Source temperature and bath temperature

Classes by temperature

Water analyses sort springs into four classes by temperature at the point of emergence:

Cold mineral spring: below 25 °C
Low-temperature spring: 25–34 °C
Hot spring: 34–42 °C
High-temperature spring: 42 °C and above

About six in ten of Japan's sources are above 42 °C, and only around two in ten emerge at a ready-to-bathe 34–42 °C. Most onsen therefore either cool or heat their water.

Ways of cooling

**Adding cold water**: simplest, but dilutes the minerals.
**Yumomi and jikan-yu** (Kusatsu): stirring with boards to cool, then bathing for exactly three minutes at around 48 °C.
**Yubatake / wooden channels** (Kusatsu, Yumura): running the source along long wooden troughs to cool in the air — and to harvest yunohana.
**Distance** (Takayu, Totsukawa): a long pipe run from source to tub for natural cooling.
**Heat exchange**: using the source's heat to warm other water, leaving the source itself cooler with its minerals intact.
**Resting in a storage tank**: cooling overnight, at some cost to freshness.

The method shows in the changing-room notice ("water added: yes/no") and on the official website.

Aspere · CC0

Kusatsu Onsen

草津温泉

Gunma · 100 Onsen to Know Japan — No. 25

AcidicSulfurSulfateChloride

Thirty-two thousand litres a minute of pH 2.1 acid water rise in the middle of town and cool as they run down wooden channels.

Asturio Cantabrio · CC BY-SA 4.0

Yumura Onsen

湯村温泉

Hyogo · 100 Onsen to Know Japan — No. 64

ChlorideSulfateBicarbonate

Water at 98°C gushes from the rocks by the river at 470 litres a minute. People boil eggs in it, then soak their feet in the same water once it has cooled.

Purplepumpkins · CC BY-SA 3.0

Takayu Onsen

高湯温泉

Fukushima · 100 Onsen to Know Japan — No. 22

Sulfur

Nine sources, 2,589 litres a minute, and not a drop of tap water or a single boiler: the milky water is cooled only by gravity on its way down the hill.

Bath temperature: Japan runs hot

Japanese tubs are usually 41–43 °C, considerably hotter than thermal pools abroad (36–38 °C). Communal bathhouses can be hotter still — Kusatsu's jikan-yu is 48 °C, and Shika-no-yu in Nasu has a 48 °C tub. Hot water gave rise to customs such as "kaburi-yu" (pouring water over the head before entering) and "bunkatsu-yoku" (several short soaks).

At the other end, therapeutic bathing in 35–38 °C "lukewarm" water for thirty minutes or more is a tradition of its own: gentle on the body, letting time and minerals do the work.

Miyuki Meinaka · CC BY-SA 4.0

Nasu Onsenkyō

那須温泉郷

Tochigi · 100 Onsen to Know Japan — No. 30

SulfurAcidic

Six tubs from 41 to 48 °C, and before you get in you pour 200 ladles of hot water over your head. The rules of Shika-no-yu in Nasu-Yumoto, a bath in use since 630.

浅野ます道 · CC BY 3.0

Shimobe Spa

下部温泉

Yamanashi · 100 Onsen to Know Japan — No. 45

SimpleSulfur

Water at 30 °C rises from the rock beneath your feet in 'Shingen's hidden spring'. An hour in the cool bath, then a warm-up in the new hot source: a toji village with two kinds of water.

As6022014 · Public domain

Iya Onsen

祖谷温泉

Tokushima · 100 Onsen to Know Japan — No. 83

SulfurSimple

A cable car drops you about 170 metres down a 42-degree cliff in five minutes, to a riverside bath where the source runs in at 38 °C.

How this atlas records it

"Source temperature" on an area page is the temperature of the source(s) as recorded, given as a range where there are several. Bath temperature appears separately, per facility and bath, only where an in-depth article confirmed it. Don't judge a spring "hot" by its source alone — a 98 °C source may feed a 41 °C tub.

Glossary

Jikan-yu (timed bathing): Kusatsu's traditional method: three minutes in very hot water under a bath leader's direction.
Kaburi-yu: Pouring water over the head before entering, to avoid a sudden change in blood pressure.

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