What do people get wrong about uchimizu, sprinkling water on the ground?
This answer is a draft. It stays unpublished until legal review and source verification are complete.
On summer evenings in Japan you still see shopkeepers and grandmothers throwing a ladle of water across the pavement in front of the house. It looks like a ritual — and it partly is — but uchimizu is evaporative cooling with 400 years of practice behind it, and Tokyo's government has run city-wide uchimizu campaigns since 2003. Done at the wrong time, though, it makes the evening more humid, not cooler.
How it works and how to do it
| Item | Detail |
|---|---|
| Principle | Evaporating 1 litre of water absorbs about 2,400 kJ — enough to cool a large patch of pavement by several degrees. The cooled surface then stops radiating heat into the room and the street |
| Measured effect | Tokyo trials: ground surface −10 to −15°C, air at 1.5 m −1 to −2°C for 30–60 minutes after wetting in the evening |
| Best time | Early morning (6–8 am) and evening (after 17:00, when the sun is off the surface). Water on ground still in full sun evaporates too fast for a lasting effect |
| Where | Shaded or cooling ground: the entrance path, the balcony floor, the north side, under the eaves. Balconies: the concrete floor and the outdoor unit's surroundings (not the unit itself) |
| How much | 1–2 litres per square metre — a ladle or a watering can. Puddles don't help; a film does |
| Which water | Leftover bath water, rainwater from a barrel, air-conditioner drain water, rinse water — the campaign rule is "secondary water only". Never tap water during a drought advisory |
| Don'ts | Not on a neighbour's wall, doorstep or laundry; not on a road where cars pass (splash); not on wooden decks daily (rot); not with soapy water on plants |
- The etiquette origin: uchimizu began as purification and welcome — wetting the entrance path before guests arrived — and as dust control on unpaved streets. The cooling was understood too; Edo-period manuals recommend it for evening comfort.
- Around the air conditioner: cooling the area around the outdoor unit (shading it, wetting the concrete nearby) can improve its efficiency slightly; spraying water onto the unit itself is discouraged by manufacturers.
- Balcony and veranda routine: many households combine sudare (shade) + uchimizu (evaporation) + window opened for cross-breeze in the evening before turning on the air conditioner, and report the room needs less cooling.
- Community campaigns: "Mission Uchimizu" (Tokyo, since 2003) and municipal uchimizu days in August; schools and shopping streets do it together at 17:00 on a set day.
- Humidity caveat: in already humid, still evenings (above 75% RH), the cooling is small and the wet ground adds to the mugginess — skip it and ventilate instead.
[Verify] Temperature-reduction figures from Tokyo uchimizu trials; latent heat value (2,400 kJ/L) is standard physics.
---
Sources
- Tokyo Metropolitan Government — Uchimizu campaign and heat-island measures
https://www.kankyo.metro.tokyo.lg.jp/
- Ministry of the Environment — Heat-island countermeasure guideline (evaporative cooling, uchimizu)
https://www.env.go.jp/air/life/heat_island/
- Ministry of Land, Infrastructure, Transport and Tourism — Water-saving and secondary-water use in households
https://www.mlit.go.jp/mizukokudo/
⚠️ 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.
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