THE sea reacts slowly to the changing seasons. Land masses of the northern hemisphere generally experience their highest temperatures in late July and are at their coldest in the closing days of January. The ocean, however, reaches its extreme temperature values about a month later in each case. The sea water around Ireland is at its coldest at the end of February, and warms slowly to reach a maximum around now, in late August or early September. The increase in temperature from early spring to late summer shows none of the daily ups and downs that characterise the temperature of the air; the change is gradual but steady, so that around this time of the year the water on Irish coasts is at its most amenable.
The warmest water in the vicinity of Ireland is usually to be found in the southwest, near the coasts of Kerry and Cork; the sea is coldest, on the other hand, off the coast of Antrim, the temperature difference between these two zones being usually about 2 or 30C. Mid winter sea temperatures off the southwest coast are normally a little less than 100C, while in the North Irish Sea they drop to around seven degrees. At the other extreme, the summertime maximum in August reaches 15.5C along most of the south coast, but is only 13 between Fair Head and the coast of Scotland.
These figures represent the temperature some distance from the coast; close to shore, and particularly in large estuaries, the summer sea might be expected to be slightly warmer. Moreover the figures quoted are average values: in any particular year the actual sea temperature at any time may differ from the average by as much as a degree or two. The coldest years in recent times for sea temperatures around Ireland were 1963, 1979 and 1986; the warmest year by far, and not surprisingly, was 1995 when the temperature of the sea water reached an all time record of 160C in July and a balmy 17.50C in August.
It used to be that all our information about the temperature of the sea came from passing ships, as they carefully gathered the data at frequent and regular intervals during a voyage. Met Eireann has also measured the sea temperature at Malin Head twice daily since 1957, providing a continuous record except for those rare occasions when severe weather has prevented access to the local pier. In recent years, however, sea temperatures are available from weather satellites, allowing maps showing isotherms over a large area to be compiled easily and on an almost daily basis.