Back in Ireland for a little time from my usual haunts down the valley of the Rhine, I notice how much cooler it is here than on the Continent. Latitude is responsible in part, since Ireland is significantly further north than much of Germany. More important, however, is the almost complete absence of wind in Central Europe, which allows the ground, heated by the early summer sunshine, to raise with ease the temperature of the lower atmosphere.
Ireland, on the other hand, lies in the perpetual swirl of the circumpolar circulation, right in the path of a near-continuous eastward-moving stream of cloud and rain and wind.
Land and sea differ markedly in the way in which they absorb the sun's energy. Water responds slowly, and shows a very small increase in temperature for a given added increment of heat. A land surface, on the other hand, responds quickly to solar radiation, and ground temperature increases rapidly under its influence. Consequently, the daily variation of temperature is much less in places near the sea than it is in continental regions.
The same is true of the annual range of temperature - the difference between the average temperature of the warmest month and the coldest month. In Dublin, for instance, the annual range of average temperature is about 10 degrees; in Berlin it is 23 degrees, while in Vladivostok, Russia, it is a massive 35 degrees. One way of analysing figures like these, and of making them easier to classify, is to use what meteorologists call an Index of Continentality, the most frequently used being that devised in 1931 by a Swedish meteorologist called O.V. Johansson. His formula was based on figures for Thorshavn in the Faroe Islands and Verkhoyansk in the middle of Siberia. At the former, he felt, the oceanic influence was total, so his formula was constructed to assign to it an Index of Continentality of zero per cent. The latter, by contrast, was totally continental, and had a value of 100 per cent.
Based on Johansson's formula, the level of continentality in Ireland, as one might expect, is very small. Nowhere in the country, after all, is more than about 60 miles from the sea. Coastal places like Valentia, Belmullet or Malin Head turn out to have a continentality of 3 per cent, while midland areas measure about 8 per cent on the Johansson scale. These compare with indices of continentality of 60 per cent for the centre of North America, 30 per cent for most of Western Europe, and about 45 per cent for Western Russia.