Yesterday's Weather Eye was dedicated to the exquisite discomfort, to the agony and the heroic ecstasy, of that small but dedicated band of swimmers who take to the icy waters on a Christmas Day. But what exactly do they have to cope with? How cold are the stormy waters around our coastline at this time of year?
The temperature of the water near the surface of the world's oceans varies from about 1C in the vicinity of the two poles to a balmy 20 to 30 in the tropics.
The decrease in temperature proceeds more or less uniformly with latitude - except in the North Atlantic, where the pattern is noticeably distorted by the warming influence of the North Atlantic Drift; this results in a tongue of warm water extending north-eastwards past Ireland, and protruding between Scotland and Iceland.
The North Atlantic Drift makes the winter waters in these parts warmer than anywhere else in the world at the same latitude.
Whatever the time of year, 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, near Fair Head in Antrim, the temperature difference between these two zones being usually about 2C or 3 C.
Right around our island, however, the seas are at present approaching their very coldest, and sea temperatures will reach their minimum in the early days of February. We would expect the sea temperature around Christmas to range from about 11C off the southwest coast to, say, 8C or 9C in the North Irish Sea.
These are, of course, average values. In any given year, and at any particular spot, the actual sea temperature may differ from this average by as much as a degree or two.
The temperature of the sea is of obvious interest to those hardy individuals who like to swim in it, but it is also of importance to the meteorologist cossetted snugly at the bench, because of its impact on the development of weather systems.
The sea is an enormous reservoir of energy: in winter it warms the cool air passing over it, and in summer it extracts excess heat from the atmosphere. In either case it can absorb or surrender great quantities of heat without itself suffering any noticeable change in temperature.
It has been calculated, for example, that a worldwide decrease in temperature of only 1C in the topmost metre of sea-water would release sufficient heat to increase the temperature of the lowest 3,000 feet of air in the atmosphere by almost four degrees.