Land masses in the northern hemisphere are at their coldest in January and at their warmest in mid-July. The sea, however, lags behind; it reaches its extreme values of temperature about a month later in each case, being coldest in February and warmest in late August.
The increase in temperature which takes place from early spring to late summer shows none of the daily ups and downs that characterise the temperature of the air; the change is slow and gradual, but relatively steady.
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.
Their technique is very simple: they just heave a bucket over the side, haul it up full of water and measure the water temperature with a thermometer.
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, modern technology has provided a more efficient method of obtaining this information. Sensors aboard orbiting satellites react to the longwave radiation emitted by the surface of the ocean, which is in turn dependent on the temperature of the water.
By this means, maps showing the variations in sea surface temperature from place to place throughout the world can be obtained almost instantaneously.
The warmest water in the vicinity of Ireland is usually to be found in the south-west, near the coasts of Kerry and Cork; the sea is coldest, on the other hand, off the coast of Antrim, the temperature difference between those two zones being usually about 2 or 3 Celsius.
Mid-winter sea temperatures off the south-west coast are normally about 10 C, while in the north Irish Sea. they drop to around 7 C. At the other extreme, the summertime maximum in August reaches 15.5 C along most of the south coast, but is only 13 C between Rathlin Island and the Mull of Kintyre in Scotland.
Those 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 and, 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 warmest year in recent times for sea temperatures around Ireland, surprisingly, was 1995 when the temperature of the sea water reached an all-time record of 16 C in July and a balmy 17.5 C in August.