The temperature of the water near the surface of the world's oceans varies from about 1 Celsius in the vicinity of the two poles to a balmy 20 C to 30C 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 Gulf Stream.
It results in a tongue of warm water in the vicinity of Ireland, which in turn brings us higher winter temperatures, on average, than anywhere else in the world at the same latitude.
Take Cork, for instance, which enjoys an average January temperature of about 6 C.
To see how it compares with other cities at the same latitude, let us imagine ourselves circumnavigating the globe, not, as Steve Fossett tried to do of late, in a balloon, but on the surface, thermometer in hand, along the 52nd parallel.
As we travel eastwards, the same line of latitude brings us very close to London, where we note that the temperature of an average January day has dropped to 5 C. Still further east, we come upon Berlin; our average thermometer now reads zero and is still falling.
Indeed the ground may well be ankle-deep in snow, since it lies on the ground for an average of 40 days each winter in Berlin, compared to three in Cork.
The farther east one ventures, over Poland and deep into Russia, the colder it becomes, and lower and lower falls the average temperature for this time of year. It reaches its nadir in eastern Siberia, near Irkutsk, where the latitude is still 52 N, but the January temperature is a freezing minus 20 C, and that is only the average. Imagine yourself coatless in Irkutsk on a really cold Irkutskian day!
Continuing eastwards, temperatures increase only slowly across Asia, until the maritime influence of the Pacific begins to take effect. January temperatures in mid-latitudes over the Pacific are lower than those on the Atlantic; the highest at 52 N is 3 C or 4 C, before it is time to wrap up again for the cold passage across Canada.
Vancouver, Winnipeg and the Gulf of St Lawrence lie roughly in line across the North American continent, and are 2 of latitude further south than Cork. Their average temperatures in January are 3, 13 and 12 C respectively. Only as we move eastwards across the Atlantic does our average thermometer be gin to rise again, reaching its highest point of the entire journey 200 miles to the west of Ireland.
Cork, clearly, is the warmest city in the world at its latitude in January.