December days in dark grip of winter

Weather Eye/Brendan McWilliams: December days are dreary, dark and dull. There are fewer hours of sunshine than in any other month, although in one way this must be expected, since even if we were to enjoy cloud-free skies throughout, the long dark nights would curtail potential sunlight. In the event, it is a cloudy month as well, and the combined effect is an average of only one hour of sunshine per day.

John Greenleaf Whittier, of Barbara Frietchie fame, summed up the picture over a hundred years ago: "The sun that brief December day rose cheerless over hills of grey, And, darkly circled, gave at noon a sadder light than waning moon." Although it is the last month of our year, December takes its name from the fact that it was the tenth month of the old Roman calendar, which began in March. In more recent times the Saxons, quite sensibly, referred to it as Winter Monath, and when they converted to Christianity they changed its name to Helighmonath, Holy Month. To our own ancestors it was Mi na Nollag, the month of Noel from the Latin word natal, as in "nativity". It is, in other words, the month of Christmas.

Meteorologists, in this respect, are conceptually closest to the Saxons, since for them December is the first month of winter. Others, of course, have different ideas; astronomers, for example, allow autumn to run for another month, so that their astronomical winter begins just before Christmas, at the winter solstice around December 21st. But weather people trust to their thermometers, and regard winter as comprising the three coldest calendar months of the year - December, January and February. Like the other winter months, the average December in Ireland is characterised by a persistent procession of active depressions heading north-eastwards close to the coasts of Donegal and Mayo. As a result, it is often a very windy month, with more than its fair share of stormy conditions. A typical December brings gale force winds to our western coasts on eight or nine of its 31 days.

On an average December day the air temperature rises to a mere nine degrees, and those brave enough to contemplate a Christmas swim should be prepared for water temperatures of no more than 8 or 9 degrees. As winter gains in confidence and tightens up its grip, ground frost can be expected to be present on over half the mornings of the month.

The highest rainfall occurs in the mountainous parts of Kerry where the December norm is about 400 mm, although parts of the midlands and east escape with less than 100 mm.

And although January and February are the big months for snow, it can happen in December too; the records tell us that sleet or snow occurs on an average of three or four days this month in the northern parts of our island.

Most of the time these falls are not disruptive - but the risk is always there, especially in December's closing days.