When is the sea warm enough to swim?
Irish sea temperatures month by month, what each one feels like, and how to stay safe in cold water, wind and tides.
Sea swimming has become a year-round habit in Ireland, from the Forty Foot in Dublin to Salthill in Galway and the coves of Cork and Kerry. The water is never warm by Mediterranean standards, but it is a good deal warmer than the air in winter and, in late summer, comfortable for most people. Here is what to expect month by month.
Sea temperature through the year
The sea warms and cools slowly, so it lags the seasons by six to eight weeks. The coldest water comes in late February and March, not December, and the warmest in August and early September, not June.
| Month | Sea temperature | How it feels |
|---|---|---|
| January | 9 to 10°C | Cold. A few minutes for the experienced; a wetsuit for everyone else. |
| February | 8 to 10°C | Cold, and the air colder still. |
| March | 8 to 9°C | The coldest of the year. |
| April | 9 to 10°C | Still cold, though the sunshine helps on the beach. |
| May | 10 to 12°C | Cool. Short swims without a wetsuit if you are used to it. |
| June | 12 to 14°C | Cool but bearable; the shallows warm up on a sunny afternoon. |
| July | 14 to 16°C | Comfortable for most people for a proper swim. |
| August | 15 to 17°C | The warmest of the year, with sheltered bays a degree or two warmer still. |
| September | 14 to 16°C | Still warm, and the beaches are quiet. |
| October | 13 to 14°C | Cooling, but milder than the air on a grey day. |
| November | 11 to 13°C | Cold enough to take your breath away. |
| December | 10 to 12°C | Cold; the Christmas swim is a dip, not a swim. |
The figures are typical for open coast. Dublin Bay runs from about 8°C in March to about 15°C in August. The south and west coasts are a degree warmer in winter, thanks to the Atlantic, and shallow, sheltered bays such as those of Wexford and Waterford can reach 17 or 18°C after a warm August. The sea off the north coast is coldest.
What cold water does
Below about 15°C the body reacts to sudden immersion with cold water shock: a gasp, fast breathing and a racing heart that last a minute or two. It is the biggest danger in Irish water and it does not care how fit you are. Get in slowly, let the breathing settle before you swim, and never jump into water you have not been in before. In water under 12°C most people lose useful strength in their arms within ten to fifteen minutes, long before hypothermia sets in, so swim close to shore and get out while you still feel strong. Warm up slowly afterwards, with dry clothes and a hot drink rather than a hot shower.
Wind, waves and tides
Wind matters as much as temperature. Under 15 km/h the sea is flat; from about 28 km/h, Force 5, there are white horses and real waves, and swimming becomes hard work and then dangerous. An offshore wind flattens the surface but pushes floats, boards and tired swimmers out to sea, which is why lifeguards fly the orange windsock. The Sports weather page rates each day of the week ahead for open-water swimming, and the hourly forecast for your nearest town gives the wind through the day.
Swim at lifeguarded beaches when you can; most are staffed from June to early September. Rip currents form at beaches with surf, especially near rocks, piers and river mouths: if you are carried out, swim across the current parallel to the shore rather than against it, and raise an arm for help. Check the tide: a big spring tide moves fast around headlands and sandbars, and the safest swimming is usually around slack water at high tide.
After heavy rain
Rain washes the land into the sea. After a wet day, water quality at beaches near towns and rivers can fall for a day or two, and local authorities sometimes post a temporary warning. Give it 48 hours after a downpour at an urban beach, and check the council's water quality notices, which are published for every designated bathing area through the summer.
The best months
For a warm, easy swim, August and September. For the quietest sea, early morning in any month, before the sea breeze picks up. And for the Christmas swim, the water is 10 to 12°C, milder than it looks from the shore, but keep it short.