Sea temperature and swimming season in Portsmouth
The sea off Portsmouth never reaches 19.0°C. It peaks at 18.8°C in August and falls to 8.4°C in February. People do swim here — but in a wetsuit, or briefly, and knowing what they are doing.
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 | 9.1°C | 9.1°C | 7.6 m/s | Dangerous | 5/4 mm with boots, gloves and hood |
| February | 8.4°C | 9.2°C | 8.6 m/s | Dangerous | 6/5 mm and a very good reason |
| March | 8.7°C | 8.8°C | 5.9 m/s | Dangerous | 6/5 mm and a very good reason |
| April | 10.1°C | 12.0°C | 4.2 m/s | Dangerous | 5/4 mm with boots, gloves and hood |
| May | 12.6°C | 14.8°C | 6.1 m/s | Cold | 4/3 mm with boots |
| June | 15.4°C | 15.6°C | 6.5 m/s | Cold | 3/2 mm full suit |
| July | 17.7°C | 19.0°C | 5.5 m/s | Bracing | 3/2 mm full suit |
| August | 18.8°C | 20.5°C | 5.2 m/s | Bracing | shorty, or nothing if you are acclimatised |
| September | 18.3°C | 17.7°C ↓ | 5.1 m/s | Bracing | shorty, or nothing if you are acclimatised |
| October | 16.2°C | 15.9°C ↓ | 6.0 m/s | Bracing | 3/2 mm full suit |
| November | 13.7°C | 11.6°C ↓ | 6.9 m/s | Cold | 4/3 mm with boots |
| December | 10.8°C | 10.1°C ↓ | 7.4 m/s | Dangerous | 5/4 mm with boots, gloves and hood |
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 Europe/London local time.
| Month | Sunrise | Sunset | Daylight | Peak UV |
|---|---|---|---|---|
| January | 08:00 | 16:27 | 8h 27m | 0.6 |
| February | 07:16 | 17:20 | 10h 04m | 1.4 |
| March | 06:17 | 18:08 | 11h 51m | 2.8 |
| April | 06:09 | 19:59 | 13h 49m | 4.6 |
| May | 05:15 | 20:46 | 15h 31m | 6.1 |
| June | 04:50 | 21:19 | 16h 29m | 7.0 |
| July | 05:08 | 21:12 | 16h 04m | 7.0 |
| August | 05:52 | 20:25 | 14h 33m | 6.0 |
| September | 06:39 | 19:19 | 12h 39m | 4.2 |
| October | 07:27 | 18:12 | 10h 45m | 2.2 |
| November | 07:20 | 16:17 | 8h 58m | 0.9 |
| December | 08:00 | 15:58 | 7h 59m | 0.5 |
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 Portsmouth?
Some months, yes. The sea drops to 8.4°C in February. What thickness, month by month, for swimming and for surfing →
Other coastal cities in United Kingdom
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 19.5 km from Portsmouth. 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.