We worked out the average sea surface temperature for each month at the open-sea point nearest the weather station of 8 European countries and 29 Japanese prefectures, from NOAA's 1991–2020 averages. In August the warmest is near Naha (28.9 °C) and the coolest near Reykjavik (11.4 °C).
How we worked it out
NOAA publishes the average sea surface temperature for every month of 1991–2020 on a grid of cells a quarter of a degree across. For each weather station our country and prefecture pages use, we took the nearest cell that NOAA marks as open sea, and used it only if its centre is within 30 km of the station. We did not average cells or move to another sea.1
So each figure is the average for one patch of open sea, up to 30 km from the station. It is not the temperature at a particular beach, and it describes a typical year, not this year.
Europe: 8 capitals with open sea within 30 km
| Station city | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Nicosia (Cyprus) | 17.1 | 16.6 | 16.9 | 18.0 | 20.8 | 24.3 | 27.3 | 28.4 | 27.4 | 25.3 | 22.0 | 18.9 |
| Tallinn (Estonia) | 2.3 | 1.0 | 0.9 | 2.4 | 6.4 | 11.6 | 16.4 | 17.4 | 14.7 | 10.7 | 7.3 | 4.5 |
| Helsinki (Finland) | 2.1 | 0.8 | 0.7 | 2.2 | 6.2 | 11.4 | 16.1 | 17.2 | 14.4 | 10.3 | 7.0 | 4.2 |
| Reykjavik (Iceland) | 5.3 | 5.1 | 4.9 | 5.6 | 7.2 | 9.6 | 11.2 | 11.4 | 10.0 | 8.3 | 6.8 | 5.8 |
| Dublin (Ireland) | 9.1 | 8.1 | 7.9 | 8.7 | 10.6 | 13.1 | 14.7 | 15.1 | 14.5 | 13.4 | 12.1 | 10.6 |
| Rome (Italy) | 14.8 | 14.1 | 14.1 | 15.4 | 18.4 | 21.9 | 24.6 | 25.6 | 23.7 | 21.4 | 18.8 | 16.3 |
| Riga (Latvia) | 2.3 | 0.9 | 1.0 | 3.4 | 8.6 | 13.8 | 17.9 | 18.7 | 15.6 | 11.3 | 7.6 | 4.8 |
| Amsterdam (Netherlands) | 6.2 | 5.5 | 6.2 | 8.4 | 11.6 | 14.7 | 17.3 | 18.7 | 17.5 | 14.8 | 11.3 | 8.2 |
Degrees Celsius. Nicosia is inland: its figure is for the sea off Cyprus's north coast, about 26.1 km from the station. Rome's station is at Ciampino and Amsterdam's at Schiphol. In the Helsinki, Tallinn and Riga cells, NOAA reports sea ice in some winters, so the winter figures there are not a guide to swimming.
Japan: 29 prefectures with open sea within 30 km
| Station city (prefecture) | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Akita (Akita) | 11.3 | 9.4 | 8.8 | 10.0 | 13.6 | 18.5 | 22.3 | 25.5 | 24.4 | 20.9 | 17.4 | 14.1 |
| Aomori (Aomori) | 10.2 | 8.2 | 7.6 | 7.9 | 10.8 | 15.2 | 19.1 | 22.6 | 22.3 | 19.1 | 15.2 | 12.3 |
| Chiba (Chiba) | 13.6 | 12.5 | 13.2 | 15.0 | 17.9 | 20.9 | 23.4 | 25.9 | 24.7 | 21.6 | 18.9 | 16.0 |
| Matsuyama (Ehime) | 14.8 | 13.2 | 13.4 | 14.8 | 17.2 | 20.5 | 24.2 | 26.1 | 25.1 | 22.9 | 20.2 | 17.3 |
| Fukui (Fukui) | 13.3 | 11.5 | 11.0 | 12.7 | 16.1 | 20.1 | 23.9 | 26.8 | 25.3 | 22.0 | 19.0 | 15.9 |
| Fukuoka (Fukuoka) | 15.5 | 14.0 | 14.1 | 15.4 | 17.9 | 20.7 | 24.4 | 26.7 | 25.3 | 22.7 | 20.4 | 17.9 |
| Kobe (Hyogo) | 14.4 | 13.1 | 13.5 | 15.2 | 17.8 | 21.2 | 24.7 | 26.8 | 25.9 | 23.1 | 19.6 | 16.7 |
| Mito (Ibaraki) | 14.6 | 13.0 | 13.1 | 14.5 | 16.9 | 19.4 | 21.7 | 24.1 | 23.3 | 20.8 | 18.8 | 16.4 |
| Kanazawa (Ishikawa) | 13.0 | 11.0 | 10.5 | 12.1 | 15.8 | 19.9 | 23.7 | 26.8 | 25.2 | 21.9 | 18.8 | 15.7 |
| Kagoshima (Kagoshima) | 18.8 | 17.8 | 18.3 | 19.5 | 21.4 | 23.7 | 26.5 | 28.2 | 27.4 | 25.3 | 22.8 | 20.6 |
| Yokohama (Kanagawa) | 14.5 | 13.4 | 13.9 | 15.5 | 18.2 | 21.0 | 23.5 | 25.8 | 24.8 | 22.0 | 19.5 | 16.7 |
| Kochi (Kochi) | 18.1 | 17.1 | 17.4 | 18.5 | 20.8 | 23.4 | 26.1 | 27.7 | 27.0 | 25.1 | 22.7 | 20.0 |
| Kumamoto (Kumamoto) | 14.4 | 13.3 | 14.1 | 16.2 | 18.9 | 22.0 | 25.4 | 27.4 | 26.2 | 23.1 | 19.7 | 16.7 |
| Tsu (Mie) | 14.3 | 13.5 | 13.7 | 15.4 | 18.5 | 21.8 | 25.4 | 27.4 | 25.7 | 22.5 | 19.6 | 16.7 |
| Sendai (Miyagi) | 11.2 | 9.1 | 8.6 | 10.3 | 13.5 | 17.7 | 21.1 | 23.7 | 22.6 | 19.4 | 16.5 | 13.8 |
| Miyazaki (Miyazaki) | 19.6 | 18.6 | 18.8 | 20.1 | 21.9 | 23.9 | 26.3 | 27.9 | 27.3 | 25.5 | 23.5 | 21.3 |
| Nagasaki (Nagasaki) | 16.7 | 15.6 | 16.0 | 17.4 | 19.6 | 22.3 | 25.4 | 27.6 | 26.6 | 24.1 | 21.4 | 18.7 |
| Niigata (Niigata) | 12.3 | 10.4 | 9.7 | 10.9 | 14.5 | 19.3 | 23.1 | 26.3 | 25.1 | 21.6 | 18.2 | 15.1 |
| Oita (Oita) | 15.5 | 14.1 | 14.3 | 15.6 | 18.0 | 21.1 | 24.2 | 26.2 | 25.0 | 22.8 | 20.4 | 17.8 |
| Naha (Okinawa) | 22.6 | 21.7 | 21.8 | 22.8 | 24.7 | 26.8 | 28.7 | 28.9 | 28.3 | 27.1 | 25.4 | 23.9 |
| Osaka (Osaka) | 14.3 | 13.0 | 13.3 | 15.1 | 17.8 | 21.2 | 24.7 | 26.8 | 25.8 | 22.9 | 19.5 | 16.6 |
| Saga (Saga) | 14.3 | 13.2 | 14.0 | 16.1 | 18.9 | 22.0 | 25.4 | 27.3 | 26.1 | 23.0 | 19.6 | 16.6 |
| Matsue (Shimane) | 14.2 | 12.7 | 12.8 | 14.2 | 16.9 | 20.5 | 23.9 | 26.7 | 25.3 | 22.4 | 19.7 | 16.7 |
| Shizuoka (Shizuoka) | 16.1 | 15.2 | 15.5 | 16.9 | 19.1 | 21.6 | 24.4 | 26.5 | 25.7 | 23.3 | 20.9 | 18.3 |
| Tokushima (Tokushima) | 15.0 | 13.7 | 14.0 | 15.5 | 17.9 | 21.3 | 24.6 | 26.8 | 26.1 | 23.6 | 20.3 | 17.3 |
| Tokyo (Tokyo) | 13.6 | 12.5 | 13.2 | 15.0 | 17.9 | 20.9 | 23.4 | 25.9 | 24.7 | 21.6 | 18.9 | 16.0 |
| Tottori (Tottori) | 13.7 | 12.0 | 12.1 | 13.7 | 16.6 | 20.4 | 24.0 | 26.8 | 25.3 | 22.1 | 19.2 | 16.3 |
| Toyama (Toyama) | 12.9 | 10.7 | 10.0 | 11.3 | 15.2 | 19.9 | 23.8 | 26.9 | 25.4 | 21.9 | 18.7 | 15.6 |
| Wakayama (Wakayama) | 15.6 | 14.4 | 14.6 | 16.2 | 18.5 | 21.6 | 24.8 | 26.8 | 26.0 | 23.6 | 20.7 | 17.9 |
Degrees Celsius. Tokyo and Chiba share the same cell, in Tokyo Bay, so their figures are the same.
Why some places have no figure
For these stations NOAA's grid has no open-sea cell within 30 km: in Europe, Ankara, Belgrade, Berlin, Bratislava, Brussels, Bucharest, Budapest, Ljubljana, Luxembourg, Madrid, Oslo, Paris, Podgorica, Prague, Sofia, Stockholm, Vienna, Warsaw and Zagreb; in Japan, Aichi, Fukushima, Gifu, Gunma, Hiroshima, Hokkaido, Iwate, Kagawa, Kyoto, Nagano, Nara, Okayama, Saitama, Shiga, Tochigi, Yamagata, Yamaguchi and Yamanashi. Some of these cities are on the water, but on bays, fjords or inland seas too small for a grid of this size, so we leave them out rather than use a different sea.
Show the station, distance and grid cell for every figure
| Place | Station | Distance to cell | Cell centre (lat, lon) |
|---|---|---|---|
| Nicosia (Cyprus) | AthalassaRadiosonde | 26.1 km | 35.375, 33.375 |
| Tallinn (Estonia) | Tallinn-Harku | 25.3 km | 59.625, 24.625 |
| Helsinki (Finland) | HelsinkiKaisaniemi | 6.8 km | 60.125, 24.875 |
| Reykjavik (Iceland) | Reykjavik | 10.8 km | 64.125, -22.125 |
| Dublin (Ireland) | DublinAirport | 9.7 km | 53.375, -6.125 |
| Rome (Italy) | RomaCiampino | 26.8 km | 41.625, 12.375 |
| Riga (Latvia) | RigaLu | 18.9 km | 57.125, 24.125 |
| Amsterdam (Netherlands) | AmsterdamSchipholAp | 28.9 km | 52.375, 4.375 |
| Akita (Akita) | Akita | 21.7 km | 39.625, 139.875 |
| Aomori (Aomori) | Aomori | 10.8 km | 40.875, 140.875 |
| Chiba (Chiba) | Chiba | 20.8 km | 35.625, 139.875 |
| Matsuyama (Ehime) | Matsuyama | 14.4 km | 33.875, 132.625 |
| Fukui (Fukui) | Fukui | 11.7 km | 36.125, 136.125 |
| Fukuoka (Fukuoka) | Fukuoka | 23.6 km | 33.625, 130.125 |
| Kobe (Hyogo) | Kobe | 11.2 km | 34.625, 135.125 |
| Mito (Ibaraki) | Mito | 14.2 km | 36.375, 140.625 |
| Kanazawa (Ishikawa) | Kanazawa | 4.1 km | 36.625, 136.625 |
| Kagoshima (Kagoshima) | Kagoshima | 21.4 km | 31.375, 130.625 |
| Yokohama (Kanagawa) | Yokohama | 21.4 km | 35.375, 139.875 |
| Kochi (Kochi) | Kochi | 22.5 km | 33.375, 133.625 |
| Kumamoto (Kumamoto) | Kumamoto | 10.3 km | 32.875, 130.625 |
| Tsu (Mie) | Tsu | 15.5 km | 34.625, 136.625 |
| Sendai (Miyagi) | Sendai | 25.1 km | 38.125, 141.125 |
| Miyazaki (Miyazaki) | Miyazaki | 21.2 km | 31.875, 131.625 |
| Nagasaki (Nagasaki) | Nagasaki | 12.1 km | 32.625, 129.875 |
| Niigata (Niigata) | Niigata | 12.7 km | 37.875, 138.875 |
| Oita (Oita) | Oita | 28.5 km | 33.375, 131.875 |
| Naha (Okinawa) | Naha | 11.0 km | 26.125, 127.625 |
| Osaka (Osaka) | Osaka | 14.6 km | 34.625, 135.375 |
| Saga (Saga) | Saga | 16.9 km | 33.125, 130.375 |
| Matsue (Shimane) | Matsue | 19.5 km | 35.625, 133.125 |
| Shizuoka (Shizuoka) | Shizuoka | 11.4 km | 34.875, 138.375 |
| Tokushima (Tokushima) | Tokushima | 8.0 km | 34.125, 134.625 |
| Tokyo (Tokyo) | Tokyo | 13.5 km | 35.625, 139.875 |
| Tottori (Tottori) | Tottori | 18.5 km | 35.625, 134.125 |
| Toyama (Toyama) | Toyama | 19.8 km | 36.875, 137.125 |
| Wakayama (Wakayama) | Wakayama | 12.0 km | 34.125, 135.125 |
What these figures cannot tell you
- Open sea, not the beach. Shallow bays and beaches are often warmer in summer and colder in winter than the open sea nearby.
- Averages over 30 years. Any one year, and any one week, can differ.
- Whether it is safe to swim. That depends on currents, flags and local advice, not temperature.
- We have not been to these places and make no judgement about which is better to visit.
Each month guide shows that month's figures next to the weather and holidays.
Key takeaways
- Warmest in August: near Naha, 28.9 °C. Coolest: near Reykjavik, 11.4 °C.
- 8 European capitals and 29 Japanese prefecture stations have open sea within 30 km; the rest have no figure.
- What we would skip: planning a swim on these numbers alone. They are open-sea averages; ask locally about the beach.
Sources
Every figure comes from NOAA's sea surface temperature averages; we chose the grid cell for each station and rounded the values. See how we research.
- Sea temperature: NOAA Optimum Interpolation Sea Surface Temperature (OISST) v2.1, 0.25° grid, monthly long-term mean 1991–2020 (file sst.mon.ltm.1991-2020.nc). Data provided by the NOAA/OAR/PSL, Boulder, Colorado, USA, from their Web site at https://psl.noaa.gov/. Huang, B., C. Liu, V. Banzon, E. Freeman, G. Graham, B. Hankins, T. Smith and H.-M. Zhang (2021), Improvements of the Daily Optimum Interpolation Sea Surface Temperature (DOISST) Version 2.1, Journal of Climate 34, 2923–2939, doi:10.1175/JCLI-D-20-0166.1. Dataset page. Accessed 24 September 2026. Public domain (NOAA PSL disclaimer). The choice of grid cell and the rounding are ours; these are not official NOAA figures, and NOAA does not endorse this site.
Our calculation. We picked the nearest open-sea grid cell within 30 km of each weather station and rounded NOAA's monthly averages once, to one decimal place.