Sea temperature and swimming season in Hitachi
The sea off Hitachi is comfortable for swimming from June to November. It peaks at 25.0°C in August and drops to 14.6°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 | 15.8°C | 8.3°C ↓ | 5.0 m/s | Cold | 3/2 mm full suit |
| February | 15.2°C | 7.0°C ↓ | 5.1 m/s | Cold | 3/2 mm full suit |
| March | 14.6°C | 12.2°C ↓ | 5.0 m/s | Cold | 4/3 mm with boots |
| April | 15.7°C | 16.1°C | 4.5 m/s | Cold | 3/2 mm full suit |
| May | 17.9°C | 19.0°C | 4.1 m/s | Bracing | 3/2 mm full suit |
| June | 19.6°C | 21.1°C | 3.4 m/s | Comfortable | shorty, or nothing if you are acclimatised |
| July | 22.9°C | 25.0°C | 3.6 m/s | Very comfortable | none, or a rash vest |
| August | 25.0°C | 25.9°C | 2.9 m/s | Very comfortable | none needed |
| September | 25.0°C | 24.6°C ↓ | 4.2 m/s | Very comfortable | none needed |
| October | 22.4°C | 20.5°C ↓ | 5.1 m/s | Very comfortable | none, or a rash vest |
| November | 19.6°C | 15.9°C ↓ | 4.4 m/s | Comfortable | shorty, or nothing if you are acclimatised |
| December | 17.4°C | 12.0°C ↓ | 5.5 m/s | Bracing | 3/2 mm full suit |
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 | 06:48 | 16:44 | 9h 56m | 2.6 |
| February | 06:25 | 17:17 | 10h 52m | 4.0 |
| March | 05:48 | 17:44 | 11h 55m | 5.9 |
| April | 05:03 | 18:11 | 13h 07m | 8.0 |
| May | 04:30 | 18:37 | 14h 07m | 9.4 |
| June | 04:18 | 18:57 | 14h 39m | 10.1 |
| July | 04:30 | 18:56 | 14h 26m | 10.3 |
| August | 04:54 | 18:29 | 13h 35m | 9.6 |
| September | 05:19 | 17:46 | 12h 26m | 7.8 |
| October | 05:44 | 17:02 | 11h 18m | 5.3 |
| November | 06:14 | 16:28 | 10h 14m | 3.2 |
| December | 06:42 | 16:22 | 9h 41m | 2.3 |
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 Hitachi?
Some months, yes. The sea drops to 14.6°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 20.3 km from Hitachi. 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.