning tiers) with the JMA glossary; laundry-index scale with JWA; evacuation level definitions with the Cabinet Office guideline.
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Sources
- Japan Meteorological Agency — Explanation of forecast terms: probability of precipitation, warnings and advisories
https://www.jma.go.jp/jma/kishou/know/
- Japan Weather Association (tenki.jp) — Laundry index and living indices explained
https://tenki.jp/indexes/
- Ministry of the Environment — Heat Stroke Alert (WBGT) system
https://www.wbgt.env.go.jp/
⚠️ Editorial note: the source URLs above were identified by search. Our working environment cannot reach go.jp domains, so the contents have not been checked against the text. Verify every source before publication and record who checked it and when.
About this article
What we share here are ideas and everyday practices passed down among people living in Japan, together with general information drawn from public sources. They are not official recommendations, and they are not professional instruction. Practices vary between households and regions.
This is general information. Results vary from person to person and depend on your own conditions and surroundings.
Please decide for yourself, using your own judgement, whether any of this suits your situation. We cannot accept responsibility for any damage, loss or injury arising from following this article.
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