Keats might have been writing of the last few days:
Ah, bitter chill it was;
The owl, for all his feathers, was a-cold;
The hare limp'd trembling through the frozen grass,
And silent was the flock in woolly fold.
The reason for the cold snap can be plainly seen on any weather map. The air surging over our island as an icy breeze has its origins in the cold Arctic regions far to the north of Ireland; chilled after lying for an extended period over ice and snow, it has been carried southwards without experiencing any significant change of temperature on the way.
The temperature outside, particularly at this time of year, is dictated mainly by the path followed by 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 atmosphere without affecting it, and the air then takes its temperature by contact with the surface over which it flows.
A glance at the weather map is often sufficient to identify the broad character of the air reaching our shores, and indeed this is precisely how meteorologists compile their forecasts in such circumstances. Since the wind blows along the isobars with low pressure to the left, the orientation of these lines on the chart indicates the general direction from which the air approaches us.
When the isobars show the air to have come from a southerly direction, perhaps having originated over the warm balmy waters of the Atlantic in the vicinity of the Azores, it will be mild and humid at any time of year; conversely, as at present, air flowing down from the north in wintertime will be very cold and raw.
The seeds of the future are, as it were, buried in the recent past; the forecaster proceeds, as Warwick puts it to a dejected Henry, by
Figuring the nature of the times deceased,
The which observed, a man may prophesy,
With a near aim, of the main chance of things
As yet not come to life.
The approaching air may be either maritime or continental, depending on whether it comes to us after spending a long time over the ocean or over a large area of land, and this, too, dictates its character.
Maritime air masses are humid, with plenty of moisture available for the formation of fog or rain. Continental air is dry, and may well result in cloud-free skies; in summer it is warm, but at this time of year winds approaching Ireland from the Continent are usually very cold indeed.