Sea temperature and swimming season in Mutsu
The sea off Mutsu is comfortable for swimming from July to October. It peaks at 23.6°C in September and drops to 8.5°C in March.
Month by month
Sea temperature, the air you climb out into, and the wind that decides how that feels.
| Month | Sea | Air | Wind | Feels | What to wear |
|---|---|---|---|---|---|
| January | 10.2°C | 1.7°C ↓ | 10.9 m/s | Dangerous | 5/4 mm with boots, gloves and hood |
| February | 9.6°C | 1.8°C ↓ | 8.4 m/s | Dangerous | 5/4 mm with boots, gloves and hood |
| March | 8.5°C | 3.4°C ↓ | 7.1 m/s | Dangerous | 6/5 mm and a very good reason |
| April | 9.5°C | 10.7°C | 7.9 m/s | Dangerous | 5/4 mm with boots, gloves and hood |
| May | 11.6°C | 11.3°C ↓ | 6.2 m/s | Dangerous | 5/4 mm with boots, gloves and hood |
| June | 15.2°C | 16.4°C | 4.6 m/s | Cold | 3/2 mm full suit |
| July | 20.0°C | 20.2°C | 4.8 m/s | Comfortable | shorty, or nothing if you are acclimatised |
| August | 23.4°C | 23.9°C | 4.0 m/s | Very comfortable | none, or a rash vest |
| September | 23.6°C | 21.1°C ↓ | 6.0 m/s | Very comfortable | none, or a rash vest |
| October | 20.1°C | 14.5°C ↓ | 9.5 m/s | Comfortable | shorty, or nothing if you are acclimatised |
| November | 16.3°C | 7.7°C ↓ | 7.7 m/s | Bracing | 3/2 mm full suit |
| December | 13.1°C | 4.2°C ↓ | 10.7 m/s | Cold | 4/3 mm with boots |
Air is the average daily maximum — roughly what you would meet swimming in the middle of the day. An arrow marks months where the air is colder than the sea.
When in the day can you swim?
Daylight and burn risk by month, in Asia/Tokyo local time.
| Month | Sunrise | Sunset | Daylight | Peak UV |
|---|---|---|---|---|
| January | 06:58 | 16:30 | 9h 31m | 1.7 |
| February | 06:30 | 17:08 | 10h 38m | 2.9 |
| March | 05:47 | 17:40 | 11h 54m | 4.6 |
| April | 04:56 | 18:14 | 13h 18m | 6.7 |
| May | 04:16 | 18:46 | 14h 29m | 8.1 |
| June | 04:01 | 19:09 | 15h 08m | 8.9 |
| July | 04:14 | 19:07 | 14h 52m | 9.0 |
| August | 04:44 | 18:35 | 13h 51m | 8.2 |
| September | 05:15 | 17:45 | 12h 30m | 6.4 |
| October | 05:46 | 16:55 | 11h 09m | 4.1 |
| November | 06:22 | 16:16 | 9h 53m | 2.2 |
| December | 06:53 | 16:06 | 9h 13m | 1.5 |
Open water burns faster than a beach: the surface reflects sunlight up at you, and sunscreen washes off in the first few minutes. A long swim under a UV index above 6 needs a rash vest more than it needs more cream.
What the bands mean
| From | Description | In practice |
|---|---|---|
| above 26°C | Bath-like | Warm enough that it offers no cooling at all |
| above 22°C | Very comfortable | Swimwear, no time limit for most people |
| above 19°C | Comfortable | Pleasant once you are in; most people swim happily |
| above 16°C | Bracing | Swimmable but short; many prefer a shorty wetsuit |
| above 12°C | Cold | Wetsuit territory. Unprotected swims should be brief |
| below 12°C | Dangerous | Cold shock risk. Specialist gear and supervision |
Do you need a wetsuit in Mutsu?
Some months, yes. The sea drops to 8.5°C in March. What thickness, month by month, for swimming and for surfing →
Other coastal cities in Japan
Where these numbers come from, and what they miss
- Open water, not your beach. The measurement is a satellite cell about 28 km across, centred 12.2 km from Mutsu. Shallow bays and lagoons run warmer; exposed coasts with upwelling run colder, sometimes by several degrees.
- Surface temperature. Below a few metres the sea is often noticeably colder, and in summer a warm surface layer can sit over much colder water.
- A six-year average, not a thirty-year normal. Sea temperature varies little between years, so this is close — but it is not a climate normal, and any individual year will differ.
- Averages hide storms. A monthly mean says nothing about the week a cold current arrives.