March is a deceptive time of year. Bright sunny days may be chilly when you venture forth, while dull and blustery conditions may well disguise an unexpected mildness. And sometimes these contrasting conditions may alternate with great alacrity, producing a climatic assortment of the kind described by the rock group Crowded House some years ago:
Even when you're feelin' warm,
The temperature could drop away
Like four seasons in one day. The warmth or coolness of the weather at any given time depends mainly on the path followed by our air before it reaches Ireland. Surprisingly, perhaps, the atmosphere itself 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 it is absorbed by the ground or by the ocean underneath. The air then takes its temperature by contact with the surface over which it flows.
It is by analysing the recent history of the expected Irish air, and noting where it came from, that meteorologists predict the daily temperatures. If air comes to us from the warm balmy regions of the Atlantic in the vicinity of the Azores it will be mild and humid; if it originates in the frozen Arctic wastes it will be cold and raw. In either case the temperature of the air as it flows over Ireland will be largely independent of whether or not the sun happens to be shining on our island at the time.
Meteorologists have names for all these different kinds of air, or air masses as they like to call them: each is classified according to its region of 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 more often to be dry. Secondly, air masses are classified geographically; those affecting Ireland may be "tropical", "polar", or now and then in wintertime, a bitter "arctic".
A glance at the weather map is often sufficient to identify the likely characteristics of the air in our vicinity. When the isobars show the air to have come from a southerly direction, temperatures higher than usual can be expected; conversely, air flowing from the north, or westwards from continental Europe in the wintertime, is likely to be cold.
And in the lively isobaric patterns typical of March, changes in the wind direction may well seem to bring about - as the song says - four seasons in one day.