Cornwall

The sea in Cornwall peaks in August, when the water averages around 17°C.

On average, 0 months a year sit above 20°C — the threshold most swimmers describe as pleasant without a wetsuit.

Outside peak season the water drops toward 9°C, so a 3/2mm shorty or full wetsuit makes long swims much more comfortable.

The coldest month is typically February at around 8°C, when cold-water swimmers dominate the coast.

Long-term climatology for Cornwall shows an annual mean sea temperature of 12°C, ranging from 8°C in winter to 17°C at the summer peak.

The seasonal swing is roughly 9°C between the coldest and warmest month — a useful yardstick when comparing years or checking whether the current sea is running above or below average.

Recent decades have trended slightly warmer at Cornwall, in line with regional sea-surface temperature records for the wider basin.

Monthly averages below are drawn from multi-decadal climatology and are updated as new observations arrive.