'Halcyon days" is a Mediterranean concept. In Greek mythology the term referred to a brief period of unseasonally sunny, mild and nearly windless weather that seemed to turn up with remarkable regularity in mid-December.
The fine spell was seen to be particularly useful to the halcyon, a species of kingfisher common on the Mediterranean, which builds its floating nest on the water around this time of year.
More often than not, the wily bird, by design or accident, seemed to enjoy ideal conditions for this task, and might even, some thought, have the power itself to bring about the calm. By extension, the expression "halcyon days" came to signify any brief, fondly remembered, interlude of quiet in other wise turbulent and difficult times. This year, however, there are no halcyon days in southern Europe. The weather maps suggest that Greece and its environs are enduring a chilly and rather changeable regime at present, and the high pressure that might have brought the peace and calm is firmly ensconced over the North Sea, bringing a kind of winter summer to Britain and to Ireland.
The "normal" state of the atmosphere over Northern Europe is one of great mobility. Westerly winds predominate, and depressions move rapidly eastwards across the ocean in perennial procession, often passing very close to Ireland. It is this volatility that explains the usual prevalence of storms and rainy conditions at this time of year. But every so often, the pattern is interrupted. Sometimes the jet stream, the core of the upper winds and the steering mechanism for depressions, ceases to surge determinedly from west to east across the North Atlantic; it develops a very uncertain path, meandering first north, then south. In such a situation, it often happens that a resilient anticyclone forms at sea-level over Northern Europe under one of the northerly meanders - an anticyclone which blocks the normal track of the depressions and obliges them to veer well to the north or to the south of Ireland. Such a blocking anticyclone may persist for days or even weeks, bestowing on us quiet, settled, almost windless weather.
When accompanied by clear skies, winter anticyclones like these allow the earth to lose heat very rapidly at night, so that while the following morning may be bright and sunny, the ground is often hard with frost. But either way, the conditions are calm and unseasonally pleasant.