Type "outdoor thermal pool" into anything and you will get two kinds of picture back. One is a stone basin with steam coming off it, snow on the rim, somebody's shoulders in dark water at four in the afternoon in December. The other is a bright blue rectangle full of children in July.
Both are open-air bathing. In the map data they carry the same tag. If you are planning a November trip, that single label is the reason a search sends you to a lake in Sweden.
What the label actually covers
463 venues record open-air bathing. Sorted by what kind of place they are:
| Kind of venue | Count |
|---|---|
| Swimming pool | 289 |
| Thermal bath | 154 |
| Hot spring | 11 |
| Water park | 8 |
| Beach resort | 1 |
And sorted by what the water actually is, which is the more useful cut:
| Water | Count |
|---|---|
| Lake | 114 |
| Thermal | 98 |
| Hot spring | 44 |
| River | 12 |
| Sea | 9 |
| Constructed pool | 8 |
Read those two tables together and the shape appears: more of this set is natural cold water than is heated thermal water. A search for somewhere warm to sit outdoors is being answered, half the time, with a lake.
The three worlds, and how to tell them apart
The heated outdoor thermal basin. 154 venues, and the thing most people are actually looking for: geothermal or heated water in a pool with a sky above it, usable in January precisely because the water arrives warm. Germany has 28 of them, Austria and Slovenia most of the rest, Iceland almost all of its 22.
The Freibad. The German municipal open-air pool, and 104 of Germany's 138 entries. Chlorinated, often unheated or barely heated, magnificent in July and closed by mid-September. This is a summer institution rather than a winter one, and nothing in the tag says so.
The bathing place. A jetty, a ladder, a patch of mown grass and a lake — the Nordic and German municipal version of a beach. All 96 Swedish entries are these: 22 explicitly lakes, 6 sea, and names that give the game away by themselves — Ljungsjön is a lake, Sättrabadet and Flatenbadet are lake baths. Sweden appearing second on a list of open-air bathing is real, and it means something entirely different from Germany appearing first.
Iceland, where the label means what you hoped
Twenty-two Icelandic venues record open-air bathing, and 19 of them are thermal baths, 3 natural hot springs; by water, 11 are hot springs and 10 thermal. That is the highest concentration of genuinely warm outdoor water in the catalogue relative to the country's size, and it is a different tradition rather than a different climate: the neighbourhood pool in Iceland is outdoors and geothermally heated as a matter of course, so being outside in bad weather in warm water is unremarkable there. Guðlaug, Krosslaug, Laugarvallalaug — the -laug ending is the word.
The temperature problem
Only 25 of the 463 publish a water temperature. Of the 154 outdoor thermal baths, 22 do. Here is the entire set of published figures, which is short enough to print in full:
20, 24, 30, 32, 32, 33, 33, 33, 35, 35, 35, 36, 36, 37, 37, 38, 38, 38, 39, 39, 40, 40, 42, 45, 45 °C.
The median is 36 °C and the useful reading is at the ends. At 40 °C and above, in an outdoor basin, cold air stops being the point and becomes the pleasure — the contrast is the experience. At 30 °C or below, outdoors, in winter, you will be cold within minutes no matter how the page describes it. Somewhere around 34 °C is the line where an outdoor pool becomes a place to sit still rather than a place to swim to keep warm.
What we cannot tell you
Whether most of these are open in January. Fewer than fifty venues in the entire catalogue — not of the 463, of all 10,312 — record a season at all: 25 say summer, 19 say they operate year-round, a handful say something else. For the remaining hundreds, the honest answer is that we do not know, and a listing that implied otherwise would be sending somebody to a locked gate in the cold.
This is the ordinary shape of open data. The tag that answers a yes-or-no question about a building gets applied thousands of times; the tag that answers "and is it open in December" is more work for the mapper and gets applied fifty times.
How to read a listing before a winter trip
- Find out what the water is before anything else. Thermal, hot spring, lake, river, sea. A lake in the Alps and a geothermal basin in the Alps are the same tag and forty degrees apart.
- Look for a published temperature, and treat its absence as a warning in winter. An operator whose outdoor pool is warm in December tends to say so in large type, because it is the reason to come.
- Check the word for the season, not the opening hours. Freibad, Sommerbad, badplats, strandbad all imply summer. Ganzjährig, year-round, egész évben is the phrase that matters.
- Check whether the outdoor basin is the thermal one. Plenty of thermal baths heat the indoor pools and run the outdoor one at swimming-pool temperature, or close it entirely from October.
- Remember the walk. A warm basin thirty metres from the changing room is a different proposition in February from one you reach across an open terrace, and the operator's photographs are always taken in summer.
Why the cold matters at all
The appeal of an outdoor basin in winter is contrast, and contrast is also the oldest structure in bathing: hot, then cold, then rest. A thermal pool under an open sky does the first two at once — 38 °C water against 2 °C air on your face and shoulders — which is why these places fill up at dusk in December and stand empty on a mild afternoon in May. It is the same principle as the plunge pool next to a sauna, arranged by geography instead of plumbing.
What we can tell you is which places have water outdoors, what kind of water it is, and the 25 temperatures anybody has bothered to publish. Where an operator states a season or a winter temperature, that belongs on the venue's page with the source beside it — and for the rest, one phone call still beats a guess dressed up as a filter.