Mamaia sea temperature in September

Median on 26 September

21.4 °C

On 26 September, between

19.7 and 23.1 °C

Coldest day, 20–30 September

18.8 °C · 29 September 2022

Warmest day, 20–30 September

23.2 °C · 25 September 2023

We looked at the daily mean sea temperature from 20 to 30 September in every year from 2016–2025: 110 days, with 26 September — the 2027 race date — in the middle.

Only 9 of the 110 days were below 20 °C, all in 2022, from 22 September to 30 September.

The water cools through the window: from 20 to 30 September it dropped by 1.4 °C on average, and in 9 of 10 years the warmest day was one of the first three.

When the race was announced, some articles quoted a water temperature of about 16 °C. NOAA's data shows nothing like it at Mamaia in late September: on none of the 110 days analysed did the daily mean fall below 18.8 °C.

That doesn't rule out a colder morning. With a persistent southerly or south-westerly wind, warm surface water can be pushed offshore and colder water from below rises near the shore (upwelling), by several degrees within hours. A daily mean over a cell about 25 km across can miss such an episode at the start beach; what counts is the official reading on race morning.

IRONMAN rules allow age-group athletes a wetsuit up to and including 24.5 °C; above that, Mamaia prohibits them, and below 16 °C they are mandatory. The call is made from the official reading on race morning. On every one of the 110 days on record the water stayed at or below that.

Between 07:00 and 15:00 on 26 September the wind averaged 6–31 km/h with gusts up to 68 km/h, and the air was 8–26 °C (ERA5 reanalysis). In windy years the waves make the swim harder, and on the bike the wind matters more than the climbing.

Wind and air, year by year, on the race page →

• Your wetsuit — the final call is made on race morning.
• Tinted or mirrored goggles, plus a clear pair: Mamaia beach faces east, so with a morning start you swim towards the sun on the way out.
• Warm clothes for the morning and after the finish: during race hours the air was 8–26 °C.
• Anti-chafe cream for neck and armpits: wetsuits and salt water rub the skin raw.

• Sea temperature: NOAA OISST v2.1, NOAA's daily product blending satellite observations with ship and buoy measurements on a 0.25° grid (about 25 km).
• We used the nearest ocean grid cell, centred at 44.375° N, 28.625° E, about 15 km north of the start, and compared it with an offshore cell (44.125° N, 28.875° E): on 26 September they differed by at most 0.9 °C.
• Cross-check: for 2023, 2024, 2025, the hourly Open-Meteo Marine model gives 23.3 / 22.2 / 22.2 °C at 07:00 on 26 September, against 23.1 / 21.8 / 22.1 °C in OISST.
• Weather: ERA5 reanalysis (Copernicus) through Open-Meteo's historical API, for 07:00–15:00 at the transition area in Mamaia.
• These are satellite estimates and reanalysis, not readings at the start beach. We update the data after every September.