THE WINDOW has been a poor thermometer of late. Bright sunny days turn bitter cold as soon as you venture forth, while dull and damp conditions may well disguise an unexpected mildness. And sometimes vice versa. It seems as if January is unable to make up its mind as different weathers alternate confusingly.
The temperature outside, particularly at this time of year, is mainly dictated by the path followed by the air before it reaches Ireland. The atmosphere is not heated directly by the sun, being almost "transparent" to its short-wave solar radiation. The sun's energy, by and large, passes directly through the air without affecting it, but is absorbed by the ground or by the ocean underneath, and the air then takes its temperature by contact with the surface over which it flows.
If air comes from the east, having swept for several thousand miles across the snows of eastern Europe and the frozen wastes of Russia and Siberia it will be cold and raw - as it was a week or so ago. 11, on the other hand, it originates over the warm balmy waters of the Atlantic in the vicinity of the Azores, it will be mild and humid, as it is at present. In either case, the seeds of today's weather may have been sown several weeks ago on the steppes of Russia, or amid the waving palm trees on an island in the Caribbean.
Meteorologists have names for all these different kinds of air, or air masses as they like to call them - each is classified according to the region in which it has its origin. In the first instance, an air mass may be "maritime" or "continental", depending on whether it approaches from over the ocean or over a large tract of land. As you would expect, maritime air masses are humid and continental air masses tend to be dry. Secondly, air masses are classified geographically - those affecting Ireland may be "tropical", "polar" or occasionally at this time of year, a bitter "arctic".
A casual glance at the weather map in the past few days would have shown the isobars stretching southwestwards from Ireland towards the lower latitudes of the north Atlantic. The air moving along them in our direction, therefore, can be seen to have travelled over the ocean from a warm and sultry region of the hemisphere, and can, therefore, be classified as "tropical maritime" - a strange designation, one might think, for the air apparent on a January day.